takes from the atmosphere about a half ton of carbon per acre
annually, or fifty tons per acre in a century. Fifty tons of carbon of
the specific gravity of coal, about 1·5, spread evenly over the surface
of an acre, would make a layer nearly one-third of an inch.[243]
Humboldt makes the estimate a little higher, viz., one half-inch.
Taking the latter estimate, it would require 7,200 years to form a
bed of coal a yard thick. Dr. Heer, of Zurich, thinks that it would
not require more than 1,400 years to form a bed of coal one yard
thick;[244] while Mr. Maclaren thinks that a bed of coal one yard thick
would be formed in 1,000 years.[245] Professor Phillip, calculating
from the amount of carbon taken from the atmosphere, as determined by
Liebig, considers that if it were converted into ordinary coal with
about 75 per cent. of carbon, it would yield one inch in 127·5 years,
or a yard in 4,600 years.[246]
There is here a considerable amount of difference in regard to the time
required to form a yard of coal. The truth, however, may probably be
somewhere between the two extremes, and we may assume 5,000 years to be
about the time. In a warm period of 15,000 years we should then have
deposited a seam of coal 9 feet thick, while during a warm period of
7,000 years we would have a seam of only 4 feet.
_Reason why the Coal Strata present so little Evidence of
Ice-action._—There are two objections which will, no doubt, present
themselves to the reader’s mind. (1.) If coal be an inter-glacial
formation, why do the coal strata present so little evidence of
ice-action? If the coal-seams represent warm inter-glacial periods, the
intervening beds must represent cold or glacial periods, and if so,
they ought to contain more abundant evidence of ice-action than they
really do. (2.) In the case of the glacial epoch, almost every vestige
of the vegetation of the warm periods was destroyed by the ice of the
cold periods; why then did not the same thing take place during the
glacial epoch of the Carboniferous period?
During the glacial epoch the face of the country was in all
probability covered for ages with the most luxuriant vegetation; but
scarcely a vestige of that vegetation now remains, indeed the very soil
upon which it grew is not to be found. All that now remains is the
wreck and desolation produced by the ice-sheet that covered the country
during the cold periods of that epoch, consisting of transported blocks
of stones, polished and grooved rocks, and a confused mass of boulder
clay. Here we have in this epoch nothing tangible presenting itself
but the destructive effects of the ice which swept over the land. Why,
then, in reference to the glacial epochs of the Carboniferous age
should we have such abundant evidence of the vegetation of the warm
periods, and yet so little evidence of the effect of the ice of the
cold periods? The answer to these two objections will go a great way
to explain why we have so much coal belonging to the Carboniferous
age, and so little belonging to any other age; and it will, I think,
be found in the peculiar physical character of the country during
the Carboniferous age. The areas on which the forests of the Coal
period grew escaped the destructive power of glaciers and land-ice on
account of the flat nature of the ground. There are few points on which
geologists are more unanimous than in regard to the flat character of
the country during the Coal period.
There does not seem to be any very satisfactory evidence that the
interior of the country rose to any very great elevation. Mr.
Godwin-Austen thinks that during the Coal period there must have
been “a vast expanse of continuous horizontal surface at very slight
elevations above the sea-level.”[247] Of the widely spread terrestrial
surface of the Coal-measure period, portions, he believes, attained
a considerable elevation. But in contrast to this he states, “There
is a feature which seems to distinguish this period physically from
all subsequent periods, and which consists in the vast expanse of
continuous horizontal surface which the land area presented, bordering
on, and at very slight elevations above, the sea-level.”[248] Hugh
Miller, describing in his usual graphic way the appearance of the
country during the Coal period, says:—“It seems to have been a land
consisting of immense flats, unvaried, mayhap, by a _single hill_,
in which dreary swamps, inhabited by doleful creatures, spread out
on every hand for hundreds and thousands of miles; and a gigantic
and monstrous vegetation formed, as I have shown, the only prominent
features of the scenery.”[249]
Now, if this is in any way like a just representation of the general
features of the country during the Coal period, it was physically
impossible, no matter however severe the climate may have been,
that there could have been in this country at that period anything
approaching to continental ice, or perhaps even to glaciers of such
dimensions as would reach down to near the sea-level, where the coal
vegetation now preserved is supposed chiefly to have grown. The
condition of things which would prevail would more probably resemble
that of Siberia than that of Greenland.
The absence of all traces of ice-action in the strata of the
coal-measures can in this case be easily explained. For as by
supposition there were no glaciers, there could have been no
scratching, grooving, or polishing of the rocks; neither could there
have been any icebergs, for the large masses known as icebergs are
the terminal portions of glaciers which have reached down to the sea.
Again, there being no icebergs, there of course could have been no
grinding or scratching of the rocks forming the floor of the ocean.
True, during summer, when the frozen sea broke up, we should then
have immense masses of floating ice, but these masses would not be of
sufficient thickness to rub against the sea-bottom. But even supposing
that they did occasionally touch the bottom here and there, we could
not possibly find the evidence of this in any of the strata of the
coal-measures. We could not expect to find any scratchings or markings
on the sandstone or shale of those strata indicating the action of
ice, for at that period there were no beds of sandstone or shale, but
simply beds of sand and mud, which in future ages became consolidated
into sandstone and shale. A mass of ice might occasionally rub along
the sea-bottom, and leave its markings on the loose sand or soft mud
forming that bottom, but the next wave that passed over it would
obliterate every mark, and leave the surface as smooth as before.
Neither could we expect to find any large erratics or boulders in the
coal strata, for these must come from the land, and as by supposition
there were no glaciers or land-ice at that period, there was therefore
no means of transporting them. In Greenland the icebergs sometimes
carry large boulders, which are dropped into the sea as the icebergs
melt away; but these blocks have all either been transported on the
backs of glaciers from inland tracts, or have fallen on the field-ice
along the shore from the face of crags and overhanging precipices.
But as there were probably neither glaciers reaching to the sea, nor
perhaps precipitous cliffs along the sea-shore, there could have been
few or no blocks transported by ice and dropped into the sea of the
Carboniferous period, and of course we need not expect to find them in
the sandstone and shale which during that epoch formed the bed of the
ocean. There would no doubt be coast-line ice and ground-ice in rivers,
carrying away large quantities of gravel and stones; but these gravels
and stones would of course be all water-worn, and although found in the
strata of the coal-measures, as no doubt they actually are, they would
not be regarded as indicating the action of ice. The simple absence of
relics of ice-action in the coal-measures proves nothing whatever in
regard to whether there were cold periods during their formation or not.
This comparative absence of continental ice might be one reason why
the forests of the Carboniferous period have been preserved to a much
greater extent than those of any other age.
It must be observed, however, that the conclusions at which we have
arrived in reference to the comparative absence of continental ice
applies only to the areas which now constitute our coal-fields. The
accumulation of ice on the antarctic regions, and on some parts of
the arctic regions, might have been as great during that age as it
is at present. Had there been no continental ice there could have
been no such oscillations of sea-level as is assumed in the foregoing
theory. The leading idea of the theory, expressed in a few words,
is, that the glacial epochs of the Carboniferous age were as severe,
and the accumulation of ice as great, as during any other age, only
there were large tracts of flat country, but little elevated above the
sea-level, which were not covered by ice. These plains, during the
warm inter-glacial periods, were covered with forests of sigillariæ
and other coal trees. Portions of those forests were protected by the
submergence which resulted from the rise of the sea-level during the
cold or glacial periods and the subsequent subsidence of the land.
Those portions now constitute our coal-beds.
But that coal may be an inter-glacial formation is no mere hypothesis,
for we have in the well-known Dürnten beds—described in Chapter XV.—an
actual example of such a formation.
_Carboniferous Limestones._—As a general rule the limestones of the
Carboniferous period, like the coal, are found in beds separated by
masses of sandstone and other stratified deposits, which proves that
the corals, crinoids, and other creatures, of the remains of which it
is composed, did not live continuously on during the entire Limestone
period. These limestones are a marine formation. If the land was
repeatedly submerged the coal must of necessity have been produced in
seams with stratified deposits between, but there is no reason why the
same should have been the case with the limestones. If the climatic
condition of the sea continued the same we should not have expected
this alternate succession of life and death; but, according to the
theory of alternate cold and warm periods, such a condition follows
as a necessary consequence, for during the warm periods, when the
land was covered with a luxuriant vegetation, the sea-bottom would be
covered with mollusca, crinoids, corals, &c., fitted to live only in a
moderately warm sea; but when the cold came on those creatures would
die, and their remains, during the continuance of the cold period,
would become slowly covered over with deposits of sand and clay. On the
return of the warm period those deposits would soon become covered with
life as before, forming another bed of limestone, and this alternation
of life and death would go on as long as the glacial epoch continued.
It is true that in Derbyshire, and in the south of Ireland and some
other places, the limestone is found in one mass of several hundred
feet in thickness without any beds of sandstone or shale, but then it
is nowhere found in one continuous mass from top to bottom without any
lines of division. These breaks or divisions may as distinctly mark
a cold period as though they had been occupied by beds of sandstone.
The marine creatures ceased to exist, and when the rough surface left
by their remains became smoothed down by the action of the waves into
a flat plain, another bed would begin to form upon this floor so
soon as life again appeared. Two agencies working together probably
conspired to produce these enormous masses of limestone divided only
by breaks marking different periods of elaboration. Corals grow in
warm seas, and there only in water of a depth ranging from 20 to 30
fathoms. The cold of a period of glaciation would not only serve to
destroy them, but they would be submerged so much beyond the depth
proper for their existence that even were it possible that with the
submergence a sufficient temperature was left, they would inevitably
perish from the superincumbent mass of water. We are therefore, as
it seems to me, warranted in concluding that the separate masses of
Derbyshire limestone were formed during warm inter-glacial periods,
and that the lines of division represent cold periods of glaciation
during which the animals perished by the combined influence of cold and
pressure of water. The submergence of the coral banks in deep water on
a sea-bottom, which, like the land, was characteristically flat and
even, implies its carrying away far into the bosom of the ocean, and
consequently remote from any continent and the river-borne detritus
thereof.
CHAPTER XXVII.
PATH OF THE ICE-SHEET IN NORTH-WESTERN EUROPE AND ITS RELATIONS
TO THE BOULDER CLAY OF CAITHNESS.[250]
Character of Caithness Boulder Clay.—Theories of the Origin
of the Caithness Clay.—Mr. Jamieson’s Theory.—Mr. C. W.
Peach’s Theory.—The proposed Theory.—Thickness of Scottish
Ice-sheet.—Pentlands striated on their Summits.—Scandinavian
Ice-sheet.—North Sea filled with Land-ice.—Great Baltic
Glacier.—Jutland and Denmark crossed by Ice.—Sir R.
Murchison’s Observations.—Orkney, Shetland, and Faroe Islands
striated across.—Loess accounted for.—Professor Geikie’s
Suggestion.—Professor Geikie and B. N. Peach’s Observations
on East Coast of Caithness.—Evidence from Chalk Flints and
Oolitic Fossils in Boulder Clay.
_The Nature of the Caithness Boulder Clay._—A considerable amount of
difficulty has been felt by geologists in accounting for the origin of
the boulder clay of Caithness. It is an unstratified clay, of a deep
grey or slaty colour, resembling much that of the Caithness flags on
which it rests. It is thus described by Mr. Jamieson (Quart. Jour.
Geol. Soc., vol. xxii., p. 261):—
“The glacial drift of Caithness is particularly interesting as an
example of a boulder clay which in its mode of accumulation and
ice-scratched _débris_ very much resembles that unstratified stony mud
which occurs underneath glaciers—the ‘_moraine profonde_,’ as some call
it.
“The appearance of the drift along the Haster Burn, and in many other
places in Caithness, is in fact precisely the same as that of the old
boulder clay of the rest of Scotland, except that it is charged with
remains of sea-shells and other marine organisms.
“If want of stratification, hardness of texture, and abundance of
well-glaciated stones and boulders are to be the tests for what we call
genuine boulder clay, then much of the Caithness drift will stand the
ordeal.”
So far, therefore, as the mere appearance of the drift is concerned,
it would at once be pronounced to be true Lower Till, the product of
land-ice. But there are two circumstances connected with it which have
been generally regarded as fatal to this conclusion.
(1) The striæ on the rocks show that the ice which formed the clay
must have come from the sea, and not from the interior of the country;
for their direction is almost at right angles to what it would have
been had the ice come from the interior. Over the whole district, the
direction of the grooves and scratches, not only of the rocks but
even of the stones in the clay, is pretty nearly N.W. and S.E. “When
examining the sections along the Haster Burn,” says Mr. Jamieson, “in
company with Mr. Joseph Anderson, I remarked that the striæ on the
imbedded fragments generally agreed in direction with those of the
rocks beneath. The scratches on the boulders, as usual, run lengthways
along the stones when they are of an elongated form; and the position
of these stones, as they lie imbedded in the drift, is, as a rule, such
that their longer axes point in the same direction as do the scratches
on the solid rock beneath; showing that the same agency that scored the
rocks also ground and pushed along the drift.”
Mr. C. W. Peach informs me that he seldom or never found a stone with
two sets of striæ on it, a fact indicating, as Mr. Jamieson remarks,
that the drift was produced by one great movement invariably in the
same direction. Let it be borne in mind that the ice, which thus moved
over Caithness in this invariable track, must either have come from the
Atlantic to the N.W., or from the Moray Firth to the S.E.
(2) The boulder clay of Caithness is full of sea-shells and other
marine remains. The shells are in a broken condition, and are
interspersed like the stones through the entire mass of the clay.
Mr. Jamieson states that he nowhere observed any instance of shells
being found in an undisturbed condition, “nor could I hear,” he says,
“of any such having been found; there seems to be no such thing as a
bed of laminated silt with shells _in situ_.” The shell-fragments are
scratched and ice-worn, the same as the stones found in the clay. Not
only are the shells glaciated, but even the foraminifera, when seen
through the microscope, have a rubbed and worn appearance. The shells
have evidently been broken, striated, and pushed along by the ice at
the time the boulder clay was being formed.
_Theories regarding the Origin of the Caithness Clay._—Mr. Jamieson, as
we have seen, freely admits that the boulder clay of Caithness has the
appearance of true land-ice till, but from the N.W. and S.E. direction
of the striæ on the rocks, and the presence of sea-shells in the clay,
he has come to the conclusion that the glaciation of Caithness has been
effected by floating ice at a time when the district was submerged. I
have always felt convinced that Mr. Jamieson had not hit upon the true
explanation of the phenomena.
(1) It is physically impossible that any deposit formed by icebergs
could be wholly unstratified. Suppose a mass of the materials which
would form boulder clay is dropped into the sea from, say an iceberg,
the heavier parts, such as stones, will reach the bottom first. Then
will follow lighter materials, such as sand, then clay, and last of all
the mud will settle down over the whole in fine layers. The different
masses dropped from the various icebergs, will, no doubt, lie in
confusion one over the other, but each separate mass will show signs of
stratification. A good deal of boulder clay evidently has been formed
in the sea, but if the clay be unstratified, it must have been formed
under glaciers moving along the sea-bottom as on dry ground. Whether
_unstratified_ boulder clay may happen to be formed under water or on
dry land, it must in either case be the product of land-ice.[251] Those
who imagine that materials, differing in specific gravity like those
which compose boulder clay, dropped into water, can settle down without
assuming the stratified form, should make the experiment, and they
would soon satisfy themselves that the thing is physically impossible.
The notion that unstratified boulder clay could be formed by deposits
from floating ice, is not only erroneous, but positively pernicious,
for it tends to lead those who entertain it astray in regard to the
whole question of the origin of drift.
(2) It is also physically impossible that ice-markings, such as those
everywhere found on the rocky face of the district, and on the pebbles
and shells imbedded in the clay, could have been effected by any other
agency than that of land-ice. I need not here enter into any discussion
on this point, as this has been done at considerable length in another
place.[252] In the present case, however, it is unnecessary, because
if it can be shown that all the facts are accounted for in the most
natural manner by the theory of land-ice, no one will contend for the
floating-ice theory; for it is admitted that, with the exception of the
direction of the striæ and the presence of the shells, all the facts
agree better with the land-ice than with the floating-ice theory.
My first impression on the subject was that the glaciation of Caithness
had been effected by the polar ice-cap, which, during the severer part
of the glacial epoch, must have extended down to at least the latitude
of the north of Scotland.
On a former occasion (see the _Reader_ for 14th October, 1865) it was
shown that all the northern seas, owing to their shallowness, must,
at that period, have been blocked up with solid ice, which displaced
the water and moved along the sea-bottoms the same as on dry land. In
fact, the northern seas, including the German Ocean, being filled at
the time with glacier-ice, might be regarded as dry land. Ice of this
sort, moving along the bed of the German Ocean or North Sea, and over
Caithness, could not fail to push before it the shells and other animal
remains lying on the sea-bottom, and to mix them up with the clay
which now remains upon the land as evidence of its progress.
About two years ago I had a conversation with Mr. C. W. Peach on the
subject. This gentleman, as is well known, has long been familiar with
the boulder clay of Caithness. He felt convinced that the clay of that
country is the true Lower Till, and not a more recent deposit, as Mr.
Jamieson supposes. He expressed to me his opinion that the glaciation
of Caithness had been effected by masses of land-ice crossing the
Moray Firth from the mountain ranges to the south-east, and passing
over Caithness in its course. The difficulty which seems to beset
this theory is, that a glacier entering the Firth would not leave it
and ascend over the Caithness coast. It would take the path of least
resistance and move into the North Sea, where it would find a free
passage into deeper water. Mr. Peach’s theory is, however, an important
step in the right direction. It is a part of the truth, but I believe
not the whole truth. The following is submitted as a solution of the
question.
_The Proposed Theory._—It may now be regarded as an established fact
that, during the severer part of the glacial period, Scotland was
covered with one continuous mantle of ice, so thick as to bury under
it the Ochil, Sidlaw, Pentland, Campsie, and other moderately high
mountain ranges. For example, Mr. J. Geikie and Mr. B. N. Peach found
that the great masses of the ice from the North-west Highlands, came
straight over the Ochils of Perthshire and the Lomonds of Fife. In
fact, these mountain ridges were not sufficiently high to deflect the
icy stream either to the right hand or to the left; and the flattened
and rounded tops of the Campsie, Pentland, and Lammermoor ranges bear
ample testimony to the denuding power of ice.
Further, to quote from Mr. Jamieson, “the detached mountain of
Schehallion in Perthshire, 3,500 feet high, is marked near the top as
well as on its flanks, and this not by ice flowing down the sides of
the hill itself, but by ice pressing over it from the north. On the top
of another isolated hill, called Morven, about 3,000 feet high, and
situated a few miles to the north of the village of Ballater, in the
county of Aberdeen, I found granite boulders unlike the rock of the
hill, and apparently derived from the mountains to the west. Again,
on the highest watersheds of the Ochils, at altitudes of about 2,000
feet, I found this summer (1864) pieces of mica schist full of garnets,
which seem to have come from the Grampian Hills to the north-west,
showing that the transporting agent had overflowed even the highest
parts of the Ochil ridge. And on the West Lomonds, in Fifeshire, at
Clattering-well Quarry, 1,450 feet high, I found ice-worn pebbles of
Red Sandstone and porphyry in the _débris_ covering the Carboniferous
Limestone of the top of the Bishop Hill. Facts like these meet us
everywhere. Thus on the Perthshire Hills, between Blair Athol and
Dunkeld, I found ice-worn surfaces of rocks on the tops of hills, at
elevations of 2,200 feet, as if caused by ice pressing over them from
the north-west, and transporting boulders at even greater heights.”[253]
Facts still more important, however, in their bearing on the question
before us were observed on the Pentland range by Mr. Bennie and myself
during the summer of 1870. On ascending Allermuir, one of the hills
forming the northern termination of the Pentland range, we were not a
little surprised to find its summit ice-worn and striated. The top of
the hill is composed of a compact porphyritic felstone, which is very
much broken up; but wherever any remains of the original surface could
be seen, it was found to be polished and striated in a most decided
manner. These striæ are all in one uniform direction, nearly east and
west; and on minutely examining them with a lens we had no difficulty
whatever in determining that the ice which effected them came from the
west and not from the east, a fact which clearly shows that they must
have been made at the time when, as is well known, the entire Midland
valley was filled with ice, coming from the North-west Highlands. On
the summit of the hill we also found patches of boulder clay in hollow
basins of the rock. At one spot it was upwards of a foot in depth, and
rested on the ice-polished surface. The clay was somewhat loose and
sandy, as might be expected of a layer so thin, exposed to rain, frost,
and snow, during the long course of ages which must have elapsed since
it was deposited there. Of 100 pebbles collected from the clay, just as
they turned up, every one, with the exception of three or four composed
of hard quartz, presented a flattened and ice-worn surface; and
forty-four were distinctly striated: in short, every stone which was
capable of receiving and retaining scratches was striated. A number of
these stones must have come from the Highlands to the north-west.[254]
The height of Allermuir is 1,617 feet, and, from its position, it is
impossible that the ice could have gone over its summit, unless the
entire Midland valley, at this place, had been filled with ice to the
depth of more than 1,600 feet. The hill is situated about four or
five miles to the south of Edinburgh, and forms, as has already been
stated, the northern termination of the Pentland range. Immediately
to the north lies the broad valley of the Firth of Forth, more than
twelve miles across, offering a most free and unobstructed outlet for
the great mass of ice coming along the Midland valley from the west.
Now, when we reflect how easily ice can accommodate itself to the
inequalities of the channel along which it moves, how it can turn to
the right hand or to the left, so as to find for itself the path of
least resistance, it becomes obvious that the ice never would have gone
over Allermuir, unless not only the Midland valley at this point, but
also the whole surrounding country had been covered with one continuous
mass of ice to a depth of more than 1,600 feet. But it must not be
supposed that the height of Allermuir represents the thickness of the
ice; for on ascending Scald Law, a hill four miles to the south-west
of Allermuir, and the highest of the Pentland range, we found, in
the _débris_ covering its summit, hundreds of transported stones of
all sizes, from one to eighteen inches in diameter. We also dug up a
Greenstone boulder about eighteen inches in diameter, which was finely
polished and striated. As the height of this hill is 1,898 feet, the
mass of ice covering the surrounding country must have been at least
1,900 feet deep. But this is not all. Directly to the north of the
Pentlands, in a line nearly parallel with the east coast, and at right
angles to the path of ice from the interior, there is not, with the
exception of the solitary peak of East Lomond, and a low hill or two of
the Sidlaw range, an eminence worthy of the name of a hill nearer than
the Grampians in the north of Forfarshire, distant upwards of sixty
miles. This broad plain, extending from almost the Southern to the
Northern Highlands, was the great channel through which the ice of the
interior of Scotland found an outlet into the North Sea. If the depth
of the ice in the Firth of Forth, which forms the southern side of this
broad hollow, was at least 1,900 feet, it is not at all probable that
its depth in the northern side, formed by the Valley of Strathmore
and the Firth of Tay, which lay more directly in the path of the ice
from the North Highlands, could have been less. Here we have one vast
glacier, more than sixty miles broad and 1,900 feet thick, coming from
the interior of the country.
It is, therefore, evident that the great mass of ice entering the North
Sea to the east of Scotland, especially about the Firths of Forth
and Tay, could not have been less, and was probably much more, than
from 1,000 to 2,000 feet in thickness. The grand question now to be
considered is, What became of the huge sheet of ice after it entered
the North Sea? Did it break up and float away as icebergs? This appears
to have been hitherto taken for granted; but the shallowness of the
North Sea shows such a process to have been utterly impossible. The
depth of the sea in the English Channel is only about twenty fathoms,
and although it gradually increases to about forty fathoms at the
Moray Firth, yet we must go to the north and west of the Orkney and
Shetland Islands ere we reach the 100 fathom line. Thus the average
depth of the entire North Sea is not over forty fathoms, which is even
insufficient to float an iceberg 300 feet thick.
No doubt the North Sea, for two reasons, is now much shallower than
it was during the period in question. (1.) There would, at the time
of the great extension of the ice on the northern hemisphere, be a
considerable submergence, resulting from the displacement of the
earth’s centre of gravity.[255] (2.) The sea-bed is now probably
filled up to a larger extent with drift deposits than it was at the
ice period. But, after making the most extravagant allowance for the
additional depth gained on this account, still there could not possibly
have been water sufficiently deep to float a glacier of 1,000 or 2,000
feet in thickness. Indeed, the North Sea would have required to be
nearly ten times deeper than it is at present to have floated the
ice of the glacial period. We may, therefore, conclude with the most
perfect certainty that the ice-sheet of Scotland could not possibly
have broken up into icebergs in such a channel, but must have moved
along on the bed of the sea in one unbroken mass, and must have found
its way to the deep trough of the Atlantic, west of the Orkney and
Shetland Islands, ere it broke up and floated away in the iceberg form.
It is hardly necessary to remark that the waters of the North Sea would
have but little effect in melting the ice. A shallow sea like this,
into which large masses of ice were entering, would be kept constantly
about the freezing-point, and water of this temperature has but little
melting power, for it takes 142 lbs. of water, at 33°, to melt one
pound of ice. In fact, an icy sea tends rather to protect the ice
entering it from being melted than otherwise. And besides, owing to
fresh acquisitions of snow, the ice-sheet would be accumulating more
rapidly upon its upper surface than it would be melting at its lower
surface, supposing there were sea-water under that surface. The ice of
Scotland during the glacial period must, of necessity, have found its
way into warmer water than that of the North Sea before it could have
been melted. But this it could not do without reaching the Atlantic,
and in getting there it would have to pass round by the Orkney Islands,
along the bed of the North Sea, as land-ice.
This will explain how the Orkney Islands may have been glaciated by
land-ice; but it does not, however, explain how Caithness should have
been glaciated by that means. These islands lay in the very track of
the ice on its way to the Atlantic, and could hardly escape being
overridden; but Caithness lay considerably to the left of the path
which we should expect the ice to have taken. The ice would not leave
its channel, turn to the left, and ascend upon Caithness, unless it
were forced to do so. What, then, compelled the ice to pass over
Caithness?
_Path of the Scandinavian Ice._—We must consider that the ice from
Scotland and England was but a fraction of that which entered the
North Sea. The greater part of the ice of Scandinavia must have gone
into this sea, and if the ice of our island could not find water
sufficiently deep in which to float, far less would the much thicker
ice of Scandinavia do so. The Scandinavian ice, before it could break
up, would thus, like the Scottish ice, have to cross the bed of the
North Sea and pass into the Atlantic. It could not pass to the north,
or to the north-west, for the ocean in these directions would be
blocked up by the polar ice. It is true that along the southern shore
of Norway there extends a comparatively deep trough of from one to two
hundred fathoms. But this is evidently not deep enough to have floated
the Scandinavian ice-sheet; and even supposing it had been sufficiently
deep, the floating ice must have found its way to the Atlantic, and
this it could not have done without passing along the coast. Now, its
passage would not only be obstructed by the mass of ice continually
protruding into the sea directly at right angles to its course, but it
would be met by the still more enormous masses of ice coming off the
entire Norwegian coast-line. And, besides this, the ice entering the
Arctic Ocean from Lapland and the northern parts of Siberia, except
the very small portion which might find an outlet into the Pacific
through Behring’s Straits, would have to pass along the Scandinavian
coast in its way to the Atlantic. No matter, then, what the depth of
this trough may have been, if the ice from the land, after entering
it, could not make its escape, it would continue to accumulate till
the trough became blocked up; and after this, the great mass from the
land would move forward as though the trough had no existence. Thus,
the only path for the ice would be by the Orkney and Shetland Islands.
Its more direct and natural path would, no doubt, be to the south-west,
in the direction of our shores; and in all probability, had Scotland
been a low flat island, instead of being a high and mountainous one,
the ice would have passed completely over it. But its mountainous
character, and the enormous masses of ice at the time proceeding from
its interior, would effectually prevent this, so that the ice of
Scandinavia would be compelled to move round by the Orkney Islands.
Consequently, these two huge masses of moving ice—the one from Scotland
and the much greater one from Scandinavia—would meet in the North Sea,
probably not far from our shores, and would move, as represented in
the diagram, side by side northwards into the Atlantic as one gigantic
glacier.
Nor can this be regarded as an anomalous state of things; for in
Greenland and the antarctic continent the ice does not break up into
icebergs on reaching the sea, but moves along the sea-bottom in a
continuous mass until it reaches water sufficiently deep to float
it. It is quite possible that the ice at the present day may nowhere
traverse a distance of three or four hundred miles of sea-bottom, but
this is wholly owing to the fact that it finds water sufficiently deep
to float it before having travelled so far. Were Baffin’s Bay and
Davis’s Straits, for example, as shallow as the North Sea, the ice of
Greenland would not break up into icebergs in these seas, but cross in
one continuous mass to and over the American continent.
The median line of the Scandinavian and Scottish ice-sheets would be
situated not far from the east coast of Scotland. The Scandinavian ice
would press up as near to our coast as the resistance of the ice from
this side permitted. The enormous mass of ice from Scotland, pressing
out into the North Sea, would compel the Scandinavian ice to move round
by the Orkneys, and would also keep it at some little distance from
Scotland. Where, on the other hand, there was but little resistance
offered by ice from the interior of this country (and this might be the
case along many parts of the English coast), the Scandinavian ice might
reach the shores, and even overrun the country for some distance inland.
We have hitherto confined our attention to the action of ice proceeding
from Norway; but if we now consider what took place in Sweden and the
Baltic, we shall find more conclusive proof of the downward pressure
of Scandinavian ice on our own shores. The western half of Gothland
is striated in the direction of N.E. and S.W., and that this has been
effected by a huge mass of ice covering the country, and not by local
glaciers, is apparent from the fact observed by Robert Chambers,[256]
and officers of the Swedish Geological Survey, that the general
direction of the groovings and striæ on the rocks bears little or no
relation to the conformation of the surface, showing that the ice was
of sufficient thickness to move straight forward, regardless of the
inequalities of the ground.
At Gottenburg, on the shores of the Cattegat, and all around Lake
Wener and Lake Wetter, the ice-markings are of the most remarkable
character, indicating, in the most decided manner, that the ice came
from the interior of the country to the north-east in one vast mass.
All this mass of ice must have gone into the shallow Cattegat, a sea
not sufficiently deep to float even an ordinary glacier. The ice coming
off Gothland would therefore cross the Cattegat, and thence pass over
Jutland into the North Sea. After entering the North Sea, it would be
obliged to keep between our shores and the ice coming direct from the
western side of Scandinavia.
But this is not all. A very large proportion of the Scandinavian ice
would pass into the Gulf of Bothnia, where it could not possibly float.
It would then move south into the Baltic as land-ice. After passing
down the Baltic, a portion of the ice would probably move south into
the flat plains in the north of Germany, but the greater portion
would keep in the bed of the Baltic, and of course turn to the right
round the south end of Gothland, and thence cross over Denmark into
the North Sea. That this must have been the path of the ice is, I
think, obvious from the observations of Murchison, Chambers, Hörbye,
and other geologists. Sir Roderick Murchison found—though he does not
attribute it to land-ice—that the Aland Islands, which lie between the
Gulf of Bothnia and the Baltic, are all striated in a north and south
direction.[257]
Upsala and Stockholm, a tract of flat country projecting for some
distance into the Baltic, is also grooved and striated, not in the
direction that would be effected by ice coming from the interior of
Scandinavia, but north and south, in a direction parallel to what must
have been the course of the ice moving down the Baltic.[258] This part
of the country must have been striated by a mass of ice coming from
the direction of the Gulf of Bothnia. And that this mass must have
been great is apparent from the fact that Lake Malar, which crosses
the country from east to west, at right angles to the path of the ice,
does not seem to have had any influence in deflecting the icy stream.
That the ice came from the north and not from the south is also evident
from the fact that the northern sides of rocky eminences are polished,
rounded, and ice-worn, while the southern sides are comparatively
rough. The northern banks of Lake Malar, for example, which, of course,
face the south, are rough, while the southern banks, which must have
offered opposition to the advance of the ice, are smoothed and rounded
in a most singular manner.
Again, that the ice, after passing down the Baltic, turned to the
right along the southern end of Gothland, is shown by the direction
of the striæ and ice-groovings observed on such islands as Gothland,
Öland, and Bornholm. Sir R. Murchison found that the island of
Gothland is grooved and striated in one uniform direction from N.E.
to S.W. “These groovings,” says Sir Roderick, “so perfectly resemble
the flutings and striæ produced in the Alps by the actual movement
of glaciers, that neither M. Agassiz nor any one of his supporters
could detect a difference.” He concludes, however, that the markings
could not have been made by land-ice, because Gothland is not only a
low, flat island in the middle of the Baltic, but is “at least 400
miles distant from any elevation to which the term of mountain can be
applied.” This, of course, is conclusive against the hypothesis that
Gothland and the other islands of the Baltic could have been glaciated
by ordinary glaciers; but it is quite in harmony with the theory
that the Gulf of Bothnia and the entire Baltic were filled with one
continuous mass of land-ice, derived from the drainage of the greater
part of Sweden, Lapland, and Finland. In fact, the whole glacial
phenomena of Scandinavia are inexplicable on the hypothesis of local
glaciers.
That the Baltic was completely filled by a mass of ice moving from the
north is further evidenced by the fact that the mainland, not only at
Upsala, but at several places along the coast of Gothland, is grooved
and striated parallel to the shore, and often at right angles to the
markings of the ice from the interior, showing that the present bed of
the Baltic was not large enough to contain the icy stream. For example,
along the shores between Kalmar and Karlskrona, as described by Sir
Roderick Murchison and by M. Hörbye, the striations are parallel to the
shore. Perhaps the slight obstruction offered by the island of Öland,
situated so close to the shore, would deflect the edge of the stream at
this point over on the land. The icy stream, after passing Karlskrona,
bent round to the west along the present entrance to the Baltic, and
again invaded the mainland, and crossed over the low headland of
Christianstadt, and thence passed westward in the direction of Zealand.
[Illustration: PLATE V.
W. & A. K. Johnston, Edinb^r. and London.
CHART SHOWING THE PROBABLE PATH OF THE ICE IN NORTH-WESTERN EUROPE
DURING THE PERIOD OF MAXIMUM GLACIATION.
_The lines also represent the actual direction of the striae on the
rocks._]
This immense Baltic glacier would in all probability pass over Denmark,
and enter the North Sea somewhere to the north of the River Elbe, and
would then have to find an outlet to the Atlantic through the English
Channel, or pass in between our eastern shores and the mass from
Gothland and the north-western shores of Europe. The entire probable
path of the ice may be seen by a reference to the accompanying chart
(Plate V.) That the ice crossed over Denmark is evident from the fact
that the surface of that country is strewn with _débris_ derived from
the Scandinavian peninsula.
Taking all these various considerations into account, the conclusion is
inevitable that the great masses of ice from Scotland would be obliged
to turn abruptly to the north, as represented in the diagram, and pass
round into the Atlantic in the direction of Caithness and the Orkney
Islands.
If the foregoing be a fair representation of the state of matters,
it is physically impossible that Caithness could have escaped being
overridden by the land-ice of the North Sea. Caithness, as is well
known, is not only a low, flat tract of land, little elevated above the
sea-level, and consequently incapable of supporting large glaciers;
but, in addition, it projects in the form of a headland across the
very path of the ice. Unless Caithness could have protected itself by
pushing into the sea glaciers of one or two thousand feet in thickness,
it could not possibly have escaped the inroads of the ice of the
North Sea. But Caithness itself could not have supported glaciers of
this magnitude, neither could it have derived them from the adjoining
mountainous regions of Sutherland, for the ice of this county found a
more direct outlet than along the flat plains of Caithness.
The shells which the boulder clay of Caithness contains have thus
evidently been pushed out of the bed of the North Sea by the land-ice,
which formed the clay itself.
The fact that these shells are not so intensely arctic as those found
in some other quarters of Scotland, is no evidence that the clay was
not formed during the most severe part of the glacial epoch, for the
shells did not live in the North Sea at the time that it was filled
with land-ice. The shells must have belonged to a period prior to the
invasion of the ice, and consequently before the cold had reached its
greatest intensity. Neither is there any necessity for supposing the
shells to be pre-glacial, for these shells may have belonged to an
inter-glacial period. In so far as Scotland is concerned, it would be
hazardous to conclude that a plant or an animal is either pre-glacial
or post-glacial simply because it may happen not to be of an arctic or
of a boreal type.
The same remarks which apply to Caithness apply to a certain extent
to the headland at Fraserburgh. It, too, lay in the path of the ice,
and from the direction of the striæ on the rocks, and the presence of
shells in the clay, as described by Mr. Jamieson, it bears evidence
also of having been overridden by the land-ice of the North Sea.
In fact, we have, in the invasion of Caithness and the headland at
Fraserburgh by the land-ice of the North Sea, a repetition of what we
have seen took place at Upsala, Kalmar, Christianstadt, and other flat
tracts along the sides of the Baltic.
The scarcity, or perhaps entire absence of Scandinavian boulders in
the Caithness clay is not in any way unfavourable to the theory, for
it would only be the left edge of the North Sea glacier that could
possibly pass over Caithness; and this edge, as we have seen, was
composed of the land-ice from Scotland. We might expect, however, to
find Scandinavian blocks on the Shetland and Faroe Islands, for, as we
shall presently see, there is pretty good evidence to prove that the
Scandinavian ice passed over these islands.
_The Shetland and Faroe Islands glaciated by Land-ice._—It is also
worthy of notice that the striæ on the rocks in the Orkney, Shetland,
and Faroe Islands, all point in the direction of Scandinavia, and are
what would be effected by land-ice moving in the paths indicated
in the diagram. And it is a fact of some significance, that when we
proceed north to Iceland, the striæ, according to the observations
of Robert Chambers, seem to point towards North Greenland. Is it
possible that the entire Atlantic, from Scandinavia to Greenland, was
filled with land-ice? Astounding as this may at first appear, there
are several considerations which render such a conclusion probable.
The observations of Chambers, Peach, Hibbert, Allan, and others, show
that the rocky face of the Shetland and Faroe Islands has been ground,
polished, and striated in a most remarkable manner. That this could not
have been done by ice belonging to the islands themselves is obvious,
for these islands are much too small to have supported glaciers of any
size, and the smallest of them is striated as well as the largest.
Besides, the uniform direction of the striæ on the rocks shows that
it must have been effected by ice passing over the islands. That the
striations could not have been effected by floating icebergs at a time
when the islands were submerged is, I think, equally obvious, from the
fact that not only are the tops of the highest eminences ice-worn,
but the entire surface down to the present sea-level is smoothed and
striated; and these striations conform to all the irregularities of the
surface. This last fact Professor Geikie has clearly shown is wholly
irreconcilable with the floating-ice theory.[259] Mr. Peach[260] found
vertical precipices in the Shetlands grooved and striated, and the
same thing was observed by Mr. Thomas Allan on the Faroe Islands.[261]
That the whole of these islands have been glaciated by a continuous
sheet of ice passing over them was the impression left on the mind of
Robert Chambers after visiting them.[262] This is the theory which
alone explains all the facts. The only difficulty which besets it is
the enormous thickness of the ice demanded by the theory. But this
difficulty is very much diminished when we reflect that we have good
evidence, from the thickness of icebergs which have been met with
in the Southern Ocean,[263] that the ice moving off the antarctic
continent must be in some places considerably over a mile in thickness.
It is then not so surprising that the ice of the glacial epoch, coming
off Greenland and Northern Europe, should not have been able to float
in the North Atlantic.
_Why the Ice of Scotland was of such enormous Thickness._—The enormous
thickness of the ice in Scotland, during the glacial epoch, has been
a matter of no little surprise. It is remarkable how an island, not
more than 100 miles across, should have been covered with a sheet
of ice so thick as to bury mountain ranges more than 1,000 feet in
height, situated almost at the sea-shore. But all our difficulties
disappear when we reflect that the seas around Scotland, owing to their
shallowness, were, during the glacial period, blocked up with solid
ice. Scotland, Scandinavia, and the North Sea, would form one immense
table-land of ice, from 1,000 to 2,000 feet above the sea-level. This
table-land would terminate in the deep waters of the Atlantic by a
perpendicular wall of ice, extending probably from the west of Ireland
away in the direction of Iceland. From this barrier icebergs would be
continually breaking off, rivalling in magnitude those which are now to
be met with in the antarctic seas.
_The great Extension of the Loess accounted for._—An effect which would
result from the blocking up of the North Sea with land-ice, would be
that the waters of the Rhine, Elbe, and Thames would have to find
an outlet into the Atlantic through the English Channel. Professor
Geikie has suggested to me that if the Straits of Dover were not then
open—quite a possible thing—or were they blocked up with land-ice, say
by the great Baltic glacier crossing over from Denmark, the consequence
would be that the waters of the Rhine and Elbe would be dammed back,
and would inundate all the low-lying tracts of country to the south;
and this might account for the extraordinary extension of the Loess in
the basin of the Rhine, and in Belgium and the north of France.[264]
[Illustration: PLATE VI.
CHART SHOWING PATH OF THE ICE
W. & A. K. Johnston, Edinb^r and London.
Note.
_Curved lines shew path of Ice.
Arrows shew direction of striae
as observed by Prof. Geikie & B. N. Peach.
Short thick lines shew direction of
striae by other observers._]
_Note on the Glaciation of Caithness._
I have very lately received a remarkable confirmation of the path of
the Caithness ice in observations communicated to me by Professor
Geikie and Mr. B. N. Peach. The latter geologist says, “Near the Ord
of Caithness and on to Berriedale the striæ pass off the land and out
to sea; but near Dunbeath, 6 miles north-east of Berriedale, they
begin to creep up out of the sea on to the land and range from about
15° to 10° east of north. _Where the striæ pass out to the sea_ the
boulder clay is made up of the materials from inland and contains no
shells, but _immediately the striæ begin to creep up on to the land_
then shells begin to make their appearance; and there is a difference,
moreover, in the colour of the clay, for in the former case it is
red and incoherent, and in the latter hard and dark-coloured.” The
accompanying chart (Plate VI.) shows the outline of the Caithness coast
and the direction of the striæ as observed by Professor Geikie and Mr.
Peach, and no demonstration could be more conclusive as to the path of
the ice and the obstacles it met than these observations, supplemented
and confirmed as they are by other recorded facts to which I shall
presently allude. Had the ice-current as it entered the North Sea off
the Sutherland coast met with no obstacle it would have ploughed its
way outwards till it broke off in glaciers and floated away. But it is
clear that the great press of Scandinavian ice and the smaller mass of
land-ice from the Morayshire coast converging in the North Sea filled
up its entire bed, and these, meeting the opposing current from the
Sutherland coast, turned it back upon itself, and forced it over the
north-east part of Caithness. The farther south on the Sutherland
coast that the ice entered the sea the deeper would it be able to
penetrate into the ocean-bed before it met an opposition sufficiently
strong to turn its course, and the wider would be its sweep; but when
we come to the Sutherland coast we reach a point where the land-ice—as,
for example, near Dunbeath—is forced to bend round before it even
reaches the sea-shore, as will be seen from the accompanying diagram.
We are led to the same conclusions regarding the path of the ice in the
North Sea from the presence of oolitic fossils and chalk flints found
likewise in the boulder clay of Caithness, for these, as we shall see,
evidently must have come from the sea. At the meeting of the British
Association, Edinburgh, 1850, Hugh Miller exhibited a collection of
boreal shells with fragments of oolitic fossils, chalk, and chalk
flints from the boulder clay of Caithness collected by Mr. Dick, of
Thurso. My friend, Mr. C. W. Peach, found that the chalk flints in
the boulder clay of Caithness become more abundant as we proceed
northward, while the island of Stroma in the Pentland Firth he found
to be completely strewn with them. This same observer found, also, in
the Caithness clay stones belonging to the Oolitic and Lias formations,
with their characteristic fossils, while ammonites, belemnites, fossil
wood, &c., &c., were also found loose in the clay.[265] The explanation
evidently is, that these remains were derived from an outcrop of
oolitic and cretaceous beds in the North Sea. It is well known that
the eastern coast of Sutherlandshire is fringed with a narrow strip
of oolite, which passes under the sea, but to what distance is not
yet ascertained. Outside the Oolitic formation the chalk beds in all
probability crop out. It will be seen from a glance at the accompanying
chart (Plate VI.) that the ice which passed over the north-eastern part
of Caithness must have crossed the out-cropping chalk beds.
As has already been stated in the foregoing chapter, the headland of
Fraserburgh, north-eastern corner of Aberdeenshire, bears evidence,
both from the direction of the striæ and broken shells in the boulder
clay, of having been overridden also by land-ice from the North Sea.
This conclusion is strengthened by the fact that chalk flints and
oolitic fossils have also been abundantly met with in the clay by Dr.
Knight, Mr. James Christie, Mr. W. Ferguson, Mr. T. F. Jamieson, and
others.
CHAPTER XXVIII.
NORTH OF ENGLAND ICE-SHEET, AND TRANSPORT OF WASTDALE CRAG
BLOCKS.[266]
Transport of Blocks; Theories of.—Evidence of Continental
Ice.—Pennine Range probably striated on Summit.—Glacial
Drift in Centre of England.—Mr. Lacy on Drift of Cotteswold
Hills.—England probably crossed by Land-ice.—Mr. Jack’s
Suggestion.—Shedding of Ice North and South.—South of England
Ice-sheet.—Glaciation of West Somerset.—Why Ice-markings are
so rare in South of England.—Form of Contortion produced by
Land-ice.
Considerable difficulty has been felt in accounting for the transport
of the Wastdale granite boulders across the Pennine chain to the east.
Professors Harkness,[267] and Phillips,[268] Messrs. Searles Wood,
jun.,[269] Mackintosh,[270] and I presume all who have written on
the subject, agree that these blocks could not have been transported
by land-ice. The agency of floating ice under some form or other is
assumed by all.
We have in Scotland phenomena of an exactly similar nature. The summits
of the Ochils, the Pentlands, and other mountain ranges in the east
of Scotland, at elevations of from 1,500 to 2,000 feet, are not only
ice-marked, but strewn over with boulders derived from rocks to the
west and north-west. Many of them must have come from the Highlands
distant some 50 or 60 miles. It is impossible that these stones could
have been transported, or the summits of the hills striated, by means
of ordinary glaciers. Neither can the phenomena be attributed to the
agency of icebergs carried along by currents. For we should require to
assume not merely a submergence of the land to the extent of 2,000
feet or so,—an assumption which might be permitted,—but also that the
currents bearing the icebergs took their rise in the elevated mountains
of the Highlands (a most unlikely place), and that these currents
radiated in all directions from that place as a centre.
In short, the glacial phenomena of Scotland are wholly inexplicable
upon any other theory than that, during at least a part of the
glacial epoch, the entire island from sea to sea was covered with one
continuous mass of ice of not less than 2,000 feet in thickness.
In my paper on the Boulder Clay of Caithness (see preceding chapter),
I have shown that if the ice was 2,000 feet or so in thickness, it
must, in its motion seawards, have followed the paths indicated by the
curved lines in the chart accompanying that paper (See Plate V.). In
so far as Scotland is concerned [and Scandinavia also], these lines
represent pretty accurately not only the paths actually taken by the
boulders, but also the general direction of the ice-markings on all the
elevated mountain ridges. But if Scotland was covered to such an extent
with ice, it is not at all probable that Westmoreland and the other
mountainous districts of the North of England could have escaped being
enveloped in a somewhat similar manner. Now if we admit the supposition
of a continuous mass of ice covering the North of England, all our
difficulties regarding the transport of the Wastdale blocks across the
Pennine chain disappear. An inspection of the chart above referred to
will show that these blocks followed the paths which they ought to have
done upon the supposition that they were conveyed by continental ice.
That Wastdale Crag itself suffered abrasion by ice moving over it, in
the direction indicated by the lines in the diagram, is obvious from
what has been recorded by Dr. Nicholson and Mr. Mackintosh. They both
found the Crag itself beautifully _moutonnée_ up to its summit, and
striated in a W.S.W. and E.N.E. direction. Mr. Mackintosh states that
these scorings run obliquely up the sloping face of the crag. Ice
scratches crossing valleys and running up the sloping faces of hills
and over their summits are the sure marks of continental ice, which
meet the eye everywhere in Scotland. Dr. Nicholson found in the drift
covering the lower part of the crag, pebbles of the Coniston flags and
grits from the west.[271]
The fact that in Westmoreland the direction of the ice-markings, as a
general rule, corresponds with the direction of the main valleys, is
no evidence whatever that the country was not at one period covered
with a continuous sheet of ice; because, for long ages after the period
of continental ice, the valleys would be occupied by glaciers, and
these, of course, would necessarily leave the marks of their presence
behind. This is just what we have everywhere in Scotland. It is on
the summits of the hills and elevated ridges, where no glacier could
possibly reach, that we find the sure evidence of continental ice.
But that land-ice should have passed over the tops of hills 1,000 or
2,000 feet in height is a thing hitherto regarded by geologists as
so unlikely that few of them ever think of searching in such places
for ice-markings, or for transported stones. Although little has been
recorded on this point, I hardly think it likely that there is in
Scotland a hill under 2,000 feet wholly destitute of evidence that ice
has gone over it. If there were hills in Scotland that should have
escaped being overridden by ice, they were surely the Pentland Hills;
but these, as was shown on a former occasion,[272] were completely
buried under the mass of ice covering the flat surrounding country.
I have no doubt whatever that if the summits of the Pennine range
were carefully examined, say under the turf, evidence of ice-action,
in the form of transported stones or scratches on the rock, would be
found.[273]
Nor is the fact that the Wastdale boulders are not rounded and
ice-marked, or found in the boulder clay, but lie on the surface, any
evidence that they were not transported by land-ice. For it would not
be the stones _under_ the ice, but those falling on the upper surface
of the sheet, that would stand the best chance of being carried over
mountain ridges. But such blocks would not be crushed and ice-worn;
and it is on the surface of the clay, and not imbedded in it, that we
should expect to find them.
It is quite possible that the dispersion of the Wastdale boulders took
place at various periods. During the period of local glaciers the
blocks would be carried along the line of the valleys.
All I wish to maintain is that the transport of the blocks across
the Pennine chain is easily accounted for if we admit, what is very
probable, that the great ice-covering of Scotland overlapped the high
grounds of the North of England. The phenomenon is the same in both
places, and why not attribute it to the same cause?
There is another curious circumstance connected with the drift of
England which seems to indicate the agency of an ice-covering.
As far back as 1819, Dr. Buckland, in his Memoir on the Quartz Rock
of Lickey Hill,[274] directed attention to the fact, that on the
Cotteswold Hills there are found pebbles of hard red chalk which must
have come from the Wolds of Yorkshire and Lincolnshire. He pointed
out also that the slaty and porphyritic pebbles probably came from
Charnwood Forest, near Leicester. Professor Hull, of the Geological
Survey, considers that “almost all the Northern Drift of this part
of the country had been derived from the _débris_ of the rocks of
the Midland Counties.”[275] He came also to the conclusion that the
slate fragments may have been derived from Charnwood Forest. In the
Vale of Moreton he found erratic boulders from two feet to three
feet in diameter. The same northern character of the drift of this
district is remarked by Professor Ramsay and Mr. Aveline, in their
Memoir of the Geology of parts of Gloucestershire. In Leicestershire
and Northamptonshire the officers of the Geological Survey found in
abundance drift which must have come from Lincolnshire and Yorkshire to
the north-east.
Mr. Lucy, who has also lately directed attention to the fact that
the Cotteswold Hills are sprinkled over with boulders from Charnwood
Forest, states also that, on visiting the latter place, he found that
many of the stones contained in it had come from Yorkshire, still
further to the north-east.[276]
Mr. Searles Wood, jun., in his interesting paper on the Boulder Clay
of the North of England,[277] states that enormous quantities of the
chalk _débris_ from the Yorkshire Wold are found in Leicester, Rutland,
Warwick, Northampton, and other places to the south and south-west.
Mr. Wood justly concludes that this chalk _débris_ could not have been
transported by water. “If we consider,” he says, “the soluble nature
of chalk, it must be evident that none of this débris can have been
detached from the parent mass, either by water-action, or by any other
atmospheric agency than moving ice. The action of the sea, of rivers,
or of the atmosphere, upon chalk, would take the form of dissolution,
the degraded chalk being taken up in minute quantities by the water,
and held in suspension by it, and in that form carried away; so that
it seems obvious that this great volume of rolled chalk can have been
produced in no other way than by the agency of moving ice; and for that
agency to have operated to an extent adequate to produce a quantity
that I estimate as exceeding a layer 200 feet thick over the entire
Wold, nothing less than the complete envelopment of a large part of the
Wold by ice for a long period would suffice.”
I have already assigned my reasons for disbelieving the opinion that
such masses of drift could have been transported by floating ice; but
if we refer it to land-ice, it is obvious that the ice could not have
been in the form of local glaciers, but must have existed as a sheet
moving in a south and south-west direction, from Yorkshire, across the
central part of England. But how is this to harmonize with the theory
of glaciation, which is advanced to explain the transport of the Shap
boulders?
The explanation has, I think, been pointed out by a writer in the
_Glasgow Herald_,[278] of the 26th November, 1870, in a review of Mr.
Lucy’s paper.
In my paper on the Boulder Clay of Caithness, I had represented the ice
entering the North Sea from the east coast of Scotland and England,
as all passing round the north of Scotland. But the reviewer suggests
that the ice entering at places to the south of, say, Flamborough Head,
would be deflected southwards instead of northwards, and thus pass over
England. “It is improbable, however,” says the writer, “that this joint
ice-sheet would, as Mr. Croll supposes, all find its way round the
north of Scotland into the deep sea. The southern uplands of Scotland,
and probably also the mountains of Northumberland, propelled, during
the coldest part of the glacial period, a land ice-sheet in an eastward
direction. This sheet would be met by another streaming outward from
the south-western part of Norway—in a diametrically opposite direction.
In other words, an imaginary line might be drawn representing the
course of some particular boulder in the _moraine profonde_ from
England met by a boulder from Norway, in the same straight line. With
a dense ice-sheet to the north of this line, and an open plain to the
south, it is clear that all the ice travelling east or west from points
to the south of the starting-points of our two boulders would be ‘shed’
off to the south. There would be a point somewhere along the line, at
which the ice would turn as on a pivot—this point being nearer England
or Scandinavia, as the degree of pressure exercised by the respective
ice-sheets should determine. There is very little doubt that the point
in question would be nearer England. Further, the direction of the
joint ice-sheet could not be _due_ south unless the pressure of the
component ice-sheets should be exactly equal. In the event of that from
Scandinavia pressing with greater force, the direction would be to the
south-west. This is the direction in which the drifts described by Mr.
Lucy have travelled.”
I can perceive no physical objection to this modification of the
theory. What the ice seeks is the path of least resistance, and along
this path it will move, whether it may lie to the south or to the
north. And it is not at all improbable that an outlet to the ice would
be found along the natural hollow formed by the valleys of the Trent,
Avon, and Severn. Ice moving in this direction would no doubt pass down
the Bristol Channel and thence into the Atlantic.
Might not the shedding of the north of England ice-sheet to the north
and south, somewhere not far from Stainmoor, account for the remarkable
fact pointed out by Mr. Searles Wood, that the boulder clay, with
Shap boulders, to the north of the Wold is destitute of chalk; while,
on the other hand, the chalky boulder clay to the south of the Wold
is destitute of Shap boulders? The ice which passed over Wastdale
Crag moved to the E.N.E., and did not cross the chalk of the Wold;
while the ice which bent round to the south by the Wold came from the
district lying to the south of Wastdale Crag, and consequently did not
carry with it any of the granite from that Crag. In fact, Mr. Searles
Wood has himself represented on the map accompanying his Memoir this
shedding of the ice north and south.
These theoretical considerations are, of course, advanced for what
they are worth. Hitherto geologists have been proceeding upon the
supposition of an ice-sheet and an open North Sea; but the latter is
an impossibility. But if we suppose the seas around our island to have
been filled with land-ice during the glacial epoch, the entire glacial
problem is changed, and it does not then appear so surprising that ice
should have passed over England.
_Note on the South of England Ice-sheet._
If what has already been stated regarding the north of England be
anything like correct, it is evident that the south of England
could not possibly have escaped glaciation. If the North Sea was so
completely blocked up by Scandinavian ice, that the great mass of ice
from the Cumberland mountains entering the sea on the east coast was
compelled to bend round and find a way of escape across the centre
of England in the direction of the Bristol Channel, it is scarcely
possible that the immense mass of ice filling the Baltic Sea and
crossing over Denmark could help passing across at least a portion
of the south of England. The North Sea being blocked up, its natural
outlet into the Atlantic would be through the English Channel; and it
is not likely that it could pass through without impinging to some
extent upon the land. Already geologists are beginning to recognise the
evidence of ice in this region.
Mr. W. C. Lucy, in the _Geological Magazine_ for June, 1874, records
the finding by himself of evidences of glaciation in West Somerset,
in the form of “rounded rocky knolls,” near Minehead, like those of
glaciated districts; of a bed of gravel and clay 70 feet deep, which
he considered to be boulder clay. He also mentions the occurrence near
Portlock of a large mass of sandstone well striated, only partially
detached from the parent rock. In the same magazine for the following
month Mr. H. B. Woodward records the discovery by Mr. Usher of some
“rum stuff” near Yarcombe, in the Black Down Hills of Devonshire,
which, on investigation, proved to be boulder clay; and further, that
it was not a mere isolated patch, but occurred in several other places
in the same district. Mr. C. W. Peach informs me that on the Cornwall
coast, near Dodman Point, at an elevation of about 60 feet above
sea-level, he found the rock surface well striated and ice-polished.
In a paper on the Drift Deposits of the Bath district, read before the
Bath Natural History and Antiquarian Field Club, March 10th, 1874,
Mr. C. Moore describes the rock surfaces as grooved, with deep and
long-continued furrows similar to those usually found on glaciated
rocks, and concludes that during the glacial period they were subjected
to ice-action. This conclusion is confirmed by the fact of there being
found, immediately overlying these glaciated rocks, beds of gravel
with intercalated clay-beds, having a thickness of 30 feet, in which
mammalian remains of arctic types are abundant. The most characteristic
of which are _Elephas primigenius_, _E. antiquus_, _Rhinoceros
tichorhinus_, _Bubalus moschatus_, and _Cervus tarandus_.
There is little doubt that when the ground is better examined many
other examples will be found. One reason, probably, why so little
evidence of glaciation in the south of England has been recorded,
is the comparative absence of rock surfaces suitable for retaining
ice-markings. There is, however, one class of evidence which might
determine the question of the glaciation of the south of England as
satisfactorily as markings on the rock. The evidence to which I refer
is that of contorted beds of sand or clay. In England contortions from
the sinking of the beds are, of course, quite common, but a thoughtful
observer, who has had a little experience of ice-formed contortions,
can easily, without much trouble, distinguish the latter from the
former. Contortions resulting from the lateral pressure of the ice
assume a different form from those produced by the sinking of the beds.
In Scotland, for example, there is one well-marked form of contortion,
which not only proves the existence of land-ice, but also the direction
in which it moved. The form of contortion to which I refer is the
bending back of the stratified beds upon themselves, somewhat in
the form of a fishing-hook. This form of contortion will be better
understood from the accompanying figure.
[Illustration: Fig. 11.
SECTION OF CONTORTED DRIFT NEAR MUSSELBURGH.
_a_ Boulder Clay; _b_ Laminated Clay; _c_ Sand, Gravel, and Clay,
contorted.
Depth of Section, twenty-two feet.—H. SKAE.]
CHAPTER XXIX.
EVIDENCE FROM BURIED RIVER CHANNELS OF A CONTINENTAL PERIOD IN
BRITAIN.[279]
Remarks on the Drift Deposits.—Examination of Drift
by Borings.—Buried River Channel from Kilsyth to
Grangemouth.—Channels not excavated by Sea nor by
Ice.—Section of buried Channel at Grangemouth.—Mr. Milne
Home’s Theory.—German Ocean dry Land.—Buried River Channel
from Kilsyth to the Clyde.—Journal of Borings.—Marine
Origin of the Drift Deposits.—Evidence of Inter-glacial
Periods.—Oscillations of Sea-level.—Other buried River
Channels.
_Remarks on the Drift Deposits._—The drift and other surface deposits
of the country have chiefly been studied from sections observed on the
banks of streams, railway cuttings, ditches, foundations of buildings,
and other excavations. The great defect of such sections is that they
do not lay open a sufficient depth of surface. They may, no doubt,
represent pretty accurately the character and order of the more recent
deposits which overlie the boulder clay, but we are hardly warranted
in concluding that the succession of deposits belonging to the earlier
part of the glacial epoch, the period of the true till, is fully
exhibited in such limited sections.
Suppose, for example, the glacial epoch proper—the time of the lower
boulder clay—to have consisted of a succession of alternate cold and
warm periods, there would, in such a case, be a series of separate
formations of boulder clay; but we could hardly expect to find on the
flat and open face of the country, where the surface deposits are
generally not of great depth, those various formations of till lying
the one superimposed upon the other. For it is obvious that the till
formed during one ice-period would, as a general rule, be either swept
away or re-ground and laid down by the ice of the succeeding period.
If the very hardest rocks could not withstand the abrading power
of the enormous masses of ice which passed over the surface of the
country during the glacial epoch, it is hardly to be expected that the
comparatively soft boulder clay would be able to do so. It is probable
that the boulder clay of one period would be used as grinding materials
by the ice of the succeeding periods. The boulder clay which we find in
one continuous mass may, therefore, in many cases, have been ground off
the rocks underneath at widely different periods.
If we wish to find the boulder clays belonging to each of the
successive cold periods lying, the one superimposed on the other in
the order of time in which they were formed, we must go and search in
some deep gorge or valley, where the clay has not only accumulated
in enormous masses, but has been partially protected from the
destructive power of the ice. But it is seldom that the geologist has
an opportunity of seeing a complete section down to the rock-head in
such a place. In fact, excepting by bores for minerals, or by shafts
of pits, the surface, to a depth of one or two hundred feet, is never
passed through or laid open.
_Examination of Drift by Borings._—With the view of ascertaining if
additional light would be cast on the sequence of events, during the
formation of the boulder clay, by an examination of the journals of
bores made through a great depth of surface deposits, a collection
of about 250 bores, put down in all parts of the mining districts
of Scotland, was made. An examination of these bores shows most
conclusively that the opinion that the boulder clay, or lower till, is
one great undivided formation, is wholly erroneous.
These 250 bores, as already stated,[280] represent a total thickness
of 21,348 feet, giving 86 feet as the mean thickness of the deposits
passed through. Twenty of these bores have one boulder clay, with beds
of stratified sand or gravel beneath the clay; 25 have 2 boulder clays,
with stratified beds of sand and gravel between; 10 have 3 boulder
clays; one has 4 boulder clays; 2 have 5 boulder clays; and one has no
fewer than 6 separate masses of boulder clay, with stratified beds of
sand and gravel between; 16 have two or three separate boulder clays,
differing altogether in colour and hardness, without any stratified
beds between. We have, therefore, out of 250 bores, 75 of them
representing a condition of things wholly different from that exhibited
to the geologist in ordinary sections.
These bores bear testimony to the conclusion that the glacial epoch
consisted of a succession of cold and warm periods, and not of one
continuous and unbroken period of ice, as was at one time generally
supposed.
The full details of the character of the deposits passed through by
these bores, and their bearing on the history of the glacial epoch,
have been given by Mr. James Bennie, in an interesting paper read
before the Glasgow Geological Society,[281] to which I would refer
all those interested in the subject of surface geology. But it is not
to the mere contents of the bores that I wish at present to direct
attention, but to a new and important result, to which they have
unexpectedly led.
_Buried River Channel, Kilsyth to Grangemouth, Firth of Forth._—These
borings reveal the existence of a deep pre-glacial, or perhaps
inter-glacial, trough or hollow, extending from the Clyde above Bowling
across the country by Kilsyth, along the valley of the Forth and Clyde
Canal, to the Firth of Forth at Grangemouth. This trough is filled up
with immense deposits of mud, sand, gravel, and boulder clay. These
deposits not only fill it up, but they cover it over to such an extent
that it is absolutely impossible to find on the surface a single trace
of it; and had it not been for borings, and other mining operations,
its existence would probably never have been known. In places where the
bottom of the trough is perhaps 200 feet below the sea-level, we find
on the surface not a hollow, but often an immense ridge or elliptical
knoll of sand, gravel, or boulder clay, rising sometimes to 150 or 200
feet above the present sea-level.
I need not here enter into any minute details regarding the form,
depth, and general outline of this trough, or of the character of
the deposits covering it, these having already been described by Mr.
Bennie, but shall proceed to the consideration of circumstances which
seem to throw light on the physical origin of this curious hollow,
and to the proof which it unexpectedly affords that Scotland, during
probably an early part of the glacial epoch, stood higher in relation
to the sea-level than it does at present; or rather, as I would be
disposed to express it, the sea stood much lower than at present.
From the fact that all along the line of this trough the surface of the
country is covered with enormous beds of stratified sands and gravels
of marine origin, which proves that the sea must have at a recent
period occupied the valley, my first impression was that this hollow
had been scooped out by the sea. This conclusion appeared at first
sight quite natural, for at the time that the sea filled the valley,
owing to the Gulf-stream impinging on our western shores, a strong
current would probably then pass through from the Atlantic on the west
to the German Ocean on the east. However, considerations soon began to
suggest themselves wholly irreconcilable with this hypothesis.
The question immediately arose, if the tendency of the sea occupying
the valley is to deepen it, by wearing down its rocky bottom, and
removing the abraded materials, then why is the valley filled up to
such a prodigious extent with marine deposits? Does not the fact of the
whole valley being filled up from sea to sea with marine deposits to a
depth of from 100 to 200 feet, and in some places, to even 400 feet,
show that the tendency of the sea filling this valley is to silt it up
rather than to deepen it? What conceivable change of conditions could
account for operations so diverse?
That the sea could not have cut out this trough, is, however,
susceptible of direct proof. The height of the surface of the valley
at the watershed or highest part, about a mile to the east of
Kilsyth, where the Kelvin and the Bonny Water, running in opposite
directions,—the one west into the Clyde, and the other east into the
Carron,—take their rise, is 160 feet above the sea-level. Consequently,
before the sea could pass through the valley at present, the sea-level
would require to be raised 160 feet.
But in discussing the question as to the origin of this pre-glacial
hollow, we must suppose the surface deposits of the valley all removed,
for this hollow was formed before these deposits were laid down. Let
us take the average depth of these deposits at the watershed to be 50
feet. It follows that, assuming the hollow in question to have been
formed by the sea, the sea-level at the time must have been at least
110 feet higher than at present.
Were the surface deposits of the country entirely removed, the district
to the west and north-west of Glasgow would be occupied by a sea
which would stretch from the Kilpatrick Hills, north of Duntocher,
to Paisley, a distance of about five miles, and from near Houston to
within a short way of Kirkintilloch, a distance of more than twelve
miles. This basin would contain a few small islands and sunken rocks,
but its mean depth, as determined from a great number of surface bores
obtained over its whole area, would be not much under 70 or 80 feet.
But we shall, however, take the depth at only 50 feet. Now, if we raise
the sea-level so as to allow the water just barely to flow over the
watershed of the valley, the sea in this basin would therefore be 160
feet deep. Let us now see what would be the condition of things on the
east end of the valley. The valley, for several miles to the east of
Kilsyth, continues very narrow, but on reaching Larbert it suddenly
opens into the broad and flat carse lands through which the Forth and
Carron wind. The average depth at which the sea would stand at present
in this tract of country, were the surface removed, as ascertained from
bores, would be at least 100 feet, or about double that in the western
basin. Consequently, when the sea was sufficiently high to pass over
the watershed, the water would be here 210 feet in depth, and several
miles in breadth.
[Illustration: PLATE VII.
W. & A. K. Johnston Edinb^r. and London.
CHART OF THE MIDLAND VALLEY, SHOWING BURIED RIVER CHANNELS.
_The blue parts represent the area which would be covered by sea
were the land submerged to the extent of 200 feet. The heavy black
lines A and B represent the buried River Channels._]
But in order to have a current of some strength passing through the
valley, let us suppose the sea at the time to have stood 150 feet
higher in relation to the land than at present. This would give 40 feet
as the depth of the sea on the watershed, and 200 feet as the depth in
the western basin, and 250 feet as the depth in the eastern.
An examination of the Ordnance Survey map of the district will show
that the 200 feet contour lines which run along each side of the valley
from Kilsyth to Castlecary come, in several places, to within one-third
of a mile of each other. From an inspection of the ground, I found
that, even though the surface deposits were removed off the valley, it
would not sensibly affect the contours at those places. It is therefore
evident that though the sea may have stood even 200 feet higher than at
present, the breadth of the strait at the watershed and several other
points could not have exceeded one-third of a mile. It is also evident
that at those places the current would be flowing with the greatest
velocity, for here it was not only narrowest, but also shallowest. A
reference to Plate VII. will show the form of the basins. The stippled
portion, coloured blue, represents the area which would be covered by
the sea were the land submerged to the extent of 200 feet.
Let us take the breadth of the current in the western basin at, say,
three miles. This is two miles less than the breadth of the basin
itself. Suppose the current at the narrow parts between Kilsyth and
Castlecary to have had a velocity of, say, five miles an hour. Now, as
the mean velocity of the current at the various parts of its course
would be inversely proportionate to the sectional areas of those parts,
it therefore follows that the mean velocity of the current in the
western basin would be only 1/45th of what it was in the narrow pass
between Kilsyth and Castlecary. This would give a mile in nine hours
as the velocity of the water in the western basin. In the eastern
basin the mean velocity of the current, assuming its breadth to be the
same as in the western, would be only a mile in eleven hours. In the
central part of the current the velocity at the surface would probably
be considerably above the mean, but at the sides and bottom it would,
no doubt, be under the mean. In fact, in these two basins the current
would be almost insensible.
The effect of such a current would simply be to widen and deepen the
valley all along that part between Kilsyth and Castlecary where the
current would be flowing with considerable rapidity. But it would
have little or no effect in deepening the basins at each end, but the
reverse. It would tend rather to silt them up. If the current flowed
from west to east, the materials removed from the narrow part between
Kilsyth and Castlecary, where the velocity of the water was great,
would be deposited when the current almost disappeared in the eastern
part of the valley. Sediment carried by a current flowing at the rate
of five miles an hour, would not remain in suspension when the velocity
became reduced to less than five miles a day.
But even supposing it were shown that the sea under such conditions
could have deepened the valley along the whole distance from the Clyde
to the Forth, still this would not explain the origin of the trough
in question. What we are in search of is not the origin of the valley
itself, but the origin of a deep and narrow hollow running along
the bottom of it. A sea filling the whole valley, and flowing with
considerable velocity, would, under certain conditions, no doubt deepen
and widen it, but it would not cut out along its bottom a deep, narrow
trough, with sides often steep, and in some places perpendicular and
even overhanging.
This hollow is evidently an old river-bed scooped out of the rocky
valley by a stream, flowing probably during an early part of the
glacial period.
During the latter part of the summer of 1868, I spent two or three
weeks of my holidays in tracing the course of this buried trough from
Kilsyth to the river Forth at Grangemouth, and I found unmistakable
evidence that the eastern portion of it, stretching from the watershed
to the Forth, had been cut out, not by the sea, but by a stream which
must have followed almost the present course of the Bonny Water.
I found that this deep hollow enters the Forth a few hundred yards to
the north of Grangemouth Harbour, at the extraordinary depth of 260
feet below the present sea-level. At the period when the sea occupied
the valley of the Forth and Clyde Canal, the bottom of the trough at
this spot would therefore be upwards of 400 feet below the level of the
sea.
A short distance to the west of Grangemouth, and also at Carron,
several bores were put down in lines almost at right angles across
the trough, and by this means we have been enabled to form a pretty
accurate estimate of its depth, breadth, and shape at those places. I
shall give the details of one of those sections.
Between Towncroft Farm and the river Carron, a bore was put down to
the depth of 273 feet before the rock was reached. About 150 yards to
the north of this there is another bore, giving 234 feet as the depth
to the rock; 150 yards still further north the depth of the surface
deposits, as determined by a third bore, is 155 feet. This last bore is
evidently outside of the hollow, for one about 150 yards north of it
gives the same depth of surface, which seems to be about its average
depth for a mile or two around. About half a mile to the south of the
hollow at this place the surface deposits are 150 feet deep. From a
number of bores obtained at various points within a circuit of 1½
miles, the surface appears to have a pretty uniform depth of 150 feet
or thereby. For the particulars of these “bores” I am indebted to the
kindness of Mr. Mackay, of Grangemouth.
To the south of the trough (see Fig. 12) there is a fault running
nearly parallel to it, having a down-throw to the north, and cutting
off the coal and accompanying strata to the south. But an inspection of
the section will show that the hollow in question is no way due to the
fault, but has been scooped out of the solid strata.
[Illustration: Fig. 12.
SECTION OF BURIED RIVER-BED NEAR TOWNCROFT FARM, GRANGEMOUTH.]
The main coal wrought extensively here is cut off by the trough,
as will be seen from the section. Mr. Dawson, of Carron Iron Works,
informs me that at Carronshore pit, about a mile and a quarter above
where this section is taken, the coal was found to be completely cut
off by this trough. In one of the workings of this pit, about forty
years ago, the miners cut into the trough at 40 fathoms below the
surface, when the sand rushed in with irresistible pressure, and filled
the working. Again, about a mile below where the section is taken,
or about two miles below Carronshore, and just at the spot where the
trough enters the Firth, it was also cut into in one of the workings of
the Heuck pit at a depth of 40 fathoms from the surface. Fortunately,
however, at this point the trough is filled with boulder clay instead
of sand, and no damage was sustained. Here, for a distance of two
miles, the Main coal and “Upper Coxroad” are cut off by this hollow; or
rather, I should say this hollow has been cut through the coal-seams.
The “Under Coxroad,” lying about 14 fathoms below the position of the
“Main” coal, as will be seen in the descriptive section (Fig. 12), is
not reached by the trough, and passes undisturbed under it.
This hollow would seem to narrow considerably as it recedes westwards,
for at Carronshore pit-shaft the surface is 138 feet deep; but not much
over 150 yards to the south of this is the spot where the coal was cut
off by the trough at a depth of 40 fathoms or 240 feet. Here it deepens
upwards of 100 feet in little more than 150 yards. That it is narrow at
this place is proved by the fact, that a bore put down near Carronbank,
a little to the south, shows the surface to be only 156 feet deep.
In the section (Fig. 12) the line described as “150 _feet above
sea-level_” registers the height of the sea-level at the time when
the central valley was occupied by sea 40 feet deep at the watershed.
Now, if this hollow, which extends right along the whole length of
the valley, had been cut out by the sea, the surface of the rock 150
feet below the present surface of the ground would be the sea-bottom
at the time, and the line marked “150 _feet above sea-level_” would be
the surface of the sea. The sea would therefore be here 300 feet deep
for several miles around. It cannot be supposed that the sea acting on
a broad flat plain of several miles in extent should cut out a deep,
narrow hollow, like the one exhibited in the section, and leave the
rest of the plain a flat sea-bottom.
And it must be observed, that this is not a hollow cut merely in a
sea-beach, but one extending westward to Kilsyth. Now, if this hollow
was cut out by the sea, it must have been done, not by the waves
beating on the beach, but by a current flowing through the valley.
The strongest current that could possibly pass through the narrow
part between Kilsyth and Castlecary would be wholly insensible when it
reached Grangemouth, where the water was 300 feet deep, and several
miles broad. Consequently, it is impossible that the current could have
scooped out the hollow represented in the section.
Again, if this hollow had been scooped out by the sea, it ought to
be deepest between Kilsyth and Castlecary, where the current was
narrowest; but the reverse is actually the case. It is shallowest at
the place where the current was narrowest, and deepest at the two
ends where the current was broadest. In the case of a trough cut by
a sea current, we must estimate its depth from the level of the sea.
Its depth is the depth of the water in it while it was being scooped
out. The bottom of the trough in the highest and narrowest part of
the valley east of Kilsyth is 40 feet above the present sea-level.
Consequently, its depth at this point at the period in question, when
the sea-level was 150 feet higher than at present, would be 110 feet.
The bottom of the trough at Grangemouth is 260 feet below the present
sea-level; add to this 150 feet, and we have 410 feet as its depth here
at the time in question. If this hollow was scooped out by the sea,
how then does it thus happen that at the place where the current was
strongest and confined to a narrow channel by hills on each side, it
cut its channel to a depth of only 110 feet, whereas at the place where
it had scarcely any motion it has cut, on a flat and open plain several
miles broad, a channel to a depth of 410 feet?
But, suppose we estimate the relative amount of work performed by the
sea at Kilsyth and Grangemouth, not by the actual depth of the bottom
of the trough at these two places below the sea-level at the time that
the work was performed, but by the present actual depth of the bottom
of the trough below the rocky surface of the valley, this will still
not help us out of the difficulty. Taking, as before, the height of the
rocky bed of the valley at the watershed at 110 feet above the present
sea-level, and the bottom of the trough at 40 feet, this gives 70 feet
as the depth scooped out of the rock at that place. The depth of the
trough at Grangemouth below the rocky surface is 118 feet. Here we have
only 70 feet cut out at the only place where there was any resistance
to the current, as well as the place where it possessed any strength;
whereas at Grangemouth, where there was no resistance, and no strength
of current, 118 feet has been scooped out. Such a result as this is
diametrically opposed to all that we know of the dynamics of running
water.
We may, therefore, conclude that it is physically impossible that this
hollow could have been cut out by the sea.
Owing to the present tendency among geologists to attribute effects
of this kind to ocean-currents, I have been induced to enter thus at
much greater length than would otherwise have been necessary into the
facts and arguments against the possibility of the hollow having been
excavated by the sea. In the present case the discussion is specially
necessary, for here we have positive evidence of the sea having
occupied the valley for ages, along which this channel has been cut.
Consequently, unless it is proved that the sea could not possibly have
scooped out the channel, most geologists would be inclined to attribute
it to the sea-current which is known to have passed through the valley
rather than to any other cause.
But that it is a hollow of denudation, and has been scooped out by some
agent, is perfectly certain. By what agent, then, has the erosion been
made? The only other cause to which it can possibly be attributed is
either land-ice or river-action.
The supposition that this hollow was scooped out by ice is not more
tenable than the supposition that the work has been done by the sea.
A glacier filling up the entire valley and descending into the German
Ocean would unquestionably not only deepen the valley, but would grind
down the surface over which it passed all along its course. But such a
glacier would not cut a deep and narrow channel along the bottom of the
valley. A glacier that could do this would be a small and narrow one,
just sufficiently large to fill this narrow trough; for if it were
much broader than the trough, it would grind away its edges, and make a
broad trough instead of a narrow one. But a glacier so small and narrow
as only to fill the trough, descending from the hills at Kilsyth to the
sea at Grangemouth, a distance of fifteen miles, is very improbable
indeed. The resistance to the advance of the ice along such a slope
would cause the ice to accumulate till probably the whole valley would
be filled.[282]
There is no other way of explaining the origin of this hollow, but
upon the supposition of its being an old river-bed. But there is
certainly nothing surprising in the fact of finding an old watercourse
under the boulder clay and other deposits. Unless the present contour
of the country be very different from what it was at the earlier
part of the glacial epoch, there must have then been watercourses
corresponding to the Bonny Water and the river Carron of the present
day; and that the remains of these should be found under the present
surface deposits is not surprising, seeing that these deposits are of
such enormous thickness. When water began to flow down our valleys, on
the disappearance of the ice at the close of the glacial epoch, the
Carron and the Bonny Water would not be able to regain their old rocky
channels, but would be obliged to cut, as they have done, new courses
for themselves on the surface of the deposits under which their old
ones lay buried.
Although an old pre-glacial or inter-glacial river-bed is in itself an
object of much interest and curiosity, still, it is not on that account
that I have been induced to enter so minutely into the details of this
buried hollow. There is something of far more importance attached to
this hollow than the mere fact of its being an old watercourse. For the
fact that it enters the Firth of Forth at a depth of 260 feet below the
present sea-level, proves incontestably that at the time this hollow
was occupied by a stream, _the land must have stood at least between
200 and 300 feet higher in relation to the sea-level than at present_.
We have seen that the old surface of the country in the neighbourhood
of Grangemouth, out of which this ancient stream cut its channel,
is at least 150 feet below the present sea-level. Now, unless this
surface had been above the sea-level at that time, the stream would
not have cut a channel in it. But it has not merely cut a channel, but
cut one to a depth of 120 feet. It is impossible that this channel
could have been occupied by a river of sufficient volume to fill it.
It is not at all likely that the river which scooped it out could have
been much larger than the Carron of the present day, for the area of
drainage, from the very formation of the country, could not have been
much greater above Grangemouth than at present. An elevation of the
land would, no doubt, increase the area of the drainage of the stream
measured from its source to where it might then enter the sea, because
it would increase the length of the stream; but it would neither
increase the area of drainage, nor the length of the stream above
Grangemouth. Kilsyth would be the watershed then as it is now.
What we have here is not the mere channel which had been occupied by
the ancient Carron, but the valley in which the channel lay. It may,
perhaps, be more properly termed a buried river valley; formed, no
doubt, like other river valleys by the denuding action of rain and
river.
The river Carron at present is only a few feet deep. Suppose the
ancient Carron, which flowed in this old channel, to have been say 10
feet deep. This would show that the land in relation to the sea at that
time must have stood at least 250 feet higher than at present. If 10
feet was the depth of this old river, and Grangemouth the place where
it entered the sea, then 250 feet would be the extent of the elevation.
But it is probable that Grangemouth was not the mouth of the river; it
would likely be merely the place where it joined the river Forth of
that period. We have every reason to believe that the bed of the German
Ocean was then dry land, and that the Forth, Tay, Tyne, and other
British rivers flowing eastward, as Mr. Godwin-Austin supposes, were
tributaries to the Rhine, which at that time was a huge river passing
down the bed of the German Ocean, and entering the Atlantic to the west
of the Orkney Islands. That the German Ocean, as well as the sea-bed of
the Western Hebrides, was dry land at a very recent geological period,
is so well known, that, on this point, I need not enter into details.
We may, therefore, conclude that the river Forth, after passing
Grangemouth, would continue to descend until it reached the Rhine. If,
by means of borings, we could trace the old bed of the Forth and the
Rhine up to the point where the latter entered the Atlantic, in the
same way as we have done the Bonny Water and the Carron, we should no
doubt obtain a pretty accurate estimate as to the height at which the
land stood at that remote period. Nothing whatever, I presume, is known
as to the depth of the deposits covering the bed of the German Ocean
along what was then the course of the Rhine. It must, no doubt, be
something enormous. We are also in ignorance as to the thickness of the
deposits covering the ancient bed of the Forth. A considerable number
of bores have been put down at various parts of the Firth of Forth in
connection with the contemplated railway bridge across the Firth, but
in none of those bores has the rock been reached. Bores to a depth of
175 feet have been made without even passing through the deposits of
silt which probably overlie an enormous thickness of sand and boulder
clay. Even in places where the water is 40 fathoms deep and quite
narrow, the bottom is not rock but silt.
It is, however, satisfactory to find on the land a confirmation of
what has long been believed from evidence found in the seas around our
island, that at a very recent period the sea-level in relation to the
land must have been some hundreds of feet lower than at the present
day, and that our island must have at that time formed a part of the
great eastern continent.
A curious fact was related to me by Mr. Stirling, the manager of the
Grangemouth collieries, which seems to imply a great elevation of the
land at a period long posterior to the time when this channel was
scooped out.
In sinking a pit at Orchardhead, about a mile to the north of
Grangemouth, the workmen came upon the boulder clay after passing
through about 110 feet of sand, clay, and gravel. On the upper surface
of the boulder clay they found cut out what Mr. Stirling believes
to have been an old watercourse. It was 17 feet deep, and not much
broader. The sides of the channel appear to have been smooth and
water-worn, and the whole was filled with a fine sharp sand beautifully
stratified. As this channel lay about 100 feet below the present
sea-level, it shows that if it actually be an old watercourse, it must
have been scooped out at a time when the land in relation to the sea
stood at least 100 feet higher than at present.
_Buried River Channel from Kilsyth to the Clyde._—In all probability
the western half of this great hollow, extending from the watershed
at Kilsyth to the Clyde, is also an old river channel, probably
the ancient bed of the Kelvin. This point cannot, however, be
satisfactorily settled until a sufficient number of bores have been
made along the direct line of the hollow, so as to determine with
certainty its width and general form and extent. That the western
channel is as narrow as the eastern is very probable. It has been
found that its sides at some places, as, for example, at Garscadden,
are very steep. At one place the north side is actually an overhanging
buried precipice, the bottom of which is about 200 feet below the
sea-level. We know also that the coal and ironstone in that quarter are
cut through by the trough, and the miners there have to exercise great
caution in driving their workings, in case they might cut into it. The
trough along this district is filled with sand, and is known to the
miners of the locality as the “sand-dyke.” To cut into running sand at
a depth of 40 or 50 fathoms is a very dangerous proceeding, as will be
seen from the details given in Mr. Bennie’s paper[283] of a disaster
which occurred about twenty years ago to a pit near Duntocher, where
this trough was cut into at a depth of 51 fathoms from the surface.
The depth of this hollow, below the present sea-level at Drumry, as
ascertained by a bore put down, is 230 feet. For several miles to the
east the depth is nearly as great. Consequently, if this hollow be an
old river-bed, the ancient river that flowed in it must have entered
the Clyde at a depth of more than 200 feet below the present sea-level;
and if so, then it follows that the rocky bed of the ancient Clyde must
lie buried under more than 200 feet of surface deposits from Bowling
downwards to the sea. Whether this is the case or not we have no means
at present of determining. The manager to the Clyde Trustees informs
me, however, that in none of the borings or excavations which have
been made has the rock ever been reached from Bowling downwards. The
probability is, that this deep hollow passes downwards continuously to
the sea on the western side of the island as on the eastern.[284]
The following journals of a few of the borings will give the reader an
idea of the character of the deposits filling the channels. The beds
which are believed to be boulder clay are printed in italics:—
BORINGS MADE THROUGH THE DEPOSITS FILLING THE WESTERN CHANNEL.
Bore, Drumry Farm, on Lands of Garscadden.
ft. ins.
Surface soil 2 6
Sand and gravel 3 6
Dry sand 11 0
Blue mud 8 6
Light mud and sand beds 13 0
Sand 31 6
Sand and mud 8 0
Sand and gravel 19 6
Sand 8 6
Gravel 24 4
Sand 5 0
Gravel 9 6
Sand 71 6
Sand (coaly) 1 0
Sand 9 0
Sand (coaly) 1 0
Sand 10 3
Red clay and gravel 4 8
Sand 1 5
Gravel 2 0
Sand 2 8
Gravel 10 6
Sand 1 6
Gravel 8 10
_Clay stones and gravel_ 33 3
————————
297 10
Bore on Mains of Garscadden, one mile north-east of Drumry.
ft. ins.
Surface soil 1 0
Blue clay and stones 60 0
Red clay and stones 18 0
Soft clay and sand beds 7 0
Gravel 6 0
Large gravel 9 0
Sand and gravel 7 0
Hard gravel 1 6
Sand and gravel 16 6
Dry sand 30 0
Black sand 2 0
Dry sand 33 0
Wet sand 8 0
Light mud 5 0
Sand 3 0
Gravel 5 6
Sandstone, black 0 6
Blue clay and stones 1 4
Whin block 0 10
Sandy clay 4 6
————————
219 8
Bore nearly half a mile south-west of Millichen.
ft. ins.
Sandy clay 5 0
_Brown clay and stones_ 17 0
Mud 15 0
Sandy mud 31 0
Sand and gravel with water 28 0
Sandy clay and gravel 17 0
Sand 5 0
Mud 6 0
Sand 14 0
Gravel 30 0
_Brown sandy clay and stones_ 30 0
Hard red gravel 4 6
Light mud and sand 1 8
_Light clay and stones_ 6 6
_Light clay and whin block_ 26 0
Fine sandy mud 36 0
_Brown clay and gravel and stones_ 14 4
_Bark clay and stones_ 68 0
————————
355 0
Bore at West Millichen, about 100 yards east of farm-house.
ft. ins.
Soil 1 6
_Muddy sand and stones_ 4 6
Soft mud 4 0
Sand and gravel 45 0
_Sandy mud and stones_ 20 6
Coarse gravel 11 6
Clay and gravel 1 4
Fine mud 7 0
Sand and gravel 2 0
Sandy mud 30 6
_Brown sandy clay and stones_ 25 0
Sand and gravel 6 0
_Brown sandy clay and stones_ 12 0
Sand 2 0
_Brown sandy clay and stones_ 4 0
Mud 5 0
Mud and sand 10 9
Sand and stones 2 9
_Blue clay and stones_ 5 0
————————
200 4
BORINGS MADE THROUGH THE DEPOSITS FILLING THE EASTERN CHANNEL.
No. 1. Between Towncroft Farm and Carron River—200 yards from
river. Height of surface, 12 feet above sea-level.
Feet.
Surface sand 6
Blue mud 4
Sand 4
Gravel 3
Sand 33
Red clay 46
_Soft blue till_ 17
_Hard blue till_ 140
Sand 20
———
273
No. 2. About 150 yards north of No. 1. Height of surface, 12
feet above sea-level.
Feet.
Surface sand 6
Blue mud 3
Shell bed 1
Gravel 2
Blue mud 8
Gravel 3
Blue muddy sand 15
Red clay 49
_Blue till and stones_ 20
Sand 20
_Hard blue till and stones_ 24
Sand 2
_Hard blue till and stones_ 40
Sand 7
_Hard blue till_ 24
———
234
No. 3. About 150 yards north of No. 2. Height of surface,
12 feet above sea-level.
Feet.
Surface sand 6
Soft mud with shells 11
Blue mud and sand (hard) 3
Channel (rough gravel) 3
Fine sand 8
Running sand (red and fine) 17
Red clay 30
_Soft till_ 36
Sand (pure) 2
_Soft till and sand_ 17
Gravel 8
_Hard blue till_ 14
———
155
No. 4. About 100 yards from No. 1.
Feet.
Surface 5
Blue mud 5
Black sand 3
Gravel 3
_Red clay and stones_ 34
Red clay 44
_Soft blue till_ 32
_Hard blue till and stones_ 104
Grey sand not passed through 22
———
252
Rock-head not reached.
No. 5. About 50 yards north of No. 4.
Feet.
Surface 6
Blue mud 3
Shell bed 1
Channel 2
Blue mud 8
Channel 3
Blue mud and sand 15
Red clay and sand 10
Red clay 49
_Blue till and stones_ 20
Sand 20
_Hard blue till and stones_ 24
Sand 2
_Hard blue till and stones_ 40
Sand 7
_Hard blue till_ 24
———
211
No. 6. Between Heuck and Carron River.
Feet.
Sandy clay 7
Mud 16
_Brown sandy clay and stones_ 3
Mud 36
Brown clay 39
_Blue till and stones_ 54
———
155
The question arises as to what is the origin of the stratified sands
and gravels filling up the buried river channels. Are they of marine or
of freshwater origin? Mr. Dugald Bell[285] and Mr. James Geikie[286]
are inclined to believe that as far as regards those filling the
western channel they are of lacustrine origin; that they were formed
in lakes, produced by the damming back of the water resulting from the
melting of the ice. I am, however, for the following reasons, inclined
to agree with Mr. Bennie’s opinion that they are of marine origin.
It will be seen, by a comparison of the journals of the borings made
through the deposits in the eastern channel with those in the western,
that they are of a similar character; so that, if we suppose those in
the western channel to be of freshwater origin, we may from analogy
infer the same in reference to the origin of those in the eastern
channel. But, as we have already seen, the deposits extend to the Firth
of Forth at Grangemouth, where they are met with at a depth of 260 feet
below sea-level. Consequently, if we conclude them to be of freshwater
origin, we are forced to the assumption, not that the water formed by
the melted ice was dammed back, but that the sea itself was dammed
back, and that by a wall extending to a depth of not less than two or
three hundred feet, so as to allow of a lake being formed in which the
deposits might accumulate; assuming, of course, that the absolute level
of the land was the same then as it is now.
But as regards the stratified deposits of Grangemouth, we have direct
evidence of their marine origin down to the bottom of the Red Clay that
immediately overlies the till and its intercalated beds, which on an
average is no less than 85 feet, and in some cases 100 feet, below the
present surface. From this deposit, Foraminifera, indicating an arctic
condition of sea, were determined by Mr. David Robertson. Marine shells
were also found in this bed, and along with them the remains of a
seal, which was determined by Professor Turner to be of an exceedingly
arctic type, thus proving that these deposits were not only marine but
glacial.
Direct fossil evidence as to the character of the deposits occupying
the western basin, is, however, not so abundant, but this may be owing
to the fact that during the sinking of pits, no special attention
has been paid to the matter. At Blairdardie, in sinking a pit-shaft
through these deposits, shells were found in a bed of sand between two
immense masses of boulder clay. The position of this bed will be better
understood from the following section of the pit-shaft:—
Feet.
Surface soil 4½
Blue clay 9
Hard stony clay 69
Sand with, a few _shells_ 3
Stony clay and boulders 46½
Mud and running sand 11
Hard clay, boulders, and broken rock 27
———
170
But as the shells were not preserved, we have, of course, no means of
determining whether they were of marine or of freshwater origin.
In another pit, at a short distance from the above, _Cyprina Islandica_
was found in a bed at the depth of 54 feet below the surface.[287]
In a paper read by Mr. James Smith, of Jordanhill, to the Geological
Society, April 24th, 1850,[288] the discovery is recorded of a
stratified bed containing _Tellina proxima_ intercalated between two
distinct boulder clays. The bed was discovered by Mr. James Russell in
sinking a well at Chapelhall, near Airdrie. Its height above sea-level
was 510 feet. The character of the shell not only proves the marine
origin of the bed, but also the existence of a submergence to that
extent during an inter-glacial period.