EARLIEST SIGNS OF MAN'S APPEARANCE IN EUROPE.
Chronological Relations of the Close of the Glacial Period and
the earliest geological Signs of the Appearance of Man.
Effects of Glaciers and Icebergs in polishing and scoring Rocks.
Scandinavia once encrusted with Ice like Greenland.
Outward Movement of Continental Ice in Greenland.
Mild Climate of Greenland in the Miocene Period.
Erratics of Recent Period in Sweden.
Glacial State of Sweden in the Pleistocene Period.
Scotland formerly encrusted with Ice.
Its subsequent Submergence and Re-elevation.
Latest Changes produced by Glaciers in Scotland.
Remains of the Mammoth and Reindeer in Scotch Boulder Clay.
Parallel Roads of Glen Roy formed in Glacier Lakes.
Comparatively modern Date of these Shelves.
The chronological relations of the human and glacial periods were
frequently alluded to in the last chapter, and the sections obtained
near Bedford, and at Hoxne, in Suffolk, and a general view of the
Norfolk cliffs, have taught us that the earliest signs of Man's
appearance in the British isles, hitherto detected, are of post-glacial
date. We may now therefore inquire whether the peopling of Europe by
the human race and by the mammoth and other mammalia now extinct, was
brought about during the concluding phases of the glacial epoch.
Although it may be impossible in the present state of our knowledge to
come to a positive conclusion on this head, I know of no inquiry better
fitted to clear up our views respecting the geological state of the
northern hemisphere at the time when the fabricators of the flint
implements of the Amiens type flourished. I shall therefore now proceed
to consider the chronological relations of that ancient people with
the final retreat of the glaciers from the mountains of Scandinavia,
Scotland, Wales, and Switzerland.
SUPERFICIAL MARKINGS AND DEPOSITS LEFT BY GLACIERS AND ICEBERGS.
In order fully to discuss this question, I must begin by referring
to some of the newest theoretical opinions entertained on the glacial
question. When treating of this subject in the "Principles of Geology,"
chapter 15, and in the "Manual (or Elements) of Geology," chapter 11, I
have stated that the whole mass of the ice in a glacier is in constant
motion, and that the blocks of stone detached from boundary precipices,
and the mud and sand swept down by avalanches of snow, or by rain from
the surrounding heights, are lodged upon the surface and slowly borne
along in lengthened mounds, called in Switzerland moraines. These
accumulations of rocky fragments and detrital matter are left at the
termination of the glacier, where it melts in a confused heap called the
"terminal moraine," which is unstratified, because all the blocks, large
and small, as well as the sand and the finest mud, are carried to
equal distances and quietly deposited in a confused mass without being
subjected to the sorting power of running water, which would convey the
finer materials farther than the coarser ones, and would produce, as the
strength of the current varied from time to time in the same place, a
stratified arrangement.
In those regions where glaciers reach the sea, and where large masses of
ice break off and float away, moraines, such as I have just alluded to,
may be transported to indefinite distances, and may be deposited on the
bottom of the sea wherever the ice happens to melt. If the liquefaction
take place when the berg has run aground and is stationary, and if there
be no current, the heap of angular and rounded stones, mixed with sand
and mud, may fall to the bottom in an unstratified form called "till" in
Scotland, and which has been shown in the last chapter to abound in the
Norfolk cliffs; but should the action of a current intervene at certain
points or at certain seasons, then the materials will be sorted as they
fall, and arranged in layers according to their relative weight and
size. Hence there will be passages from till to stratified clay, gravel,
and sand.
Some of the blocks of stone with which the surfaces of glaciers are
loaded, falling occasionally through fissures in the ice, get fixed and
frozen into the bottom of the moving mass, and are pushed along under
it. In this position, being subjected to great pressure, they scoop
out long rectilinear furrows or grooves parallel to each other on the
subjacent solid rock. Smaller scratches and striae are made on the
polished surface by crystals or projecting edges of the hardest
minerals, just as a diamond cuts glass.
In all countries the fundamental rock on which the boulder formation
reposes, if it consists of granite, gneiss, marble, or other hard stone
capable of permanently retaining any superficial markings which may have
been imprinted upon it, is smoothed or polished, and exhibits parallel
striae and furrows having a determinate direction. This prevailing
direction, both in Europe and North America, is evidently connected with
the course taken by the erratic blocks in the same district, and is
very commonly from north to south, or if it be twenty or thirty or more
degrees to the east or west of north, still always corresponds to the
direction in which the large angular and rounded stones have travelled.
These stones themselves also are often furrowed and scratched on more
than one side, like those already spoken of as occurring in the glacial
drift of Bedford, and in that of Norfolk.
When we contemplate the area which is now exposed to the abrading action
of ice, or which is the receptacle of moraine matter thrown down from
melting glaciers or bergs, we at once perceive that the submarine area
is the most extensive of the two. The number of large icebergs
which float annually to great distances in the northern and southern
hemispheres is extremely great, and the quantity of stone and mud which
they carry about with them enormous. Some floating islands of ice have
been met with from 2 to 5 miles in length, and from 100 to 225 feet in
height above water, the submerged portion, according to the weight
of ice relatively to sea water, being from six to eight times more
considerable than the part which is visible. Such masses, when they run
aground on the bottom of the sea, must exert a prodigious mechanical
power, and may polish and groove the subjacent rocks after the manner of
glaciers on the land. Hence there will often be no small difficulty
in distinguishing between the effects of the submarine and supramarine
agency of ice.
SCANDINAVIA ONCE COVERED WITH ICE, AND A CENTRE OF DISPERSION OF
ERRATICS.
In the north of Europe, along the borders of the Baltic, where the
boulder formation is continuous for hundreds of miles east and west, it
has been long known that the erratic blocks, often of very large size,
are of northern origin. Some of them have come from Norway and Sweden,
others from Finland, and their present distribution implies that they
were carried southwards, for a part at least of their way, by floating
ice, at a time when much of the area over which they are scattered was
under water. But it appears from the observations of Boetlingk, in
1840, and those of more recent inquirers, that while many blocks have
travelled to the south, others have been carried northwards, or to the
shores of the Polar Sea, and others north-eastward, or to those of
the White Sea. In fact, they have wandered towards all points of
the compass, from the mountains of Scandinavia as a centre, and the
rectilinear furrows imprinted by them on the polished surfaces of the
mountains where the rocks are hard enough to retain such markings,
radiate in all directions, or point outwards from the highest land, in a
manner corresponding to the course of the erratics above mentioned.*
(* Sir R.I. Murchison, in his "Russia and the Ural
Mountains" (1845) has indicated on a map not only the
southern limits of the Scandinavian drift, but by arrows the
direction in which "it proceeded eccentrically from a common
central region.")
Before the glacial theory was adopted, the Swedish and Norwegian
geologists speculated on a great flood, or the sudden rush of an
enormous body of water charged with mud and stones, descending from the
central heights or watershed into the adjoining lower lands. The erratic
blocks were supposed in their downward passage to have smoothed and
striated the rock surfaces over which they were forced along.
It would be a waste of time, in the present state of science, to
controvert this hypothesis, as it is now admitted that even if the rush
of a diluvial current, invented for the occasion and wholly without
analogy in the known course of nature, be granted, it would be
inadequate to explain the uniformity, parallelism, persistency,
and rectilinearity of the so-called glacial furrows. It is moreover
ascertained that heavy masses of rock, not fixed in ice, and moving as
freely as they do when simply swept along by a muddy current, do not
give rise to such scratches and furrows.
M. Kjerulf of Christiania, in a paper lately communicated to the
Geological Society of Berlin,* has objected, and perhaps with reason,
to what he considers the undue extent to which I have, in some of
my writings, supposed the mountains of northern Europe, to have been
submerged during the glacial period.
(* "Zeitschrift der Deutschen Geologischen Gesellschaft"
Berlin 1860.)
He remarks that the signs of glacial action on the Scandinavian
mountains ascend as high as 6000 feet, whereas fossil marine shells of
the same period never reach elevations exceeding 600 feet. The land, he
says, may have been much higher than it now is, but it has evidently not
been much lower since the commencement of the glacial period, or marine
shells would be traceable to more elevated points. In regard to the
absence of marine shells, I shall point out in the sequel how small is
the dependence we can place on this kind of negative evidence, if we
desire to test by it the extent to which the land has been submerged.
I cannot therefore consent to limit the probable depression and
re-elevation of Scandinavia to 600 feet. But that the larger part of
the glaciation of that country has been supramarine, I am willing to
concede. In support of this view M. Kjerulf observes that the direction
of the furrows and striae, produced by glacial abrasion, neither
conforms to a general movement of floating ice from the Polar regions,
nor to the shape of the existing valleys, as it would do if it had been
caused by independent glaciers generated in the higher valleys after
the land had acquired its actual shape. Their general arrangement and
apparent irregularities are, he contends, much more in accordance with
the hypothesis of there having been at one time a universal covering
of ice over the whole of Norway and Sweden, like that now existing in
Greenland, which, being annually recruited by fresh falls of snow,
was continually pressing outwards and downwards to the coast and lower
regions, after crossing many of the lower ridges, and having no relation
to the minor depressions, which were all choked up with ice and reduced
to one uniform level.
CONTINENTAL ICE OF GREENLAND.
In support of this view, he appeals to the admirable description of
the continental ice of Greenland, lately published by Dr. H. Rink of
Copenhagen,* who resided three or four years in the Danish settlements
in Baffin's Bay, on the west coast of Greenland, between latitudes 69
and 73 degrees north.
(* "Journal of Royal Geographical Society" volume 23 1853
page 145.)
"In that country, the land," says Dr. Rink, "may be divided into two
regions, the 'inland' and the 'outskirts.' The 'inland,' which is 800
miles from west to east, and of much greater length from north to south,
is a vast unknown continent, buried under one continuous and colossal
mass of permanent ice, which is always moving seaward, but a small
proportion only of it in an easterly direction, since nearly the
whole descends towards Baffin's Bay." At the heads of the fjords which
intersect the coast, the ice is seen to rise somewhat abruptly from the
level of the sea to the height of 2000 feet, beyond which the ice of
the interior rises continuously as far as the eye can reach, and to
an unknown altitude. All minor ridges and valleys are levelled and
concealed, but here and there steep mountains protrude abruptly from the
icy slope, and a few superficial lines of stones or moraines are visible
at seasons when no recent snow has fallen. [23]
Although all the ice is moving seaward, the greatest quantity is
discharged at the heads of certain large fjords, usually about 4 miles
wide, which, if the climate were milder, would be the outlet of as many
great rivers. Through these the ice is now protruded in huge blocks,
several miles wide, and from 1000 to 1500 feet in height or thickness.
When these masses reach the fjords, they do not melt or break up into
fragments, but continue their course in a solid form in the salt water,
grating along the rocky bottom, which they must polish and score at
depths of hundreds and even of more than 1000 feet. At length, when
there is water enough to float them, huge portions, having broken off,
fill Baffin's Bay with icebergs of a size exceeding any which could
be produced by ordinary valley glaciers. Stones, sand, and mud are
sometimes included in these bergs which float down Baffin's Bay. At some
points, where the ice of the interior of Greenland reaches the coast,
Dr. Rink saw mighty springs of clayey water issuing from under the edge
of the ice even in winter, showing the grinding action of the glacial
mass mixed with sand on the subjacent surface of the rocks.
The "outskirts," where the Danish colonies are stationed, consist of
numerous islands, of which Disco island is the largest in latitude 70
degrees north, and of many peninsulas, with fjords from 50 to 100 miles
long, running into the land, and through which the ice above alluded
to passes on its way to the bay. This area is 30, 000 square miles in
extent, and contains in it some mountains 4000 feet to 5000 feet high.
The perpetual snow usually begins at the height of 2000 feet, below
which level the land is for the most part free from snow between June
and August, and supports a vegetation of several hundred species of
flowering plants, which ripen their seeds before the winter. There
are even some places where phanerogamous plants have been found at an
elevation of 4500 feet; a fact which, when we reflect on the immediate
vicinity of so large and lofty a region of continental ice in the same
latitude, well deserves the attention of the geologist, who should
also bear in mind, that while the Danes are settled to the west in the
"outskirts," there exists, due east of the most southern portion of this
ice-covered continent, at the distance of about 1200 miles, the home of
the Laplanders with their reindeer, bears, wolves, seals, walruses, and
whales. If, therefore, there are geological grounds for suspecting that
Scandinavia or Scotland or Wales was ever in the same glacial condition
as Greenland now is, we must not imagine that the contemporaneous
fauna and flora were everywhere poor and stunted, or that they may
not, especially at the distance of a few hundred miles in a SOUTHWARD
direction, have been very luxuriant. [24]
Another series of observations made by Captain Graah, during a survey of
Greenland between 1823 and 1829, and by Dr. Pingel in 1830-32, adds not
a little to the geological interest of the "outskirts," in their bearing
on glacial phenomena of ancient date. Those Danish investigators, with
one of whom, Dr. Pingel, I conversed at Copenhagen in 1834, ascertained
that the whole coast from latitude 60 to about 70 degrees north has been
subsiding for the last four centuries, so that some ancient piles driven
into the beach to support the boats of the settlers have been gradually
submerged, and wooden buildings have had to be repeatedly shifted
farther inland.*
(* "Principles of Geology" chapter 30.)
In Norway and Sweden, instead of such a subsiding movement, the land is
slowly rising; but we have only to suppose that formerly, when it was
covered like Greenland with continental ice, it sank at the rate of
several feet in a century, and we shall be able to explain why marine
deposits are found above the level of the sea, and why these generally
overlie polished and striated surfaces of rock.
We know that Greenland was not always covered with snow and ice, for
when we examine the Tertiary strata of Disco Island (of the Upper
Miocene period) we discover there a multitude of fossil plants, which
demonstrate that, like many other parts of the arctic regions, it
formerly enjoyed a mild and genial climate. Among the fossils brought
from that island, latitude 70 degrees north, Professor Heer has
recognised Sequoia Langsdorfii, a coniferous species which flourished
throughout a great part of Europe in the Miocene period, and is very
closely allied to the living Sequoia sempervirens of California. The
same plant has been found fossil by Sir John Richardson within the
arctic circle, far to the west on the Mackenzie River, near the entrance
of Bear River, also by some Danish naturalists in Iceland to the east.
The Icelandic surturbrand, or lignite, of this age has also yielded
a rich harvest of plants, more than thirty-one of them, according to
Steenstrup and Heer, in a good state of preservation, and no less than
fifteen specifically identical with Miocene plants of Europe. Thirteen
of the number are arborescent; and amongst others is a tulip-tree
(Liriodendron), with its fruit and characteristic leaves, a plane
(Platanus), a walnut, and a vine, affording unmistakable evidence of
a climate in the parallel of the arctic circle which precludes the
supposition of glaciers then existing in the neighbourhood, still less
any general crust of continental ice, like that of Greenland.*
(* Heer, "Recherches sur la Vegetation du Pays tertiaire"
etc. 1861 page 178.)
As the older Pliocene flora of the Tertiary strata of Italy, like the
shells of the Coralline Crag, before adverted to, Chapter 12, indicate
a temperature milder than that now prevailing in Europe, though not
so warm as that of the Upper Miocene period, it is probable that the
accumulation of snow and glaciers on the mountains and valleys of
Greenland did not begin till after the commencement of the Pliocene
period, and may not have reached its maximum until the close of that
period.
Norway and Sweden appear to have passed through all the successive
phases of glaciation which Greenland has experienced, and others which
that country will one day undergo, if the climate which it formerly
enjoyed should ever be restored to it. There must have been first a
period of separate glaciers in Scandinavia, then a Greenlandic state of
continental ice, and thirdly, when that diminished, a second period of
enormous separate glaciers filling many a valley now wooded with fir
and birch. Lastly, under the influence of the Gulf Stream, and various
changes in the height and extent of land in the arctic circle, a melting
of nearly all the permanent ice between latitudes 60 and 70 north,
corresponding to the parallels of the continental ice of Greenland,
has occurred, so that we have now to go farther north than latitude 70
degrees before we encounter any glacier coming down to the sea coast.
Among other signs of the last retreat of the extinct glaciers, Kjerulf
and other authors describe large transverse moraines left in many of the
Norwegian and Swedish glens.
CHRONOLOGICAL RELATIONS OF THE HUMAN AND GLACIAL PERIODS IN SWEDEN.
We may now consider whether any, and what part, of these changes in
Scandinavia may have been witnessed by Man. In Sweden, in the immediate
neighbourhood of Upsala, I observed, in 1834, a ridge of stratified
sand and gravel, in the midst of which occurs a layer of marl, evidently
formed originally at the bottom of the Baltic, by the slow growth of
the mussel, cockle, and other marine shells of living species intermixed
with some proper to fresh water. The marine shells are all of dwarfish
size, like those now inhabiting the brackish waters of the Baltic; and
the marl, in which myriads of them are embedded, is now raised more than
100 feet above the level of the Gulf of Bothnia. Upon the top of this
ridge (one of those called osars in Sweden) repose several huge erratics
consisting of gneiss, for the most part unrounded, from 9 to 16 feet in
diameter, and which must have been brought into their present position
since the time when the neighbouring gulf was already characterised by
its peculiar fauna. Here, therefore, we have proof that the transport of
erratics continued to take place, not merely when the sea was inhabited
by the existing Testacea, but when the north of Europe had already
assumed that remarkable feature of its physical geography, which
separates the Baltic from the North Sea, and causes the Gulf of Bothnia
to have only one-fourth of the saltness belonging to the ocean.
I cannot doubt that these large erratics of Upsala were brought into
their present position during the Recent period, not only because of
their moderate elevation above the sea-level in a country where the land
is now rising every century, but because I observed signs of a great
oscillation of level which had taken place at Sodertelje, south of
Stockholm (about 45 miles distant from Upsala), after the country had
been inhabited by Man. I described, in the "Philosophical Transactions"
for 1835, the section there laid open in digging a level in 1819,
which showed that a subsidence followed by a re-elevation of land, each
movement amounting to more than 60 feet, had occurred since the time
when a rude hut had been built on the ancient shore. The wooden frame
of the hut, with a ring of hearthstones on the floor, and much charcoal,
were found, and over them marine strata, more than 60 feet thick,
containing the dwarf variety of Mytilus edulis, and other brackish-water
shells of the Bothnian Gulf. Some vessels put together with wooden pegs,
of anterior date to the use of metals, were also embedded in parts of
the same marine formation, which has since been raised, so that the
upper beds are more than 60 feet above the sea-level, the hut being thus
restored to about its original position relatively to the sea.
We have seen in the account of the Danish kitchen-middens of the Recent
period that even at the comparatively late period of their origin the
waters of the Baltic had been rendered more salt than they are now. The
Upsala erratics may belong to nearly the same era as these. But were
we to go back to a long antecedent epoch, or to that of the Belgian and
British caves with their extinct animals, and the signs they afford of a
state of physical geography departing widely from the present, or to the
era of the implement-bearing alluvium of St. Acheul, we might expect
to find Scandinavia overwhelmed with glaciers, and the country
uninhabitable by Man. At a much remoter period the same country was in
the state in which Greenland now is, overspread with one uninterrupted
coating of continental ice, which has left its peculiar markings on the
highest mountains. This period, probably anterior to the earliest traces
yet brought to light of the human race, may have coincided with the
submergence of England, and the accumulation of the boulder-clay of
Norfolk, Suffolk, and Bedfordshire, before mentioned. It has already
been stated that the syenite and some other rocks of the Norfolk till
seem to have come from Scandinavia, and there is no era when icebergs
are so likely to have floated them so far south as when the whole of
Sweden and Norway were enveloped in a massive crust of ice; a state
of things the existence of which is deduced from the direction of the
glacial furrows, and their frequent unconformity to the shape of the
minor valleys.
GLACIAL PERIOD IN SCOTLAND [25].
Professor Agassiz, after his tour in Scotland in 1840, announced
the opinion that erratic blocks had been dispersed from the Scottish
mountains as from an independent centre, and that the capping of ice had
been of extraordinary thickness.*
(* Agassiz, "Proceedings of the Geological Society" 1840 and
"Edinburgh Philosophical Journal" 49 page 79.)
Mr. Robert Chambers, after visiting Norway and Sweden, and comparing the
signs of glacial action observed there with similar appearances in the
Grampians, came to the conclusion that the Highlands both of Scandinavia
and Scotland had once been "moulded in ice," and that the outward and
downward movement and pressure of the frozen mass had not only smoothed,
polished, and scratched the rocks, but had, in the course of ages,
deepened and widened the valleys, and produced much of that denudation
which has commonly been ascribed exclusively to aqueous action. The
glaciation of the Scotch mountains was traced by him to the height of at
least 3000 feet.*
(* "Ancient Sea Margins" Edinburgh 1848. Glacial Phenomena
"Edinburgh New Philosophical Journal" April 1853 and January
1855.)
Mr. T.F. Jamieson, of Ellon, in Aberdeenshire, has recently brought
forward an additional body of facts in support of this theory. According
to him the Grampians were at the period of extreme cold enveloped "in
one great winding sheet of snow and ice," which reached everywhere to
the coast-line, the land being then more elevated than it is now. He
describes the glacial furrows sculptured on the solid rocks as pointing
in Aberdeenshire to the south-east, those of the valley of the Forth at
Edinburgh, from west to east, and higher up the same valley at Stirling,
from north-west to south-east, as they should do if the ice had followed
the lines of what is now the principal drainage. The observations of Sir
James Hall, Mr. Maclaren, Mr. Chambers, and Dr. Fleming, are cited by
him in confirmation of this arrangement of the glacial markings, while
in Sutherland and Ross-shire he shows that the glacial furrows along the
north coast point northwards, and in Argyleshire westwards, always in
accordance with the direction of the principal glens and fjords.
Another argument is also adduced by him in proof of the ice having
exerted its mechanical force in a direction from the higher and more
inland country to the lower region and sea-coast. Isolated hills and
minor prominences of rock are often polished and striated on the land
side, while they remain rough and jagged on the side fronting the sea.
This may be seen both on the east and west coast. Mention is also
made of blocks of granite which have travelled from south to north
in Aberdeenshire, of which there would have been no examples had the
erratics been all brought by floating ice from the arctic regions
when Scotland was submerged. It is also urged against the doctrine of
attributing the general glaciation to submergence, that the glacial
grooves, instead of radiating as they do from a centre, would, if they
had been due to ice coming from the north, have been parallel to the
coast-line, to which they are now often almost at right angles. The
argument, moreover, which formerly had most weight in favour of floating
ice, namely, that it explained why so many of the stones did not conform
to the contour and direction of the minor hills and valleys, is now
brought forward, and with no small effect, in favour of the doctrine of
continental ice on the Greenlandic scale, which, after levelling up the
lesser inequalities, would occasionally flow in mighty ice-currents, in
directions often at a high angle to the smaller ridges and glens.
The application to Scandinavia and Scotland of this theory makes it
necessary to reconsider the validity of the proofs formerly relied on
as establishing the submergence of a great part of Scotland beneath
the sea, at some period subsequent to the commencement of the glacial
period. In all cases where marine shells overlie till, or rest on
polished and striated surfaces of rock, the evidence of the land having
been under water, and having been since upheaved, remains unshaken; but
this special proof rarely extends to heights exceeding 500 feet. In the
basin of the Clyde we have already seen that Recent strata occur 25 feet
above the sea-level, with existing species of marine testacea, and with
buried canoes, and other works of art. At the higher level of 50
feet occurs the well-known raised beach of the western coast, which,
according to Mr. Jamieson, contains, near Fort William and on Loch Fyne
and elsewhere, an assemblage of shells implying a colder climate than
that of the 25-foot terrace, or that of the present sea; just as, in the
valley of the Somme, the higher-level gravels are supposed to belong to
a colder period than the lower ones, and still more decidedly than that
of the present era. At still greater elevations, older beds containing
a still more arctic group of shells have been observed at Airdrie, 14
miles south-east of Glasgow, 524 feet above the level of the sea. They
were embedded in stratified clays, with the unstratified boulder till
both above and below them, and in the overlying unstratified drift were
some boulders of granite which must have come from distances of 60 miles
at the least.*
(* Smith of Jordanhill, "Quarterly Journal of the Geological
Society" volume 6 1850 page 387.)
The presence of Tellina calcarea, and several other northern shells,
implies a climate colder than that of the present Scottish seas. In the
north of Scotland, marine shells have been found in deposits of the same
age in Caithness and in Aberdeenshire at heights of 250 feet, and on the
shores of the Moray Firth, as at Gamrie in Banff, at an elevation of 350
feet; and the stratified sands and beds of pebbles which belong to the
same formation ascend still higher--to heights of 500 feet at least.*
(* Prestwich, "Proceedings of the Geological Society" volume
2 page 545; Jamieson, "Quarterly Journal of the Geological
Society" volume 16 1860.)
At much greater heights, stratified masses of drift occur in which
hitherto no organic remains, whether of marine or freshwater animals,
have ever been found. It is still an undecided question whether the
origin of all such deposits in the Grampians can be explained without
the intervention of the sea. One of the most conspicuous examples has
been described by Mr. Jamieson as resting on the flank of a hill called
Meal Uaine, in Perthshire, on the east side of the valley of the Tummel,
just below Killiecrankie. It consists of perfectly horizontal strata,
the lowest portion of them 300 feet above the river and 600 feet above
the sea. From this elevation to an altitude of nearly 1200 feet the
same series of strata is traceable, continuously, up the slope of the
mountain, and some patches are seen here and there even as high as 1550
feet above the sea. They are made up in great part of finely laminated
silt, alternating with coarser materials, through which stones from 4 to
5 feet in length are scattered. These large boulders, and some smaller
ones, are polished on one or more sides, and marked with glacial striae.
The subjacent rocks, also, of gneiss, mica slate, and quartz, are
everywhere grooved and polished as if by the passage of a glacier.*
(* Jamieson, "Quarterly Journal of the Geological Society"
volume 16 1860 page 360.)
At one spot a vertical thickness of 130 feet of this series of strata is
exposed to view by a mountain torrent, and in all more than 2000 layers
of clay, sand, and gravel were counted, the whole evidently accumulated
under water. Some beds consist of an impalpable mud, like putty,
apparently derived from the grinding down of felspar, and resembling the
mud produced by the grinding action of modern glaciers.
Mr. Jamieson, when he first gave an account of this drift, inferred, in
spite of the absence of marine shells, that it implied the submergence
of Scotland beneath the ocean after the commencement of the glacial
period, or after the era of continental ice indicated by the subjacent
floor of polished and grooved rock. This conclusion would require
a submergence of the land as far up as 1550 feet above the present
sea-level, after which a great re-upheaval must have occurred. But the
same author, having lately revisited the valley of the Tummel, suggests
another possible, and I think probable, explanation of the same
phenomena. The stratified drift in question is situated in a deep
depression between two buttresses of rock, and if an enormous glacier be
supposed to have once filled the valley of the Tummel to the height
of the stratified drift, it may have dammed up the mouth of a mountain
torrent by a transverse barrier, giving rise to a deep pond, in which
beds of clay and sand brought down by the waters of the torrent were
deposited. Charpentier in his work on the Swiss glaciers has described
many such receptacles of stratified matter now in progress, and due
to such blockages, and he has pointed out the remnants of ancient and
similar formations left by extinct glaciers of an earlier epoch. He
specially notices that angular stones of various dimensions, often
polished and striated, which rest on the glacier and are let fall when
the torrent undermines the side of the moving ice, descend into the
small lake and become interstratified with the gravel and fine sediment
brought down by the torrent into the same.*
(* Charpentier, "Essai sur les Glaciers" page 63 1841.)
The evidence of the former sojourn of the sea upon the land after the
commencement of the glacial period was formerly inferred from the height
to which erratic blocks derived from distant regions could be traced,
besides the want of conformity in the glacial furrows to the present
contours of many of the valleys. Some of these phenomena may now, as we
have seen, be accounted for by assuming that there was once a crust of
ice resembling that now covering Greenland.
The Grampians in Forfarshire and in Perthshire are from 3000 to
4000 feet high. To the southward lies the broad and deep valley of
Strathmore, and to the south of this again rise the Sidlaw Hills to the
height of 1500 feet and upwards. On the highest summits of this chain,
formed of sandstone and shale, and at various elevations, I have
observed huge angular fragments of mica-schist, some 3 and others 15
feet in diameter, which have been conveyed for a distance of at least
15 miles from the nearest Grampian rocks from which they could have been
detached. Others have been left strewed over the bottom of the large
intervening vale of Strathmore.*
(* "Proceedings of the Geological Society" volume 3 page
344.)
It may be argued that the transportation of such blocks may have been
due not to floating ice, but to a period when Strathmore was filled up
with land ice, a current of which extended from the Perthshire Highlands
to the summit of the Sidlaw Hills, and the total absence of marine or
freshwater shells from all deposits, stratified or unstratified, which
have any connection with these erratics in Forfarshire and Perthshire
may be thought to favour such a theory.
But the same mode of transport can scarcely be imagined for those
fragments of mica-schist, one of them weighing from 8 to 10 tons, which
were observed much farther south by Mr. Maclaren on the Pentland Hills,
near Edinburgh, at the height of 1100 feet above the sea, the nearest
mountain composed of this formation being 50 miles distant.*
(* Maclaren, "Geology of Fife" etc. page 220.)
On the same hills, also, at all elevations, stratified gravels occur
which, although devoid of shells, it seems hardly possible to refer to
any but a marine origin.
Although I am willing, therefore, to concede that the glaciation of the
Scotch mountains, at elevations exceeding 2000 feet, may be explained
by land ice, it seems difficult not to embrace the conclusion that a
subsidence took place not merely of 500 or 600 feet, as demonstrated by
the marine shells, but to a much greater amount, as shown by the present
position of erratics and some patches of stratified drift. The absence
of marine shells at greater heights than 525 feet above the sea, will be
treated of in a future chapter. It may in part, perhaps, be ascribed
to the action of glaciers, which swept out marine strata from all the
higher valleys, after the re-emergence of the land.
LATEST CHANGES PRODUCED BY GLACIERS IN SCOTLAND.
We may next consider the state of Scotland after its emergence from the
glacial sea, when we cannot fail to be approaching the time when Man
co-existed with the mammoth and other mammalia now extinct. In a paper
which I published in 1840, on the ancient glaciers of Forfarshire, I
endeavoured to show that some of these existed after the mountains and
glens had acquired precisely their present shape,* and had left moraines
even in the minor valleys, just where they would now leave them were the
snow and ice again to gain ground.
(* "Proceedings of the Geological Society" volume 3 page
337.)
I described also one remarkable transverse mound, evidently the terminal
moraine of a retreating glacier, which crosses the valley of the South
Esk, a few miles above the point where it issues from the Grampians, and
about 6 miles below the Kirktown of Clova. Its central part, at a place
called Glenarm, is 800 feet above the level of the sea. The valley is
about half a mile broad, and is bounded by steep and lofty mountains,
but immediately above the transverse barrier it expands into a wide
alluvial plain, several miles broad, which has evidently once been a
lake. The barrier itself, about 150 feet high, consists in its lower
part of till with boulders, 50 feet thick, precisely resembling the
moraine of a Swiss glacier, above which there is a mass of stratified
sand, varying in thickness from 50 to 100 feet, which has the appearance
of consisting of the materials of the moraine rearranged in a stratified
form, possibly by the waters of a glacier lake. The structure of the
barrier has been laid open by the Esk, which has cut through it a deep
passage about 400 yards wide.
I have also given an account of another striking feature in the physical
geography of Perthshire and Forfarshire, which I consider to belong
to the same period; namely, a continuous zone of boulder clay, forming
ridges and mounds from 50 to 70 feet high (the upper part of the mounds
usually stratified), enclosing numerous lakes, some of them several
miles long, and many ponds and swamps filled with shell-marl and peat.
This band of till, with Grampian boulders and associated river-gravel,
may be traced continuously for a distance of 34 miles, with a width of
3 1/2 miles, from near Dunkeld, by Coupar, to the south of Blairgowrie,
then through the lowest part of Strathmore, and afterwards in a straight
line through the greatest depression in the Sidlaw Hills, from Forfar to
Lunan Bay.
Although no great river now takes its course through this line of
ancient lakes, moraines, and river gravel, yet it evidently marks
an ancient line by which, first, a great glacier descended from the
mountains to the sea, and by which, secondly, at a later period, the
principal water drainage of this country was effected. The subsequent
modification in geography is comparable in amount to that which has
taken place since the higher level gravels of the valley of the Somme
were formed, or since the Belgian caves were filled with mud and
bone-breccia.
[Illustration: Figure 35. Oval And Flattish Pebbles In Deserted Channels]
(FIGURE 35. OVAL AND FLATTISH PEBBLES IN DESERTED CHANNELS.)
Mr. Jamieson has remarked, in reference to this and some other extinct
river-channels of corresponding date, that we have the means of
ascertaining the direction in which the waters flowed by observing the
arrangement of the oval and flattish pebbles in their deserted channels;
for in the bed of a fast-flowing river such pebbles are seen to dip
towards the current, as represented in Figure 35, such being the
position of greatest resistance to the stream.*
(* Jamieson, "Quarterly Journal of the Geological Society"
volume 16 1860 page 349.)
If this be admitted, it follows that the higher or mountainous country
bore the same relation to the lower lands, at the time when a great
river passed through this chain of lakes, as it does at present.
We also seem to have a test of the comparatively modern origin of the
mounds of till which surround the above-mentioned chain of lakes
(of which that of Forfar is one), in the species of organic remains
contained in the shell-marl deposited at their bottom. All the mammalia
as well as shells are of recent species. Unfortunately, we have no
information as to the fauna which inhabited the country at the time
when the till itself was formed. There seem to be only three or
four instances as yet known in all Scotland of mammalia having been
discovered in boulder clay.
Mr. R. Bald has recorded the circumstances under which a single
elephant's tusk was found in the unstratified drift of the valley of the
Forth, with the minuteness which such a discovery from its rarity well
deserved. He distinguishes the boulder clay, under the name of "the old
alluvial cover," from that more modern alluvium, in which the whales
of Airthrie, described in Chapter 3, were found. This cover he says is
sometimes 160 feet thick. Having never observed any organic remains in
it, he watched with curiosity and care the digging of the Union Canal
between Edinburgh and Falkirk, which passed for no less than 28
miles almost continuously through it. Mr. Baird, the engineer who
superintended the works, assisted in the inquiry, and at one place
only in this long section did they meet with a fossil, namely, at
Cliftonhall, in the valley of the Almond. It lay at a depth of between
15 and 20 feet from the surface, in very stiff clay, and consisted of an
elephant's tusk, 39 inches long and 13 in circumference, in so fresh
a state that an ivory turner purchased it and turned part of it into
chessmen before it was rescued from destruction. The remainder is still
preserved in the museum at Edinburgh, but by exposure to the air it has
shrunk considerably.*
(* "Memoirs of the Wernerian Society" Edinburgh volume 4
page 58.)
In 1817, two other tusks and some bones of the elephant, as we learn
from the same authority (Mr. Bald), were met with, 3 1/2 feet long and
13 inches in circumference, lying in an horizontal position, 17 feet
deep in clay, with marine shells, at Kilmaurs, in Ayrshire. The species
of shells are not given.*
(* Ibid. volume 4 page 63.)
In another excavation through the Scotch boulder clay, made in digging
the Clyde and Forth Junction Railway, the antlers of a reindeer were
found at Croftamie, in Dumbartonshire, in the basin of the river
Endrick, which flows into Loch Lomond. They had cut through 12 feet
of till with angular and rounded stones, some of large size, and then
through 6 feet of underlying clay, when they came upon the deer's horns,
18 feet from the surface, and within a foot of the sandstone on which
the till rested. At the distance of a few yards, and in the same
position, but a foot or two deeper, were observed marine shells, Cyprina
islandica, Astarte elliptica, A. compressa, Fusus antiquus, Littorina
littorea, and a Balanus. The height above the level of the sea was
between 100 and 103 feet. The reindeer's horn was seen by Professor
Owen, who considered it to be that of a young female of the large
variety, called by the Hudson's Bay trappers the caribou.
The remains of elephants, now in the museums of Glasgow and Edinburgh,
purporting to come from the superficial deposits of Scotland have been
referred to Elephas primigenius. In cases where tusks alone have been
found unaccompanied by molar teeth, such specific determinations may be
uncertain; but if any one specimen be correctly named, the occurrence
of the mammoth and reindeer in the Scotch boulder-clay, as both these
quadrupeds are known to have been contemporary with Man, favours the
idea which I have already expressed, that the close of the glacial
period in the Grampians may have coincided in time with the existence of
Man in those parts of Europe where the climate was less severe, as, for
example, in the basins of the Thames, Somme, and Seine, in which the
bones of many extinct mammalia are associated with flint implements of
the antique type.
PARALLEL ROADS OF GLEN ROY IN SCOTLAND.
[Illustration: Plate 2. Glen Roy and Glen Spean]
(PLATE 2. VIEW OF THE MOUTHS OF GLEN ROY AND GLEN SPEAN,
BY SIR T. DICK LAUDER.
VV. Hill of Bohuntine.
VVV. Glen Roy.
V(inverted)V. Mealderry.
V. Entrance of Glen Spean
VV(superscript)V. Point of division between Glens Roy
and Spean.)
Perhaps no portion of the superficial drift of Scotland can lay claim to
so modern an origin on the score of the freshness of its aspect, as that
which forms what are called the Parallel Roads of Glen Roy. If they do
not belong to the Recent epoch, they are at least posterior in date to
the present outline of mountain and glen, and to the time when every one
of the smaller burns ran in their present channels, though some of them
have since been slightly deepened. The almost perfect horizontality,
moreover, of the roads, one of which is continuous for about 20 miles
from east to west, and 12 miles from north to south, shows that since
the era of their formation no change has taken place in the relative
levels of different parts of the district.
[Illustration: Figure 36. Map of Glen Roy]
(FIGURE 36. MAP OF THE PARALLEL ROADS OF GLEN ROY OR LOCHABER.
A. five miles distant south-west from this point is
Fort William, where the Lochy joins an arm of the sea,
called Loch Eil.
Vertical lines. Cols or watersheds at the heads of the
glens--once the westward outlet of the lakes.
Dots. Conspicuous delta deposits as laid down by
Mr. T.F. Jamieson.)
Glen Roy is situated in the Western Highlands, about 10 miles
east-north-east of Fort William, near the western end of the great glen
of Scotland, or Caledonian Canal, and near the foot of the highest of
the Grampians, Ben Nevis. (See map, Figure 36.) Throughout nearly its
whole length, a distance of more than 10 miles, three parallel roads
or shelves are traced along the steep sides of the mountains, as
represented in the annexed view, Plate 2, by the late Sir T. Dick
Lauder, each maintaining a perfect horizontality, and continuing at
exactly the same level on the opposite sides of the glen. Seen at a
distance, they appear like ledges, or roads, cut artificially out of the
sides of the hills; but when we are upon them, we can scarcely recognise
their existence, so uneven is their surface, and so covered with
boulders. They are from 10 to 60 feet broad, and merely differ from the
side of the mountain by being somewhat less steep.
On closer inspection, we find that these terraces are stratified in
the ordinary manner of alluvial or littoral deposits, as may be seen at
those points where ravines have been excavated by torrents. The parallel
shelves, therefore, have not been caused by denudation, but by the
deposition of detritus, precisely similar to that which is dispersed in
smaller quantities over the declivities of the hills above. These hills
consist of clay-slate, mica schist, and granite, which rocks have been
worn away and laid bare at a few points immediately above the parallel
roads. The lowest of these roads is about 850 feet above the level of
the sea, the next about 212 feet higher, and the third 82 feet above
the second. There is a fourth shelf, which occurs only in a contiguous
valley called Glen Gluoy, which is 12 feet above the highest of all the
Glen Roy roads, and consequently about 1156 feet above the level of the
sea.*
(* Another detached shelf also occurs at Kilfinnan. (See
Map, Figure 36.))
One only, the lowest of the three roads of Glen Roy, is continued
throughout Glen Spean, a large valley with which Glen Roy unites. (See
Plate 2 and map, Figure 36.) As the shelves, having no slope towards
the sea like ordinary river terraces, are always at the same absolute
height, they become continually more elevated above the river in
proportion as we descend each valley; and they at length terminate very
abruptly, without any obvious cause, or any change either in the shape
of the ground or in the composition or hardness of the rocks.
I should exceed the limits of this work, were I to attempt to give a
full description of all the geographical circumstances attending these
singular terraces, or to discuss the ingenious theories which have been
severally proposed to account for them by Dr. Macculloch, Sir T. Lauder,
and Messrs. Darwin, Agassiz, Milne, and Chambers. There is one point,
however, on which all are agreed, namely, that these shelves are ancient
beaches, or littoral formations, accumulated round the edges of one or
more sheets of water which once stood for a long time successively at
the level of the several shelves.
[Illustration: Figure 37. Section Through Side of Loch]
(FIGURE 37. SECTION THROUGH SIDE OF LOCH.
AB. Supposed original surface of rock.
CD. Roads or shelves in the outer alluvial covering of the hill.)
It is well known, that wherever a lake or marine fjord exists surrounded
by steep mountains subject to disintegration by frost or the action of
torrents, some loose matter is washed down annually, especially during
the melting of snow, and a check is given to the descent of this
detritus at the point where it reaches the waters of the lake. The waves
then spread out the materials along the shore, and throw some of them
upon the beach; their dispersing power being aided by the ice, which
often adheres to pebbles during the winter months, and gives buoyancy
to them. The annexed diagram (Figure 37) illustrates the manner in which
Dr. MacCulloch and Mr. Darwin suppose "the roads" to constitute mere
excrescences of the superficial alluvial coating which rests upon the
hillside, and consists chiefly of clay and sharp unrounded stones.
Among other proofs that the parallel roads have really been formed along
the margin of a sheet of water, it may be mentioned, that wherever an
isolated hill rises in the middle of the glen above the level of any
particular shelf, as in Mealderry, Plate 2, a corresponding shelf is
seen at the same level passing round the hill, as would have happened if
it had once formed an island in a lake or fjord. Another very remarkable
peculiarity in these terraces is this; each of them comes in some
portion of its course to a col, or parting ridge, between the heads of
glens, the explanation of which will be considered in the sequel.
Those writers who first advocated the doctrine that the roads were the
ancient beaches of freshwater lakes, were unable to offer any probable
hypothesis respecting the formation and subsequent removal of barriers
of sufficient height and solidity to dam up the water. To introduce
any violent convulsion for their removal was inconsistent with the
uninterrupted horizontality of the roads, and with the undisturbed
aspect of those parts of the glens where the shelves come suddenly to an
end.
Mr. Agassiz and Dr. Buckland, desirous, like the defenders of the lake
theory, to account for the limitation of the shelves to certain glens,
and their absence in contiguous glens, where the rocks are of the same
composition, and the slope and inclination of the ground very similar,
first started the theory that these valleys were once blocked up by
enormous glaciers descending from Ben Nevis, giving rise to what are
called, in Switzerland and in the Tyrol, glacier-lakes. In corroboration
of this view, they contended that the alluvium of Glen Roy, as well as
of other parts of Scotland, agrees in character with the moraines of
glaciers seen in the Alpine valleys of Switzerland. It will readily be
conceded that this hypothesis was preferable to any previous lacustrine
theory, by accounting more easily for the temporary existence and entire
disappearance of lofty transverse barriers, although the height required
for the supposed dams of ice appeared very enormous.
Before the idea of glacier-lakes had been suggested by Agassiz, Mr.
Darwin examined Glen Roy, and came to the opinion that the shelves were
formed when the glens were still arms of the sea, and, consequently,
that there never were any seaward barriers. According to him, the
land emerged during a slow and uniform upward movement, like that now
experienced throughout a large part of Sweden and Finland; but there
were certain pauses in the upheaving process, at which times the waters
of the sea remained stationary for so many centuries as to allow of the
accumulation of an extraordinary quantity of detrital matter, and the
excavation, at many points immediately above the sea-level, of deep
notches and bare cliffs in the hard and solid rock.
This theory I adopted in 1841 ("Elements," 2nd edition), as appearing to
me less objectionable than any other then proposed. The phenomena most
difficult to reconcile with it are, first, the abrupt cessation of the
roads at certain points in the different glens; secondly, their unequal
number in different valleys connecting with each other, there being
three, for example, in Glen Roy, and only one in Glen Spean; thirdly,
the precise horizontality of level maintained by the same shelf over a
space many leagues in length, requiring us to assume, that during a
rise of 1156 feet no one portion of the land was raised even a few yards
above another; fourthly, the coincidence of level already alluded to of
each shelf with a col, or the point forming the head of two glens, from
which the rain-waters flow in opposite directions. This last-mentioned
feature in the physical geography of Lochaber Mr. Darwin endeavoured to
explain in the following manner. He called these cols "land-straits,"
and regarding them as having been anciently sounds or channels between
islands, he pointed out that there is a tendency in such sounds to be
silted up, and always the more so in proportion to their narrowness. In
a chart of the Falkland Islands, by Captain Sulivan, R.N., it appears
that there are several examples there of straits where the soundings
diminish regularly towards the narrowest part. One is so nearly dry that
it can be walked over at low water, and another, no longer covered by
the sea, is supposed to have recently dried up in consequence of a small
alteration in the relative level of sea and land. "Similar straits,"
observes Mr. Chambers, "hovering, in character, between sea and land,
and which may be called fords, are met with in the Hebrides. Such, for
example, is the passage dividing the islands of Lewis and Harris, and
that between North Uist and Benbecula, both of which would undoubtedly
appear as cols, coinciding with a terrace or raised beach, all round the
islands if the sea were to subside."*
(* R. Chambers, "Ancient Sea Margins" page 114.)
The first of the difficulties above alluded to, namely, the
non-extension of the shelves over certain parts of the glens, might
be explained, said Mr. Darwin, by supposing in certain places a quick
growth of green turf on a good soil, which prevented the rain from
washing away any loose materials lying on the surface. But wherever the
soil was barren, and where green sward took long to form, there may have
been time for the removal of the gravel. In one case an intermediate
shelf appears for a short distance (three quarters of a mile) on the
face of the mountain called Tombhran, between the two upper shelves,
and is seen nowhere else. It occurs where there was the longest space of
open water, and where the waves may have acquired a more than ordinary
power to heap up detritus.
The unequal number of the shelves in valleys communicating with each
other, and in which the boundary rocks are similar in composition, and
the general absence of any shelves at corresponding altitudes in glens
on the opposite watershed, like that of the Spey, and in valleys where
the waters flow eastward, are difficulties attending the marine theory
which have never yet been got over. Mr. T.F. Jamieson, before cited,
has, during a late visit to Lochaber, in 1861, observed many facts
highly confirmatory of the hypothesis of glacier-lakes which, as I have
already stated, was originally advanced by Mr. Agassiz. In the first
place, he found much superficial scoring and polishing of rocks, and
accumulation of boulders at those points where signs of glacial action
ought to appear, if ice had once dammed up the waters of the glens in
which the "roads" occur. Ben Nevis may have sent down its glaciers from
the south, and Glen Arkaig from the north, for the mountains at the head
of the last-mentioned glen are 3000 feet high, and may, together with
other tributary glens, have helped to choke up the great Caledonian
valley with ice, so as to block up for a time the mouths of the
Spean, Roy, and Gluoy. The temporary conversion of these glens into
glacier-lakes is the more conceivable, because the hills at their upper
ends not being lofty nor of great extent, they may not have been filled
with ice at a time when great glaciers were generated in other adjoining
and much higher regions.
Secondly. The shelves, says Mr. Jamieson, are more precisely defined
and unbroken than any of the raised beaches or acknowledged ancient
coast-lines visible on the west of Scotland, as in Argyllshire, for
example.
Thirdly. At the level of the lower shelf in Glen Roy, at points
where torrents now cut channels through the shelf as they descend the
hill-side, there are small delta-like extensions of the shelf, perfectly
preserved, as if the materials, whether fine or coarse, had originally
settled there in a placid lake, and had not been acted upon by tidal
currents, mingling them with the sediment of other streams. These deltas
are too entire to allow us to suppose that they have at any time since
their origin been exposed to the waves of the sea.
Fourthly. The alluvium on the cols or watersheds, before alluded to, is
such as would have been formed if the waters of the rivers had been made
to flow east, or out of the upper ends of the supposed glacier-lakes,
instead of escaping at the lower ends, in a westerly direction, where
the great blockages of ice are assumed to have occurred.
In addition to these arguments of Mr. Jamieson, I may mention that
in Switzerland, at present, no testacea live in the cold waters of
glacier-lakes; so that the entire absence of fossil shells, whether
marine or freshwater, in the stratified materials of each shelf, would
be accounted for if the theory above mentioned be embraced.
When I examined "the parallel roads" in 1825, in company with Dr.
Buckland, neither this glacier theory nor Mr. Darwin's suggestion of
ancient sea-margins had been proposed, and I have never since revisited
Lochaber. But I retain in my memory a vivid recollection of the
scenery and physical features of the district, and I now consider the
glacier-lake theory as affording by far the most satisfactory solution
of this difficult problem. The objection to it, which until lately
appeared to be the most formidable, and which led Mr. Robert Chambers
in his "Sea Margins," to reject it entirely, was the difficulty of
conceiving how the waters could be made to stand so high in Glen Roy as
to allow the uppermost shelf to be formed. Grant a barrier of ice in the
lower part of the glen of sufficient altitude to stop the waters from
flowing westward, still, what prevented them from escaping over the col
at the head of Glen Glaster? This col coincides exactly in level, as Mr.
Milne Home first ascertained, with the second or middle shelf of Glen
Roy. The difficulty here stated appears now to be removed by supposing
that the higher lines or roads were formed before the lower ones, and
when the quantity of ice was most in excess. We must imagine that at the
time when the uppermost shelf of Glen Roy was forming in a shallow lake,
the lower part of that glen was filled up with ice, and, according to
Mr. Jamieson, a glacier from Loch Treig then protruded itself across
Glen Spean and rested on the flank of the hill on the opposite side in
such a manner as effectually to prevent any water from escaping over the
Glen Glaster col. The proofs of such a glacier having actually existed
at the point in question consist, he says, in numerous cross striae
observable in the bottom of Glen Spean, and in the presence of moraine
matter in considerable abundance on the flanks of the hill extending
to heights above the Glen Glaster col. When the ice shrank into less
dimensions the second shelf would be formed, having its level determined
by the col last mentioned, Glen Spean in the meantime being filled with
a glacier. Finally, the ice blockage common to glens Roy, Spean, and
Laggan, which consisted probably of a glacier from Ben Nevis, gave rise
to the lowest and most extensive lake, the waters of which escaped over
the pass of Muckul or the col at the head of Loch Laggan, which, as Mr.
Jamieson has now ascertained: agrees precisely in level with the lowest
of all the shelves, and where there are unequivocal signs of a river
having flowed out for a considerable period.
Dr. Hooker has described some parallel terraces, very analogous in their
aspect to those of Glen Roy, as existing in the higher valleys of
the Himalaya, of which his pencil has given us several graphic
illustrations. He believes these Indian shelves to have originated on
the borders of glacier-lakes, the barriers of which were usually
formed by the ice and moraines of lateral or tributary glaciers, which
descended into and crossed the main valley, as we have supposed in the
case of Glen Roy; but others he ascribes to the terminal moraine of the
principal glacier itself, which had retreated during a series of milder
seasons, so as to leave an interval between the ice and the terminal
moraine. This interspace caused by the melting of ice becomes filled
with water and forms a lake, the drainage of which usually takes place
by percolation through the porous parts of the moraine, and not by a
stream overflowing that barrier. Such a glacier-lake Dr. Hooker actually
found in existence near the head of the Yangma valley in the Himalaya.
It was moreover partially bounded by recently formed marginal terraces
or parallel roads, implying changes of level in the barrier of ice and
moraine matter.*
(* Hooker, "Himalayan Journal" volume 1 page 242; 2 pages
119, 121, 166. I have also profited by the author's personal
explanations.)
It has been sometimes objected to the hypothesis of glacier-lakes, as
applied to the case of Glen Roy, that the shelves must have taken a very
long period for their formation. Such a lapse of time, it is said, might
be consistent with the theory of pauses or stationary periods in the
rise of the land during an intermittent upward movement, but it is
hardly compatible with the idea of so precarious and fluctuating
a barrier as a mass of ice. But the reader will have seen that the
permanency of level in such glacier-lakes has no necessary connection
with minor changes in the height of the supposed dam of ice. If a
glacier descending from higher mountains through a tributary glen enters
the main valley in which there happens to be no glacier, the river is
arrested in its course and a lake is formed. The dam may be constantly
repaired and may vary in height several hundreds of feet without
affecting the level of the lake, so long as the surplus waters escape
over a col or parting ridge of rock. The height at which the waters
remain stationary is determined solely by the elevation of the col, and
not by the barrier of ice, provided the barrier is higher than the col.
But if we embrace the theory of glacier-lakes, we must be prepared
to assume not only that the sea had nothing to do with the original
formation of the "parallel roads," but that it has never, since the
disappearance of the lakes, risen in any one of the glens up to the
level of the lowest shelf, which is about 850 feet high; for in that
case the remarkable persistency and integrity of the roads and deltas,
before described, must have been impaired.
We have seen that 50 miles to the south of Lochaber, the glacier
formations of Lanarkshire with marine shells of arctic character have
been traced to the height of 524 feet. About 50 miles to the south-east
in Perthshire are those stratified clays and sands, near Killiecrankie,
which were once supposed to be of submarine origin, and which in that
case would imply the former submergence of what is now dry land to the
extent of 1550 feet, or several hundred feet beyond the highest of the
parallel roads. Even granting that these laminated drifts may have had
a different origin, as above suggested, there are still many facts
connected with the distribution of erratics and the striation of rocks
in Scotland which are not easily accounted for without supposing the
country to have sunk, since the era of continental ice, to a greater
depth than 525 feet, the highest point to which marine shells have yet
been traced.
After what was said of the pressure and abrading power of a general
crust of ice, like that now covering Greenland, it is almost superfluous
to say that the parallel roads must have been of later date than such a
state of things, for every trace of them must have been obliterated
by the movement of such a mass of ice. It is no less clear that as no
glacier-lakes can now exist in Greenland [26], so there could have been
none in Scotland, when the mountains were covered with one great crust
of ice. It may, however, be contended that the parallel roads were
produced when the general crust of ice first gave place to a period
of separate glaciers, and that no period of deep submergence ever
intervened in Lochaber after the time of the lakes. Even in that case,
however, it is difficult not to suppose that the Glen Roy country
participated in the downward movement which sank part of Lanarkshire
525 feet beneath the sea, subsequently to the first great glaciation of
Scotland. Yet that amount of subsidence might have occurred, and even a
more considerable one, without causing the sea to rise to the level
of the lowest shelf, or to a height of 850 feet above the present
sea-level.
This is a question on which I am not prepared at present to offer a
decided opinion.
Whether the horizontality of the shelves or terrace-lines is really as
perfect as has been generally assumed is a point which will require to
be tested by a more accurate trigonometrical survey than has yet been
made. The preservation of precisely the same level in the lowest line
throughout the glens of Roy, Spean, and Laggan, for a distance of 20
miles east and west, and 10 or 12 miles north and south, would be very
wonderful if ascertained with mathematical precision. Mr. Jamieson,
after making in 1862 several measurements with a spirit-level, has been
led to suspect a rise in the lowest shelf of one foot in a mile in a
direction from west to east, or from the mouth of Glen Roy to a point
6 miles east of it in Glen Spean. To confirm such observations, and to
determine whether a similar rate of rise continues eastward, as far as
the pass of Muckul, would be most important.
On the whole, I conclude that the Glen Roy terrace-lines and those of
some neighbouring valleys, were formed on the borders of glacier-lakes,
in times long subsequent to the principal glaciation of Scotland. They
may perhaps have been nearly as late, especially the lowest of the
shelves, as that portion of the Pleistocene period in which Man
co-existed in Europe with the mammoth.