BOULDER FORMATION--_continued_.
Difficulty of interpreting the phenomena of drift before the glacial
hypothesis was adopted--Effects of intense cold in augmenting the
quantity of alluvium--Analogy of erratics and scored rocks in North
America and Europe--Bayfield on shells in drift of Canada--Great
subsidence and re-elevation of land from the sea, required to account
for glacial appearances--Why organic remains so rare in northern
drift--Mastodon giganteus in United States--Many shells and some
quadrupeds survived the glacial cold--Alps an independent centre of
dispersion of erratics--Alpine blocks on the Jura--Whether transported
by glaciers or floating ice--Recent transportation of erratics from
the Andes to Chiloe--Meteorite in Asiatic drift.
It will appear from what was said in the last chapter of the marine shells
characterizing the boulder formation, that nine-tenths or more of them
belong to species still living. The superficial position of "the drift" is
in perfect accordance with its imbedded organic remains, leading us to
refer its origin to a modern period. If, then, we encounter so much
difficulty in the interpretation of monuments relating to times so near our
own--if in spite of their recent date they are involved in so much
obscurity--the student may ask, not without reasonable alarm, how we can
hope to decipher the records of remote ages.
To remove from the mind as far as possible this natural feeling of
discouragement, I shall endeavour in this chapter to prove that what seems
most strikingly anomalous, in the "erratic formation," as some call it, is
really the result of that glacial action which has already been alluded to.
If so, it was to be expected that so long as the true origin of so singular
a deposit remained undiscovered, erroneous theories and terms would be
invented in the effort to solve the problem. These inventions would
inevitably retard the reception of more correct views which a wider field
of observation might afterwards suggest.
The term "diluvium" was for a time the popular name of the boulder
formation, because it was referred by some geologists to the deluge.
Others retained the name as expressive of their opinion that a series of
diluvial waves raised by hurricanes and storms, or by earthquakes, or by
the sudden upheaval of land from the bed of the sea, had swept over the
continents, carrying with them vast masses of mud and heavy stones, and
forcing these stones over rocky surfaces so as to polish and imprint
upon them long furrows and striae.
But no explanation was offered why such agency should have been
developed more energetically in modern times than at former periods of
the earth's history, or why it should be displayed in its fullest
intensity in northern latitudes; for it is important to insist on the
fact, that the boulder formation is a _northern_ phenomenon. Even the
southern extension of the drift, or the large erratics found in the Alps
and the surrounding lands, especially their occurrence round the highest
parts of the chain, offers such an exception to the general rule as
confirms the glacial hypothesis; for it shows that the transportation of
stony fragments to great distances, and the striation, polishing, and
grooving of solid floors of rock, are here again intimately connected
with accumulations of perennial snow and ice.
That there is some intimate connection between a cold or northern climate
and the various geological appearances now commonly called glacial, cannot
be doubted by any one who has compared the countries bordering the Baltic
with those surrounding the Mediterranean. The smoothing and striation of
rocks, and the erratics, are traced from the sea-shore to the height of
3000 feet above the level of the Baltic, whereas such phenomena are wholly
wanting in countries bordering the Mediterranean; and their absence is
still more marked in the equatorial parts of Asia, Africa, and America; but
when we cross the southern tropic, and reach Chili and Patagonia, we again
encounter the boulder formation, between the latitude 41 deg. S. and Cape Horn,
with precisely the same characters which it assumes in Europe. The evidence
as to climate derived from the organic remains of the drift is, as we have
seen, in perfect harmony with the conclusions above alluded to, the former
habits of the species of mollusca being accurately ascertainable, inasmuch
as they belong to species still living, and known to have at present a wide
range in northern seas.
But if we are correct in assuming that the northern hemisphere was
considerably colder than now during the period under consideration, owing
probably to the greater area and height of arctic lands, and to the
quantity of icebergs which such a geographical state of things would
generate, it may be well to reflect before we proceed farther on the entire
modification which extreme cold would produce in the operation of those
causes spoken of in the sixth chapter as most active in the formation of
alluvium. A large part of the materials derived from the detritus of rocks,
which in warm climates would go to form deltas, or would be regularly
stratified by marine currents, would, under arctic influences, assume a
superficial and alluvial character. Instead of mud being carried farther
from a coast than sand, and sand farther out than pebbles,--instead of
dense stratified masses being heaped up in limited areas,--nearly the whole
materials, whether coarse or fine, would be conveyed by ice to equal
distances, and huge fragments, which water alone could never move, would be
borne for hundreds of miles without having their edges worn or fractured;
and the earthy and stony masses, when melted out of the frozen rafts, would
be scattered at random over the submarine bottom, whether on mountain tops
or in low plains, with scarcely any relation to the inequalities of the
ground, settling on the crests or ridges of hills in tranquil water as
readily as in valleys and ravines. Occasionally, in those deep and
uninhabited parts of the ocean, never reached by any but the finest
sediment in a normal state of things, the bottom would become densely
overspread by gravel, mud, and boulders.
In the Western Hemisphere, both in Canada and as far south as the 40th and
even 38th parallel of latitude in the United States, we meet with a
repetition of all the peculiarities which distinguish the European boulder
formation. Fragments of rock have travelled for great distances from north
to south; the surface of the subjacent rock is smoothed, striated, and
fluted; unstratified mud or _till_ containing boulders is associated with
strata of loam, sand, and clay, usually devoid of fossils. Where shells are
present, they are of species still living in northern seas, and half of
them identical with those already enumerated as belonging to European drift
10 degrees of latitude farther north. The fauna also of the glacial epoch
in North America is less rich in species than that now inhabiting the
adjacent sea, whether in the Gulf of St. Lawrence, or off the shores of
Maine, or in the Bay of Massachusetts. At the southern extremity of its
course, moreover, it presents an analogy with the drift of the south of
Ireland, by blending with a more southern fauna, as for example at Brooklyn
near New York, in lat. 41 deg. N., where, according to MM. Redfield and Desor,
_Venus mercenaria_ and other southern species of shells begin to occur as
fossils in the drift.
The extension on the American continent of the range of erratics during the
Pleistocene period to lower latitudes than they reached in Europe, agrees
well with the present southward deflection of the isothermal lines, or
rather the lines of equal winter temperature. Formerly, as now, a more
extreme climate and a more abundant supply of floating ice prevailed on the
western side of the Atlantic.
Another resemblance between the distribution of the drift fossils in Europe
and North America has yet to be pointed out. In Norway, Sweden, and
Scotland, as in Canada and the United States, the marine shells are
confined to very moderate elevations above the sea (between 100 and 700
feet), while the erratic blocks and the grooved and polished surfaces of
rock extend to elevations of several thousand feet.
[Illustration: Fig. 118. Cross section.
K. Mr. Ryland's house.
_h_. Clay and sand of higher grounds, with _Saxicava_, &c.
_g_. Gravel with boulders.
_f_. Mass of _Saxicava rugosa_, 12 feet thick.
_e_. Sand and loam with _Mya truncata_, _Scalaria Groenlandica_, &c.
_d_. Drift, with boulders of syenite, &c.
_c_. Yellow sand.
_b_. Laminated clay, 25 feet thick.
A. Horizontal lower Silurian strata.
B. Valley re-excavated.]
I described in 1839 the fossil shells collected by Captain Bayfield from
strata of drift at Beauport near Quebec, in lat. 47 deg., and drew from them
the inference that they indicated a more northern climate, the shells
agreeing in great part with those of Uddevalla in Sweden.[134-A] The shelly
beds attain at Beauport and the neighbourhood a height of 200, 300, and
sometimes 400 feet above the sea, and dispersed through some of them are
large boulders of granite, which could not have been propelled by a violent
current, because the accompanying fragile shells are almost all entire.
They seem, therefore, said Captain Bayfield, writing in 1838, to have been
dropped down from melting ice, like similar stones which are now annually
deposited in the St. Lawrence.[134-B] I visited this locality in 1842, and
made the annexed section, fig. 118., which will give an idea of the general
position of the drift in Canada and the United States. I imagine that the
whole of the valley B was once filled up with the beds _b_, _c_, _d_, _e_,
_f_, which were deposited during a period of subsidence, and that
subsequently the higher country (_h_) was submerged and overspread with
drift. The partial re-excavation of B took place when this region was again
uplifted above the sea to its present height. Among the twenty-three
species of fossil shells collected by me from these beds at Beauport, all
were of recent northern species, except one, which is unknown as living,
and may be extinct (see fig. 119.). I also examined the same formation
farther up the valley of the St. Lawrence, in the suburbs of Montreal,
where some of the beds of loam are filled with great numbers of the
_Mytilus edulis_, or our common European mussel, retaining both its valves
and purple colour. This shelly deposit, containing _Saxicava rugosa_ and
other characteristic marine shells, also occurs at an elevated point on
the mountain of Montreal, 450 feet above the level of the sea.[135-A]
[Illustration: Fig. 119. _Astarte Laurentiana._
_a._ Outside.
_b._ Inside of right valve.
_c._ Inside of left valve.]
In my account of Canada and the United States, published in 1845, I
announced the conclusion to which I had then arrived, that to explain
the position of the erratics and the polished surfaces of rocks, and
their striae and flutings, we must assume first a gradual submergence of
the land in North America, after it had acquired its present outline of
hill and valley, cliff and ravine, and then its re-emergence from the
ocean. When the land was slowly sinking, the sea which bordered it was
covered with islands of floating ice coming from the north, which, as
they grounded on the coast and on shoals, pushed along such loose
materials of sand and pebbles as lay strewed over the bottom. By this
force all angular and projecting points were broken off, and fragments
of hard stone, frozen into the lower surface of the ice, had power to
scoop out grooves in the subjacent solid rock. The sloping beach, as
well as the floor of the ocean, might be polished and scored by this
machinery; but no flood of water, however violent, or however great the
quantity of detritus or size of the rocky fragments swept along by it,
could produce such long, perfectly straight and parallel furrows, as are
everywhere visible in the Niagara district, and generally in the region
north of the 40th parallel of latitude.[135-B]
By the hypothesis of such a slow and gradual subsidence of the land we may
account for the fact that almost everywhere in N. America and Northern
Europe the boulder formation rests on a polished and furrowed surface of
rock,--a fact by no means obliging us to imagine, as some think, that the
polishing and grooving action was, as a whole, anterior in date to the
transportation of the erratics. During the successive depression of high
land, varying originally in height from 1000 to 3000 feet above the
sea-level, every portion of the surface would be brought down by turns to
the level of the ocean, so as to be converted first into a coast-line, and
then into a shoal; and at length, after being well scored by the stranding
upon it of thousands of icebergs, might be sunk to a depth of several
hundred fathoms. By the constant depression of land, the coast would recede
farther and farther from the successively formed zones of polished and
striated rock, each outer zone becoming in its turn so deep under water as
to be no longer grated upon by the heaviest icebergs. Such sunken areas
would then simply serve as receptacles of mud, sand, and boulders dropped
from melting ice, perhaps to a depth scarcely, if at all, inhabited by
testacea and zoophytes. Meanwhile, during the formation of the unstratified
and unfossiliferous mass in deeper water, the smoothing and furrowing of
shoals and beaches is still going on elsewhere upon and near the coast in
full activity. If at length the subsidence should cease, and the direction
of the movement of the earth's crust be reversed, the sunken area covered
with drift would be slowly reconverted into land. The boulder deposit,
before emerging, would then for a time be brought within the action of the
waves, tides, and currents, so that its upper portion, being partially
disturbed, would have its materials re-arranged and stratified. Streams
also flowing from the land would in some places throw down layers of
sediment upon the _till_. In that case, the order of superposition will be,
first and uppermost, sand, loam, and gravel occasionally fossiliferous;
secondly, an unstratified and unfossiliferous mass, for the most part of
much older date than the preceding, with angular erratics, or with boulders
interspersed; and, thirdly, beneath the whole, a surface of polished and
furrowed rock. Such a succession of events seems to have prevailed very
widely on both sides of the Atlantic, the travelled blocks having been
carried in general from the North Pole southwards, but mountain chains
having in some cases served as independent centres of dispersion, of which
the Alps present the most conspicuous example.
It is by no means rare to meet with boulders imbedded in drift which are
worn flat on one or more of their sides, the surface being at the same time
polished, furrowed, and striated. They may have been so shaped in a glacier
before they reached the sea, or when they were fixed in the bottom of an
iceberg as it ran aground. We learn from Mr. Charles Martins that the
glaciers of Spitzbergen project from the coast into a sea between 100 and
400 feet deep; and that numbers of striated pebbles or blocks are there
seen to disengage themselves from the overhanging masses of ice as they
melt, so as to fall at once into deep water.[136-A]
That they should retain such markings when again upraised above the sea
ought not to surprise us, when we remember that rippled sands, and the
cracks in clay dried between high and low water, and the foot-tracks of
animals and rain-drops impressed on mud, and other superficial markings,
are all found fossil in rocks of various ages.
On the other hand, it is not difficult to account for the absence in many
districts of striated and scored pebbles and boulders in glacial deposits,
for they may have been exposed to the action of the waves on a coast while
it was sinking beneath or rising above the sea. No shingle on an ordinary
sea-beach exhibits such striae, and at a very short distance from the
termination of a glacier every stone in the bed of the torrent which gushes
out from the melting ice is found to have lost its glacial markings by
being rolled for a distance even of a few hundred yards.
The usual dearth of fossil shells in glacial clays well fitted to preserve
organic remains may, perhaps, be owing, as already hinted, to the absence
of testacea in the deep sea, where the undisturbed accumulation of boulders
melted out of very large bergs may take place. In the AEgean and other parts
of the Mediterranean, the zero of animal life, according to Prof. E.
Forbes, is approached at a depth of about 300 fathoms. In tropical seas it
would descend farther down, just as vegetation ascends higher on the
mountains of hot countries. Near the pole, on the other hand, the same zero
would be reached much sooner both on the hills and in the sea. If the ocean
was filled with floating bergs, and a low temperature prevailed in the
northern hemisphere during the glacial period, even the shallow part of the
sea might have been uninhabitable, or very thinly peopled with living
beings. It may also be remarked that the melting of ice in some fiords in
Norway freshens the water so as to destroy marine life, and famines have
been caused in Iceland by the stranding of icebergs drifted from the
Greenland coast, which have required several years to melt, and have not
only injured the hay harvest by cooling the atmosphere, but have driven
away the fish from the shore by chilling and freshening the sea.
If the cold of the glacial epoch came on slowly, if it was long before it
reached its greatest intensity, and again if it abated gradually, we may
expect to find the earliest and latest formed drift less barren of organic
remains than that deposited during the coldest period. We may also expect
that along the southern limits of the drift during the whole glacial epoch,
there would be an intimate association of transported matter of northern
origin with fossil-bearing sediment, whether marine or freshwater,
belonging to more southern seas, rivers, and continents.
That in the United States, the _Mastodon giganteus_ was very abundant after
the drift period is evident from the fact that entire skeletons of this
animal are met with in bogs and lacustrine deposits occupying hollows in
the drift. They sometimes occur in the bottom even of small ponds recently
drained by the agriculturist for the sake of the shell marl. I examined one
of these spots at Geneseo in the state of New York, from which the bones,
skull, and tusk of a Mastodon had been procured in the marl below a layer
of black peaty earth, and ascertained that all the associated freshwater
and land shells were of a species now common in the same district. They
consisted of several species of _Lymnea_, of _Planorbis bicarinatus_,
_Physa heterostropha_, &c.
In 1845 no less than six skeletons of the same species of Mastodon were
found in Warren County, New Jersey, 6 feet below the surface, by a farmer
who was digging out the rich mud from a small pond which he had drained.
Five of these skeletons were lying together, and a large part of the bones
crumbled to pieces as soon as they were exposed to the air. But nearly the
whole of the other skeleton, which lay about 10 feet apart from the rest,
was preserved entire, and proved the correctness of Cuvier's conjecture
respecting this extinct animal, namely, that it had twenty ribs like the
living elephant. From the clay in the interior within the ribs, just where
the contents of the stomach might naturally have been looked for, seven
bushels of vegetable matter were extracted. I submitted some of this matter
to Mr. A. Henfrey of London for microscopic examination, and he informs me
that it consists of pieces of small twigs of a coniferous tree of the
Cypress family, probably the young shoots of the white cedar, _Thuja
occidentalis_, still a native of North America, on which therefore we may
conclude that this extinct Mastodon once fed.
Another specimen of the same quadruped, the most complete and probably the
largest ever found, was exhumed in 1845 in the town of Newburg, New York,
the length of the skeleton being 25 feet, and its height 12 feet. The
anchylosing of the last two ribs on the right side afforded Dr. John C.
Warren a true gauge for the space occupied by the intervertebrate
substance, so as to enable him to form a correct estimate of the entire
length. The tusks when discovered were 10 feet long, but a part only could
be preserved. The large proportion of animal matter in the tusk, teeth, and
bones of some of these fossil mammalia is truly astonishing. It amounts in
some cases, as Dr. C. T. Jackson has ascertained by analysis, to 27 per
cent., so that when all the earthy ingredients are removed by acids, the
form of the bone remains as perfect, and the mass of animal matter is
almost as firm, as in a recent bone subjected to similar treatment.
It would be rash, however, to infer from such data that these quadrupeds
were mired in _modern_ times, unless we use that term strictly in a
geological sense. I have shown that there is a fluviatile deposit in the
valley of the Niagara, containing shells of the genera _Melania_,
_Lymnea_, _Planorbis_, _Valvata_, _Cyclas_, _Unio_, and _Helix_, &c.,
all of recent species, from which the bones of the great Mastodon have
been taken in a very perfect state. Yet the whole excavation of the
ravine, for many miles below the Falls, has been slowly effected since
that fluviatile deposit was thrown down.
Whether or not, in assigning a period of more than 30,000 years for the
recession of the Falls from Queenstown to their present site, I have over
or under estimated the time required for that operation, no one can doubt
that a vast number of centuries must have elapsed before so great a series
of geographical changes were brought about as have occurred since the
entombment of this elephantine quadruped. The freshwater gravel which
incloses it is decidedly of much more modern origin than the drift or
boulder clay of the same region.[138-A]
Other extinct animals accompany the _Mastodon giganteus_ in the
post-glacial deposits of the United States, among which the _Castoroides
ohioensis_, Foster and Wyman, a huge rodent allied to the beaver, and
the _Capybara_ may be mentioned. But whether the "loess," and other
freshwater and marine strata of the Southern States, in which skeletons
of the same Mastodon are mingled with the bones of the Megatherium,
Mylodon, and Megalonyx, were contemporaneous with the drift, or were of
subsequent date, is a chronological question still open to discussion.
It appears clear, however, from what we know of the tertiary fossils of
Europe--and I believe the same will hold true in North America--that
many species of testacea and some mammalia, which existed prior to the
glacial epoch, survived that era. As European examples among the
warm-blooded quadrupeds, the _Elephas primigenius_ and _Rhinoceros
tichorinus_ may be mentioned. As to the shells, whether fresh water,
terrestrial, or marine, they need not be enumerated here, as allusion
will be made to them in the sequel, when the pliocene tertiary fossils
of Suffolk are described. The fact is important, as refuting the
hypothesis that the cold of the glacial period was so intense and
universal as to annihilate all living creatures throughout the globe.
That the cold was greater for a time than it is now in certain parts of
Siberia, Europe, and North America, will not be disputed; but, before we
can infer the universality of a colder climate, we must ascertain what was
the condition of other parts of the northern, and of the whole southern,
hemisphere at the time when the Scandinavian, British, and Alpine erratics
were transported into their present position. It must not be forgotten that
a great deposit of drift and erratic blocks is now in full progress of
formation in the southern hemisphere, in a zone corresponding in latitude
to the Baltic, and to Northern Italy, Switzerland, France, and England.
Should the uneven bed of the southern ocean be hereafter converted by
upheaval into land, the hills and valleys will be strewed over with
transported fragments, some derived from the antarctic continent, others
from islands covered with glaciers, like South Georgia, which must now be
centres of the dispersion of drift, although situated in a latitude,
agreeing with that of the Cumberland mountains in England.
Not only are these operations going on between the 45th and 60th parallels
of latitude south of the line, while the corresponding zone of Europe is
free from ice; but, what is still more worthy of remark, we find in the
southern hemisphere itself, only 900 miles distant from South Georgia,
where the perpetual snow reaches to the sea-beach, lands covered with
forests, as in Terra del Fuego. There is here no difference of latitude to
account for the luxuriance of vegetation in one spot, and the absolute want
of it in the other; but among other refrigerating causes in South Georgia
may be enumerated the countless icebergs which float from the antarctic
zone, and which chill, as they melt, the waters of the ocean, and the
surrounding air, which they fill with dense fogs.
I have endeavoured in the "Principles of Geology," chapters 7. and 8., to
point out the intimate connexion of climate and the physical geography of
the globe, and the dependence of the mean annual temperature, not only on
the height of the dry land, but on its distribution in high or low
latitudes at particular epochs. If, for example, at certain periods of the
past, the antarctic land was less elevated and less extensive than now,
while that at the north pole was higher and more continuous, the conditions
of the northern and southern hemispheres might have been the reverse of
what we now witness in regard to climate, although the mountains of
Scandinavia, Scotland, and Switzerland, may have been less elevated than at
present. But if in both of the polar regions a considerable area of
elevated dry land existed, such a concurrence of refrigerating conditions
in both hemispheres might have created for a time an intensity of cold
never experienced since; and such probably was the state of things during
that period of submergence to which I have alluded in this chapter.
_Alpine erratics._--Although the arctic regions constitute the great centre
from which erratics have travelled southwards in all directions in Europe
and North America, yet there are some mountains, as I have already stated,
like those of North Wales and the Alps, which have served as separate and
independent centres for the dispersion of blocks. In illustration of this
fact, the Alps deserve particular attention, not only from their magnitude,
but because they lie beyond the ordinary limits of the "northern drift" of
Europe, being situated between the 44th and 47th degrees of north latitude.
On the flanks of these mountains, and on the Subalpine ranges of hills or
plains adjoining them, those appearances which have been so often alluded
to, as distinguishing or accompanying the drift, between the 50th and 70th
parallels of north latitude, suddenly reappear, to assume in a more
southern country their most exaggerated form. Where the Alps are highest,
the largest erratic blocks have been sent forth, as, for example, from the
regions of Mont Blanc and Monte Rosa, into the adjoining parts of France,
Switzerland, Austria, and Italy, while in districts where the great chain
sinks in altitude, as in Carinthia, Carniola, and elsewhere, no such rocky
fragments, or a few only and of smaller bulk, have been detached and
transported to a distance.
In the year 1821, M. Venetz first announced his opinion that the Alpine
glaciers must formerly have extended far beyond their present limits, and
the proofs appealed to by him in confirmation of this doctrine were
afterwards acknowledged by M. Charpentier, who strengthened them by new
observations and arguments, and declared, in 1836, his conviction that the
glaciers of the Alps must once have reached as far as the Jura, and have
carried thither their moraines across the great valley of Switzerland. M.
Agassiz, after several excursions in the Alps with M. Charpentier, and
after devoting himself some years to the study of glaciers, published, in
1840, an admirable description of them, and of the marks which attest the
former action of great masses of ice over the entire surface of the Alps
and the surrounding country.[140-A] He pointed out that the surface of
every large glacier is strewed over with gravel and stones detached from
the surrounding precipices by frost, rain, lightning, or avalanches. And he
described more carefully than preceding writers the long lines of these
stones, which settle on the sides of the glacier, and are called the
lateral moraines; those found at the lower end of the ice being called
terminal moraines. Such heaps of earth and boulders every glacier pushes
before it when advancing, and leaves behind it when retreating. When the
Alpine glacier reaches a lower and warmer situation, about 3000 or 4000
feet above the sea, it melts so rapidly that, in spite of the downward
movement of the mass, it can advance no farther. Its precise limits are
variable from year to year, and still more so from century to century; one
example being on record of a recession of half a mile in a single year. We
also learn from M. Venetz, that whereas, between the eleventh and fifteenth
centuries, all the Alpine glaciers were less advanced than now, they began
in the seventeenth and eighteenth centuries to push forward so as to cover
roads formerly open, and to overwhelm forests of ancient growth.
These oscillations enable the geologist to note the marks which they leave
behind them as they retrograde, and among these the most prominent, as
before stated, are the terminal moraines, or mounds of unstratified earth
and stones, often divided by subsequent floods into hillocks, which cross
the valley like ancient earth-works, or embankments made to dam up the
river. Some of these transverse barriers were formerly pointed out by
Saussure below the glacier of the Rhone, as proving how far it had once
transgressed its present boundaries. On these moraines we see many large
angular fragments, which, having been carried along on the surface of the
ice, have not had their edges worn off by friction; but the greater number
of the boulders, even those of large size, have been well rounded, not by
the power of water, but by the mechanical force of the ice, which has
pushed them against each other, or against the rocks flanking the valley.
Others have fallen down the numerous fissures which intersect the glacier,
where, being subject to the pressure of the whole mass of ice, they have
been forced along, and either well rounded or ground down into sand, or
even the finest mud, of which the moraine is largely constituted.
As the terminal moraines are the most prominent of all the monuments left
by a receding glacier, so are they the most liable to obliteration; for
violent floods or debacles are often occasioned in the Alps by the sudden
bursting of what are called glacier-lakes. These temporary sheets of water
are caused by the damming up of a river by a glacier which has increased
during a succession of cold seasons, and, descending from a tributary into
the main valley, has crossed it from side to side. On the failure of this
icy barrier, the accumulated waters are let loose, which sweep away and
level all transverse mounds of gravel and loose boulder below, and spread
their materials in confused and irregular beds over the river-plain.
Another mark of the former action of glaciers, in situations where they
exist no longer, is the polished, striated, and grooved surfaces of rocks
already alluded to. Stones which lie underneath the glacier and are pushed
along by it, sometimes adhere to the ice, and as the mass glides slowly
along at the rate of a few inches, or at the utmost two or three feet, per
day, abrade, groove, and polish the rock, and the larger blocks are
reciprocally grooved and polished by the rock on their lower sides. As the
forces both of pressure and propulsion are enormous, the sand, acting like
emery, polishes the surface; the pebbles, like coarse gravers, scratch and
furrow it; and the large stones scoop out grooves in it. Another effect
also of this action, not yet adverted to, is called "roches moutonnees."
Projecting eminences of rock are smoothed and worn into the shape of
flattened domes, where the glaciers have passed over them.
Although the surface of almost every kind of rock, when exposed in the open
air, wastes away by decomposition, yet some retain for ages their polished
and furrowed exterior; and, if they are well protected by a covering of
clay or turf, these marks of abrasion seem capable of enduring for ever.
They have been traced in the Alps to great heights above the present
glaciers, and to great horizontal distances beyond them.
There are also found, on the sides of the Swiss valleys, round and deep
holes, with polished sides, such holes as waterfalls make in the solid
rock, but in places remote from running waters, and where the form of the
surface will not permit us to suppose that any cascade could ever have
existed. Similar cavities are common in hard rocks, such as gneiss, in
Sweden, where they are called _giant caldrons_, and are sometimes 10 feet
and more in depth; but in the Alps and Jura they often pass into
spoon-shaped excavations and prolonged gutters. We learn from M. Agassiz
that hollows of this form are now cut out by streams of water, which flow
along the surface of glaciers, and then fall into fissures which are open
to the bottom. Here, forming a cascade, the stream cuts a round cavity in
the rock with the gravel and sand, which it either finds there or carries
down with it, and causes to rotate; and, as it usually happens that the
glacier is advancing, a locomotive cascade is produced, which converts the
first circular hole into a deep groove.
Another effect of a glacier is to lodge a ring of stones round the summit
of a conical peak which may happen to project through the ice. If the
glacier is lowered greatly by melting, these circles of large angular
fragments, which are called "perched blocks," are left in a singular
situation near the top of a steep hill or pinnacle, the lower parts of
which may be destitute of boulders.
_Alpine blocks on the Jura._--Now some or all the marks above
enumerated,--the moraines, erratics, polished surfaces, domes, striae,
caldrons, and perched rocks, are observed in the Alps at great heights
above the present glaciers, and far below their actual extremities; also
in the great valley of Switzerland, 50 miles broad; and almost
everywhere on the Jura, a chain which lies to the north of this valley.
The average height of the Jura is about one third that of the Alps, and
is now entirely destitute of glaciers, yet it presents almost everywhere
similar moraines, and the same polished and grooved surfaces, and
water-worn cavities. The erratics, moreover, which cover it, present a
phenomenon which has astonished and perplexed the geologist for more
than half a century. No conclusion can be more incontestible than that
these angular blocks of granite, gneiss, and other crystalline
formations, came from the Alps, and that they have been brought for a
distance of 50 miles and upwards across one of the widest and deepest
valleys of the world, so that they are now lodged on the hills and
valleys of a chain composed of limestone and other formations,
altogether distinct from those of the Alps. Their great size and
angularity, after a journey of so many leagues, has justly excited
wonder; for hundreds of them are as large as cottages; and one in
particular, celebrated under the name of Pierre a Bot, rests on the side
of a hill about 900 feet above the lake of Neufchatel, and is no less
than 40 feet in diameter.
It will be remarked that these blocks on the Jura offer an exception to the
rule before laid down, as applicable in general to erratics, since they
have gone from south to north. Some of the largest masses of granite and
gneiss have been found to contain 50,000 and 60,000 cubic feet of stone,
and one limestone block near Devens, which has travelled 30 miles, contains
161,000 cubic feet, its angles being sharp and unworn.[143-A]
Von Buch, Escher, and Studer have shown, from an examination of the
mineral composition of the boulders, that those on the western Jura,
near Neufchatel, have come from the region of Mont Blanc and the Valais;
those on the middle parts of the Jura from the Bernese Oberland; and
those on the eastern Jura from the Alps of the small cantons, Glaris,
Schwytz, Uri, and Zug. The blocks, therefore, of these three great
districts have been derived from parts of the Alps nearest to the
localities in the Jura where we now find them, as if they had crossed
the great valley in a direction at right angles to its length: the most
western stream having followed the course of the Rhone; the central,
that of the Aar; and the eastern, that of the two great rivers, Reuss
and Limmat. The non-intermixture of these groups of travelled fragments,
except near their confines, was always regarded as most enigmatical by
those who adopted the opinion of Saussure, that they were all whirled
along by a rapid current of muddy water rushing from the Alps.
M. Charpentier first suggested, as before mentioned, that the Swiss
glaciers once reached continuously to the Jura, and conveyed to them
these erratics; but at the same time he conceived that the Alps were
formerly higher than now. M. Agassiz, on the other hand, instead of
introducing distinct and separate glaciers, imagines that the whole
valley of Switzerland was filled with ice, and that one great sheet of
it extended from the Alps to the Jura, when the two chains were of the
same height as now relatively to each other. Such an hypothesis labours
under this difficulty, that the difference of altitude, when distributed
over a space of 50 miles, gives an inclination of no more than two
degrees, or far less than that of any known glaciers. It has, however,
since received the able support of Professor James Forbes, in his
excellent work on the Alps, published in 1843.
In the theory which I formerly advanced, jointly with Mr. Darwin[143-B],
it was suggested that the erratics may have been transferred by floating
ice to the Jura, at the time when the greater part of that chain, and the
whole of the Swiss valley to the south, was under the sea. At that period
the Alps may have attained only half their present altitude, and may yet
have constituted a chain as lofty as the Chilian Andes, which, in a
latitude corresponding to Switzerland, now send down glaciers to the head
of every sound, from which icebergs, covered with blocks of granite, are
floated seaward.[144-A] Opposite that part of Chili where the glaciers
abound is situated the island of Chiloe, 100 miles in length, with a
breadth of 30 miles, running parallel to the continent. The channel which
separates it from the main land is of considerable depth, and 25 miles
broad. Parts of its surface, like the adjacent coast of Chili, are
overspread with recent marine shells, showing an upheaval of the land
during a very modern period; and beneath these shells is a boulder deposit,
in which Mr. Darwin found large travelled blocks. One group of fragments
were of granite, which had evidently come from the Andes, while in another
place angular blocks of syenite were met with. Their arrangement may have
been due to successive crops of icebergs issuing from different sounds, to
the heads of which glaciers descend from the Andes. These icebergs, taking
their departure year after year from distinct points, may have been
stranded repeatedly, in equally distinct groups, in bays or creeks of
Chiloe, and on islets off the coast, so as afterwards to appear, some on
hills and others in valleys, when that country and the bed of the adjacent
sea had been upheaved. A continuance in future of the elevatory movement,
in the region of the Andes and of Chiloe, might cause the former chain to
rival the Alps in altitude, and give to Chiloe a height equal to that of
the Jura. The same rise might dry up the channel between Chiloe and the
main land, so that it would then represent the great valley of Switzerland.
In the course of these changes, all parts of Chiloe and the intervening
strait, having in their turn been a sea-shore, may have been polished and
scratched by coast-ice, and by innumerable icebergs running aground and
grating on the bottom.
If we apply this hypothesis to Switzerland and the Jura, we are by no means
precluded from the supposition that, in proportion as the land acquired
additional height, and the bed of the sea emerged, the Jura itself may have
had its glaciers; and those existing in the Alps, which had at first
extended to the sea, may, during some part of the period of upheaval, have
been prolonged much farther into the valleys than now. At a later period,
when the climate grew milder, these glaciers may have entirely disappeared
from the Jura, and may have receded in the Alps to their present limits,
leaving behind them in both districts those moraines which now attest the
former extension of the ice.[144-B]
_Meteorites in drift._--Before concluding my remarks on the northern drift
of the Old World, I shall refer to a fact recently announced, the discovery
of a meteoric stone at a great depth in the alluvium of Northern Asia.
Erman, in his Archives of Russia for 1841 (p. 314.), cites a very
circumstantial account drawn up by a Russian miner of the finding of a mass
of meteoric iron in the auriferous alluvium of the Altai. Some small
fragments of native iron were first met with in the gold-washings of
Petropawlowsker in the Mrassker Circle; but though they attracted
attention, it was supposed that they must have been broken off from the
tools of the workmen. At length, at the depth of 31 feet 5 inches from the
surface, they dug out a piece of iron weighing 17-1/2 pounds, of a
steel-grey colour, somewhat harder than ordinary iron, and, on analysing
it, found it to consist of native iron, with a small proportion of nickel,
as usual in meteoric stones. It was buried in the bottom of the deposit
where the gravel rested on a flaggy limestone. Much brown iron ore, as well
as gold, occurs in the same gravel, which appears to be part of that
extensive auriferous formation in which the bones of the mammoth, the
_Rhinoceros tichorhinus_, and other extinct quadrupeds abound. No
sufficient data are supplied to enable us to determine whether it be of
Post-Pliocene or Newer Pliocene date.
We ought not, I think, to feel surprise that we have not hitherto succeeded
in detecting the signs of such aerolites in older rocks, for, besides their
rarity in our own days, those which fell into the sea (and it is with
marine strata that geologists have usually to deal), being chiefly composed
of native iron, would rapidly enter into new chemical combinations, the
water and mud being charged with chloride of sodium and other salts. We
find that anchors, cannon, and other cast-iron implements which have been
buried for a few hundred years off our English coast have decomposed in
part or entirely, turning the sand and gravel which enclosed them into a
conglomerate, cemented together by oxide of iron. In like manner meteoric
iron, although its rusting would be somewhat checked by the alloy of
nickel, could scarcely ever fail to decompose in the course of thousands of
years, becoming oxide, sulphuret or carbonate of iron, and its origin being
then no longer distinguishable. The greater the antiquity of rocks,--the
oftener they have been heated and cooled, permeated by gases or by the
waters of the sea, the atmosphere or mineral springs,--the smaller must be
the chance of meeting with a mass of native iron unaltered; but the
preservation of the ancient meteorite of the Altai, and the presence of
nickel in these curious bodies, renders the recognition of them in deposits
of remote periods less hopeless than we might have anticipated.
FOOTNOTES:
[134-A] Geol. Trans. 2d series, vol. vi. p. 135. Mr. Smith of Jordanhill
had arrived at similar conclusions as to climate from the shells of the
Scotch Pleistocene deposits.
[134-B] Proceedings of Geol. Soc. No. 63. p. 119.
[135-A] Travels in N. America, vol. ii. p. 141.
[135-B] Ibid. p. 99. chap. xix.
[136-A] Bulletin Soc. Geol. de France, tom. iv. 2de ser. p. 1121.
[138-A] See Travels in N. America, vol. i. chap. ii.
[140-A] Agassiz, Etudes sur les Glaciers.
[143-A] Archiac, Hist. des Progres, &c. vol. ii. p. 249.
[143-B] See Elements of Geology, 2d ed. 1841.
[144-A] Darwin's Journal, p. 283.
[144-B] More recently Sir R. Murchison, having revisited the Alps, has
declared his opinion that "the great granitic blocks of Mont Blanc were
translated to the Jura when the intermediate country was under
water."--Paper read to Geol. Soc. London, May 30, 1849.