RELATION TO THE HUMAN PERIOD.
Extinct Glaciers of Switzerland.
Alpine Erratic Blocks on the Jura.
Not transported by floating Ice.
Extinct Glaciers of the Italian Side of the Alps.
Theory of the Origin of Lake-Basins by the erosive Action of
Glaciers considered.
Successive phases in the Development of Glacial Action in the Alps.
Probable Relation of these to the earliest known Date of Man.
Correspondence of the same with successive Changes in the Glacial
Condition of the Scandinavian and British Mountains.
Cold Period in Sicily and Syria.
EXTINCT GLACIERS OF SWITZERLAND.
We have seen in the preceding chapters that the mountains of
Scandinavia, Scotland, and North Wales have served, during the glacial
period, as so many independent centres for the dispersion of erratic
blocks, just as at present the ice-covered continent of North Greenland
is sending down ice in all directions to the coast, and filling Baffin's
Bay with floating bergs, many of them laden with fragments of rocks.
Another great European centre of ice-action during the Pleistocene
period was the Alps of Switzerland, and I shall now proceed to consider
the chronological relations of the extinct Alpine glaciers to those of
more northern countries previously treated of. [32]
The Alps lie far south of the limits of the northern drift described in
the foregoing pages, being situated between the 44th and 47th degrees of
north latitude. On the flanks of these mountains, and on the sub-Alpine
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
and assume, in a southern region, a truly arctic development. 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 Switzerland 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.*
(* Agassiz, "Etudes sur les Glaciers et Systeme Glaciaire.")
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 a 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 a
glacier leaves behind it as it retrogrades; 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 earthworks, or embankments
made to dam up a 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
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 sometimes occasioned in the Alps by
the sudden bursting of glacier-lakes, or those temporary sheets of water
before alluded to which 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,
being let loose, sweep away and level many a transverse mound of gravel
and loose boulders 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 before described. 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 2 or 3 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.
Lastly, projecting eminences of rock, called "roches moutonnees," 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 to 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.
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 ERRATIC BLOCKS ON THE JURA.
Now some or all the marks above enumerated,--the moraines, erratics,
polished surfaces, domes, striae, 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 it is now entirely destitute of glaciers; yet it
presents almost everywhere moraines, and polished and grooved surfaces
of rocks. 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 incontestable 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, composed of gneiss, 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.
But there are some far-transported masses of granite and gneiss which
are still larger, and which have been found to contain 50,000 and 60,000
cubic feet of stone; and one limestone block at Devens, near Bex, which
has travelled 30 miles, contains 161,000 cubic feet, its angles being
sharp and unworn.
Von Buch, Escher, and Studer inferred, from an examination of the
mineral composition of the boulders, that those resting on the Jura,
opposite the lakes of Geneva and Neufchatel, have come from the region
of Mont Blanc and the Valais, as if they had followed the course of
the Rhone to the lake of Geneva, and had then pursued their way
uninterruptedly in a northerly direction.
M. Charpentier, who conceived the Alps in the period of greatest cold to
have been higher by several thousand feet than they are now, had already
suggested that the Alpine glaciers once reached continuously to the
Jura, conveying thither the large erratics in question.*
(* D'Archiac, "Histoire des Progres" etc. volume 2 page
249.)
M. Agassiz, on the other hand, instead of introducing distinct and
separate glaciers, imagined that the whole valley of Switzerland might
have been filled with ice, and that one great sheet of it extended from
the Alps to the Jura, the two chains being of the same height as
now relatively to each other. To this idea it was objected that the
difference of altitude, when distributed over a space of 50 miles, would
give an inclination of two degrees only, or far less than that of any
known glacier. In spite of this difficulty, the hypothesis has since
received the support of Professor James Forbes in his very able work on
the Alps published in 1843.
In 1841, I advanced jointly with Mr. Darwin* the theory 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.
(* See "Elements of Geology" 2nd edition 1841.)
We pointed out that if at that period the Alps had attained only half
their present altitude they would yet have constituted a chain as lofty
as the Chilean 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. Opposite that part
of Chile 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 Chile, 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 blocks
of granite and syenite which had evidently come from the Andes.
A continuance in future of the elevatory movement now observed to be
going on in this 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.
Sir Roderick I. Murchison, after making several important geological
surveys of the Alps, proposed in 1849 a theory agreeing essentially with
that suggested by Mr. Darwin and myself, namely that the erratics were
transported to the Jura at a time when the great strath of Switzerland
and many valleys receding far into the Alps were under water. He thought
it impossible that the glacial detritus of the Rhone could ever have
been carried to the Lake of Geneva and beyond it by a glacier, or that
so vast a body of ice issuing from one narrow valley could have spread
its erratics over the low country of the cantons of Vaud, Fribourg,
Berne, and Soleure, as well as the slopes of the Jura, comprising a
region of about 100 miles in breadth from south-west to north-east, as
laid down in the map of Charpentier. He therefore imagined the granitic
blocks to have been translated to the Jura by ice-floats when the
intermediate country was submerged.*
(* "Quarterly Journal of the Geological Society" volume 6
1850 page 65.)
It may be remarked that this theory, provided the water be assumed to
have been salt or brackish, demands quite as great an oscillation in the
level of the land as that on which Charpentier had speculated, the only
difference being that the one hypothesis requires us to begin with a
subsidence of 2500 or 3000 feet, and the other with an elevation to the
same amount. We should also remember that the crests or watersheds of
the Alps and Jura are about 80 miles apart, and if once we suppose them
to have been in movement during the glacial period it is very probable
that the movements at such a distance may not have been strictly
uniform. If so the Alps may have been relatively somewhat higher, which
would have greatly facilitated the extension of Alpine glaciers to the
flanks of the less elevated chain.
Five years before the publication of the memoir last mentioned, M. Guyot
had brought forward a great body of new facts in support of the original
doctrine of Charpentier, that the Alpine glaciers once reached as far
as the Jura and that they had deposited thereon a portion of their
moraines.*
(* "Bulletin de la Societe des Sciences Naturelles de
Neufchatel" 1845.)
The scope of his observations and argument was laid with great clearness
before the British public in 1852 by Mr. Charles Maclaren, who had
himself visited Switzerland for the sake of forming an independent
opinion on a theoretical question of so much interest and on which so
many eminent men of science had come to such opposite conclusions.*
(* "Edinburgh New Philosophical Magazine" October 1852.)
M. Guyot had endeavoured to show that the Alpine erratics, instead
of being scattered at random over the Jura and the great plain of
Switzerland, are arranged in a certain determinate order strictly
analogous to that which ought to prevail if they had once constituted
the lateral, medial, and terminal moraines of great glaciers. The rocks
chiefly relied on as evidence of this distribution consist of three
varieties of granite, besides gneiss, chlorite-slate, euphotide,
serpentine, and a peculiar kind of conglomerate, all of them foreign
alike to the great Strath between the Alps and Jura and to the structure
of the Jura itself. In these two regions limestones, sandstones, and
clays of the Secondary and Tertiary formations alone crop out at the
surface, so that the travelled fragments of Alpine origin can easily be
distinguished and in some cases the precise localities pointed out from
whence they must have come.
[Illustration: Figure 42. Map of Ancient Glacier]
(FIGURE 42. MAP SHOWING THE SUPPOSED COURSE OF THE ANCIENT AND NOW
EXTINCT GLACIER OF THE RHONE, AND THE DISTRIBUTION OF THE ERRATIC
BLOCKS AND DRIFT CONVEYED BY IT TO THE GREAT VALLEY OF SWITZERLAND
AND THE JURA.)
The accompanying map or diagram (Figure 42) slightly altered from one
given by Mr. Maclaren will enable the reader more fully to appreciate
the line of argument relied on by M. Guyot. The dotted area is that over
which the Alpine fragments were spread by the supposed extinct glacier
of the Rhone. The site of the present reduced glacier of that name is
shown at A. From that point the boulders may first be traced to B, or
Martigny, where the valley takes an abrupt turn at right angles to its
former course. Here the blocks belonging to the right side of the river
or derived from c d e have not crossed over to the left side at B, as
they should have done had they been transported by floating ice, but
continue to keep to the side to which they belonged, assuming that they
once formed part of a right lateral moraine of a great extinct glacier.
That glacier, after arriving at the lower end of the long narrow valley
of the upper Rhone at F, filled the Lake of Geneva, F, I, with ice. From
F, as from a great vomitory, it then radiated in all directions bearing
along with it the moraines with which it was loaded and spreading them
out on all sides over the great plain. But the principal icy mass moved
straight onwards in a direct line towards the hill of Chasseron, G
(precisely opposite F), where the Alpine erratics attain their maximum
of height on the Jura, that is to say 2015 English feet above the level
of the Lake of Neufchatel or 3450 feet above the sea. The granite blocks
which have ascended to this eminence G came from the east shoulder of
Mont Blanc h, having travelled in the direction B, F, G.
When these and the accompanying blocks resting on the south-eastern
declivity of the Jura are traced from their culminating point G in
opposite directions, whether westward towards Geneva or eastwards
towards Soleure, they are found to decline in height from the middle of
the arc G towards the two extremities I and K, both of which are at a
lower level than G, by about 1500 feet. In other words the ice of the
extinct glacier, having mounted up on the sloping flanks of the Jura in
the line of greatest pressure to its highest elevation, began to decline
laterally in the manner of a pliant or viscous mass with a gentle
inclination till it reached two points distant from each other no less
than 100 miles. [33]
In further confirmation of this theory M. Guyot observed that fragments
derived from the right bank of the great valley of the Rhone c d e are
found on the right side of the great Swiss basin or Strath as at l and
m, while those derived from the left bank p h occur on the left side of
the basin or on the Jura between G and I; and those again derived from
places farthest up on the left bank and nearest the source of the Rhone,
as n o, occupy the middle of the great basin, constituting between m
and K what M. Guyot calls the frontal or terminal moraine of the eastern
prolongation of the old glacier.
A huge boulder of talcose granite, now at Steinhoff, 10 miles east from
K, or Soleure, containing 61,000 French cubic feet, or equal in bulk
to a mass measuring 40 feet in every direction, was ascertained by
Charpentier from its composition to have been derived from n, one of the
highest points on the left side of the Rhone valley far above Martigny.
From this spot it must have gone all round by F, which is the only
outlet to the deep valley, so as to have performed a journey of no less
than 150 miles!
GENERAL TRANSPORTATION OF ERRATICS IN SWITZERLAND DUE TO GLACIERS AND
NOT TO FLOATING ICE.
It is evident that the above described restriction of certain fragments
of peculiar lithological character to that bank of the Rhone where the
parent rocks are alone met with and the linear arrangement of the
blocks in corresponding order on the opposite side of the great plain of
Switzerland, are facts which harmonise singularly well with the theory
of glaciers while they are wholly irreconcilable with that of floating
ice. Against the latter hypothesis all the arguments which Charpentier
originally brought forward in opposition to the first popular doctrine
of a grand debacle or sudden flood rushing down from the Alps to the
Jura might be revived. Had there ever been such a rush of muddy water,
said he, the blocks carried down the basins of the principal Swiss
rivers, such as the Rhone, Aar, Reuss, and Limmat, would all have been
mingled confusedly together instead of having each remained in separate
and distinct areas as they do and should do according to the glacial
hypothesis.
M. Morlot presented me in 1857 with an unpublished map of Switzerland in
which he had embodied the results of his own observations and those
of MM. Guyot, Escher, and others, marking out by distinct colours the
limits of the ice-transported detritus proper to each of the great
river-basins. The arrangement of the drift and erratics thus
depicted accords perfectly well with Charpentier's views and is quite
irreconcilable with the supposition of the scattered blocks having been
dispersed by floating ice when Switzerland was submerged.
As opposed to the latter hypothesis, I may also state that nowhere as
yet have any marine shells or other fossils than those of a terrestrial
character, such as the bones of the mammoth and a few other mammalia
and some coniferous wood, been detected in those drifts, though they are
often many hundreds of feet in thickness.
A glance at M. Morlot's map, above mentioned,* will show that the two
largest areas, indicated by a single colour, are those over which the
Rhone and the Rhine are supposed to have spread out in ancient times
their enormous moraines.
(* See map, "Quarterly Journal of the Geological Society"
volume 18 1862 page 185 Plate 18.)
One of these only, that of the Rhone, has been exhibited in our diagram,
Figure 42. The distinct character of the drift in the two cases is such
as it would be if two colossal glaciers should now come down from the
higher Alps through the valleys traversed by those rivers, leaving their
moraines in the low country. The space occupied by the glacial drift of
the Rhine is equal in dimensions or rather exceeds that of the Rhone,
and its course is not interfered with in the least degree by the Lake
of Constance, 45 miles long, any more than is the dispersion of the
erratics of the Rhone by the Lake of Geneva, about 50 miles in length.
The angular and other blocks have in both instances travelled on
precisely as if those lakes had no existence, or as if, which was no
doubt the case, they had been filled with solid ice.
During my last visit to Switzerland in 1857, I made excursions, in
company with several distinguished geologists, for the sake of testing
the relative merits of the two rival theories above referred to, and I
examined parts of the Jura above Neufchatel in company with M. Desor,
the country round Soleure with M. Langen, the southern side of the great
strath near Lausanne with M. Morlot, the basin of the Aar around Berne
with M. Escher von der Linth; and having satisfied myself that all the
facts which I saw north of the Alps were in accordance with M. Guyot's
views, I crossed to the Italian side of the great chain and became
convinced that the same theory was equally applicable to the ancient
moraines of the plains of the Po.
M. Escher pointed out to me at Trogen in Appenzel on the left bank of
the Rhine fragments of a rock of a peculiar mineralogical character,
commonly called the granite of Pontelyas, the natural position of which
is well known near Trons, 100 miles from Trogen, on the left bank of the
Rhine about 30 miles from the source of that river. All the blocks of
this peculiar granite keep to the left bank, even where the valley turns
almost at right angles to its former course near Mayenfeld below Chur,
making a sharp bend resembling that of the valley of the Rhone at
Martigny. The granite blocks, where they are traced to the low country,
still keep to the left side of the Lake of Constance. That they should
not have crossed over to the opposite river-bank below Chur is quite
inexplicable if, rejecting the aid of land-ice, we appeal to floating
ice as the transporting power.
In M. Morlot's map already cited we behold between the areas occupied
by the glacial drift of the Rhine and Rhone three smaller yet not
inconsiderable spaces distinguished by distinct colours, indicating
the peculiar detritus brought down by the three great rivers, the Aar,
Reuss, and Limmat. The ancient glacier of the first of these, the
Aar, has traversed the lakes of Brienz and Thun and has borne angular,
polished, and striated blocks of limestone and other rocks as far as
Berne and somewhat below that city. The Reuss has also stamped the
lithological character of its own mountainous region upon the lower
part of its hydrographical basin by covering it with its peculiar Alpine
drift. In like manner the old extinct glacier of the Limmat during its
gradual retreat has left monuments of its course in the Lake of Zurich
in the shape of terminal moraines, one of which has almost divided that
great sheet of water into two lakes.
The ice-work done by the extinct glaciers, as contrasted with that
performed by their dwarfed representatives of the present day, is in due
proportion to the relative volume of the supposed glaciers, whether we
measure them by the distances to which they have carried erratic
blocks or the areas which they have strewed over with drift or the hard
surfaces of rock and number of boulders which they have polished and
striated. Instead of a length of 5, 10, or 20 miles and a thickness of
200, 300, or at the utmost 800 feet, those giants of the olden time must
have been from 50 to 150 miles long and between 1000 and 3000 feet deep.
In like manner the glaciation although identical in kind is on so small
a scale in the existing Alpine glaciers as at first sight to disappoint
a Swedish, Scotch, Welsh, or North American geologist. When I visited
the terminal moraine of the glacier of the Rhone in 1859 and tried
to estimate the number of angular or rounded pebbles and blocks which
exhibited glacial polishing or scratches as compared to those bearing no
such markings, I found that several thousand had to be reckoned before I
arrived at the first which was so striated or polished as to differ
from the stones of an ordinary torrent-bed. Even in the moraines of the
glaciers of Zermatt, Viesch, and others, in which fragments of limestone
and serpentine are abundant (rocks which most readily receive and most
faithfully retain the signs of glaciation), I found, for one which
displayed such indications, several hundreds entirely free from them.
Of the most opposite character were the results obtained by me from a
similar scrutiny of the boulders and pebbles of the terminal moraine
of one of the old extinct glaciers, namely, that of the Rhone in
the suburbs of Soleure. Thus at the point K in the map, Figure 42, I
observed a mass of unstratified clay or mud, through which a variety of
angular and rubbed stones were scattered and a marked proportion of the
whole were polished and scratched and the clay rendered so compact, as
if by the incumbent pressure of a great mass of ice, that it has been
found necessary to blow it up with gunpowder in making railway cuttings
through part of it. A limestone of the age of our Portland stone
on which this old moraine rests, has its surface polished like a
looking-glass, displaying beautiful sections of fossil shells of the
genera Nerinaea and Pteroceras, while occasionally, besides finer
striae, there are deep rectilinear grooves, agreeing in direction with
the course in which the extinct glacier would have moved according to
the theory of M. Guyot, before explained.
EXTINCT GLACIERS OF THE ITALIAN SIDE OF THE ALPS.
[Illustration: Figure 43. Map Of The Moraines Of Extinct Glaciers]
(FIGURE 43. MAP OF THE MORAINES OF EXTINCT GLACIERS
EXTENDING FROM THE ALPS INTO THE PLAINS OF THE PO NEAR TURIN.
From Map of the ancient Glaciers of the Italian side of the Alps
by Signor Gabriel de Mortillet.
A. Crest or watershed of the Alps.
B. Snow-covered Alpine summits which fed the ancient glaciers.
C. Moraines of ancient or extinct glaciers.)
To select another example from the opposite or southern side of the
Alps. It will be seen in the elaborate map recently executed by Signor
Gabriel de Mortillet of the ancient glaciers of the Italian flank of
the Alps that the old moraines descend in narrow strips from the
snow-covered ridges through the principal valleys to the great basin of
the Po, on reaching which they expand and cover large circular or oval
areas. Each of these groups of detritus is observed (see map, Figure 43)
to contain exclusively the wreck of such rocks as occur in situ on
the Alpine heights of the hydrographical basins to which the moraines
respectively belong.
I had an opportunity of verifying this fact, in company with Signor
Gastaldi as my guide, by examining the erratics and boulder formation
between Susa and Turin, on the banks of the Dora Riparia, which brings
down the waters from Mont Cenis and from the Alps south-west of it. I
there observed striated fragments of dolomite and gypsum, which had come
down from Mont Cenis and had travelled as far as Avigliana; also masses
of serpentine brought from less remote points, some of them apparently
exceeding in dimensions the largest erratics of Switzerland. I
afterwards visited, in company with Signori Gastaldi and Michelotti,
a still grander display of the work of a colossal glacier of the olden
time, 20 miles north-east of Turin, the moraine of which descended
from the two highest of the Alps, Mont Blanc and Monte Rosa, and after
passing through the valley of Aosta, issued from a narrow defile above
Ivrea (see map, Figure 43). From this vomitory the old glacier poured
into the plains of the Po that wonderful accumulation of mud, gravel,
boulders, and large erratics, which extend for 15 miles from above
Ivrea to below Caluso and which when seen in profile from Turin have the
aspect of a chain of hills. In many countries, indeed, they might rank
as an important range of hills, for where they join the mountains they
are more than 1500 feet high, and retain more than half that height for
a great part of their course, rising very abruptly from the plain, often
with a slope of from 20 to 30 degrees. This glacial drift reposes near
the mountains on ancient metamorphic rocks and farther from them on
marine Pliocene strata. Portions of the ridges of till and stratified
matter have been cut up into mounds and hillocks by the action of the
river, the Dora Baltea, and there are numerous lakes, so that the entire
moraine much resembles, except in its greater height and width, the line
of glacial drift of Perthshire and Forfarshire before described. Its
complicated structure can only be explained by supposing that the
ancient glacier advanced and retreated several times and left large
lateral moraines, the more modern mounds within the limits of the older
ones, and masses of till thrown down upon the rearranged and stratified
materials of the first set of moraines. Such appearances accord well
with the hypothesis of the successive phases of glacial action in
Switzerland, to which I shall presently advert.
CONTORTED STRATA OF GLACIAL DRIFT SOUTH OF IVREA.
At Mazze near Caluso (see Figure 43), the southern extremity of this
great moraine has recently been cut through in making a tunnel for the
railway which runs from Turin to Ivrea. In the fine section thus exposed
Signor Gastaldi and I had an opportunity of observing the internal
structure of the glacial formation. In close juxtaposition to a
great mass of till with striated boulders, we saw stratified beds of
alternating gravel, sand, and loam, which were so sharply bent that many
of them had been twice pierced through in the same vertical cutting.
Whether they had been thus folded by the mechanical power of an
advancing glacier, which had pushed before it a heap of stratified
matter, as the glacier of Zermatt has been sometimes known to shove
forward blocks of stone through the walls of houses, or whether the
melting of masses of ice, once interstratified with sand and gravel, had
given rise to flexures in the manner before suggested; it is at least
satisfactory to have detected this new proof of a close connection
between ice-action and contorted stratification, such as has been
described as so common in the Norfolk cliffs and which is also very
often seen in Scotland and North America, where stratified gravel
overlies till. I have little doubt that if the marine Pliocene strata
which underlie a great part of the moraine below Ivrea were exposed to
view in a vertical section, those fundamental strata would be found not
to participate in the least degree in the plications of the sands and
gravels of the overlying glacial drift.
To return to the marks of glaciation: in the moraine at Mazze there are
many large blocks of protogine and large and small ones of limestone
and serpentine which have been brought down from Monte Rosa, through
the gorge of Ivrea, after having travelled for a distance of 50
miles. Confining my attention to a part of the moraine where pieces of
limestone and serpentine were very numerous, I found that no less than
one-third of the whole number bore unequivocal signs of glacial action;
a state of things which seems to bear some relation to the vast volume
and pressure of the ice which once constituted the extinct glacier and
to the distance which the stones had travelled. When I separated the
pebbles of quartz, which were never striated, and those of granite,
mica-schist, and diorite, which do not often exhibit glacial markings,
and confined my attention to the serpentine alone I found no less than
nineteen in twenty of the whole number polished and scratched; whereas
in the terminal moraines of some modern glaciers, where the materials
have travelled not more than 10 or 15, instead of 100 miles, scarce
one in twenty even of the serpentine pebbles exhibit glacial polish and
striation.
THEORY OF THE ORIGIN OF LAKE-BASINS BY THE EROSIVE ACTION OF GLACIERS,
CONSIDERED.
Geologists are all agreed that the last series of movements to which the
Alps owe their present form and internal structure occurred after the
deposition of the Miocene strata; and it has been usual to refer the
origin of the numerous lake-basins of Alpine and sub-Alpine regions both
in Switzerland and Northern Italy to the same movements; for it
seemed not unnatural to suppose, that forces capable of modifying the
configuration of the greatest European chain, by uplifting some of its
component Tertiary strata (those of marine origin of the Miocene period)
several thousand feet above their former level, after throwing them
into vertical and contorted positions, must also have given rise to many
superficial inequalities, in some of which large bodies of water would
collect. M. Desor, in a memoir on the Swiss and Italian lakes, suggested
that they may have escaped being obliterated by sedimentary deposition
by having been filled with ice during the whole of the glacial period.
Subsequently to the retreat of the great glaciers we know that the
lake-basins have been to a certain extent encroached upon and turned
into land by river deltas; one of which, that of the Rhone at the head
of the Lake of Geneva, is no less than 12 miles long and several miles
broad, besides which there are many torrents on the borders of the same
lake, forming smaller deltas.
M. Gabriel de Mortillet after a careful study of the glacial formations
of the Alps agreed with his predecessors that the great lakes had
existed before the glacial period, but came to the opinion in 1859
that they had all been first filled up with alluvial matter and then
re-excavated by the action of ice, which during the epoch of intense
cold had by its weight and force of propulsion scooped out the loose
and incoherent alluvial strata, even where they had accumulated to a
thickness of 2000 feet. Besides this erosion, the ice had carried the
whole mass of mud and stones up the inclined planes, from the central
depths to the lower outlets of the lakes and sometimes far beyond them.
As some of these rock-basins are 500, others more than 2000 feet deep,
having their bottoms in some cases 500, in others 1000 feet below the
level of the sea, and having areas from 20 to 50 miles in length and
from 4 to 12 in breadth, we may well be startled at the boldness of this
hypothesis.
The following are the facts and train of reasoning which induced M. de
Mortillet to embrace these views. At the lower ends of the great
Italian lakes, such as Maggiore, Como, Garda, and others, there are vast
moraines which are proved by their contents to have come from the upper
Alpine valleys above the lakes. Such moraines often repose on an older
stratified alluvium, made up of rounded and worn pebbles of precisely
the same rocks as those forming the moraines, but not derived from them,
being small in size, never angular, polished, or striated, and the whole
having evidently come from a great distance. These older alluvial strata
must, according to M. de Mortillet, be of pre-glacial date and could
not have been carried past the sites of the lakes, unless each basin had
previously been filled and levelled up with mud, sand, and gravel,
so that the river channel was continuous from the upper to the lower
extremity of each basin.
Professor Ramsay, after acquiring an intimate knowledge of the glacial
phenomena of the British Isles, had taught many years before that small
tarns and shallow rock-basins such as we see in many mountain regions
owe their origin to glaciers which erode the softer rocks, leaving
the harder ones standing out in relief and comparatively unabraded.
Following up this idea after he had visited Switzerland and without
any communication with M. de Mortillet or cognisance of his views, he
suggested in 1859 that the lake-basins were not of pre-glacial date, but
had been scooped out by ice during the glacial period, the excavation
having for the most part been effected in Miocene sandstone,
provincially called, on account of its softness, "molasse." By this
theory he dispensed with the necessity of filling up pre-existing
cavities with stratified alluvium, in the manner proposed by M. de
Mortillet.
I will now explain to what extent I agree with, and on what points I
feel compelled to differ from the two distinguished geologists above
cited. First. It is no doubt true, as Professor Ramsay remarks, that
heavy masses of ice, creeping for ages over a surface of dry land
(whether this comprise hills, plateaus, and valleys, as in the case
of Greenland, before described, or be confined to the bottoms of great
valleys, as now in the higher Alps), must often by their grinding
action produce depressions, in consequence of the different degrees
of resistance offered by rocks of unequal hardness. Thus, for example,
where quartzose beds of mica-schist alternate with clay-slate, or where
trap-dykes, often causing waterfalls in the courses of torrents,
cut through sandstone or slate--these and innumerable other common
associations of dissimilar stony compounds must give rise to a very
unequal amount of erosion and consequently to lake-basins on a small
scale. But the larger the size of any lake, the more certain it will be
to contain within it rocks of every degree of hardness, toughness, and
softness; and if we find a gradual deepening from the head towards the
central parts and a shallowing again from the middle to the lower end,
as in several of the great Swiss and Italian lakes, which are 30 or
40 miles in length, we require a power capable of acting with a
considerable degree of uniformity on these masses of varying powers of
resistance.
Secondly. Several of the great lakes are by no means in the line of
direction which they ought to have taken had they been scooped out by
the pressure and onward movement of the extinct glaciers. The Lake
of Geneva, for instance, had it been the work of ice, would have been
prolonged from the termination of the upper valley of the Rhone towards
the Jura, in the direction from F to G of the map, Figure 42, instead of
running from F to I.
Thirdly. It has been ascertained experimentally, that in a glacier, as
in a river, the rate of motion is accelerated or lessened, according to
the greater or less slope of the ground; also, that the lower strata of
ice, like those of water, move more slowly than those above them. In the
Lago Maggiore, which is more than 2600 feet deep (797 metres), the ice,
says Professor Ramsay, had to descend a slope of about 3 degrees for
the first 25 miles, and then to ASCEND for the last 12 miles (from the
deepest part towards the outlet) at an angle of 5 degrees. It is for
those who are conversant with the dynamics of glacier motion to divine
whether in such a case the discharge of ice would not be entirely
effected by the superior and faster moving strata, and whether the
lowest would not be motionless or nearly so, and would therefore exert
very little, if any, friction on the bottom.
Fourthly. But the gravest objection to the hypothesis of glacial erosion
on so stupendous a scale is afforded by the entire absence of lakes of
the first magnitude in several areas where they ought to exist if the
enormous glaciers which once occupied those spaces had possessed the
deep excavating power ascribed to them. Thus in the area laid down on
the map, Figure 43, or that covered by the ancient moraine of the Dora
Baltea, we see the monuments of a colossal glacier derived from Mont
Blanc and Monte Rosa, which descended from points nearly 100 miles
distant, and then emerging from the narrow gorge above Ivrea deployed
upon the plains of the Po, advancing over a floor of marine Pliocene
strata of no greater solidity than the Miocene sandstone and
conglomerate in which the lake-basins of Geneva, Zurich, and some others
are situated. Why did this glacier fail to scoop out a deep and wide
basin rivalling in size the lakes of Maggiore or Como, instead of merely
giving rise to a few ponds above Ivrea, which may have been due to ice
action? There is one lake, it is true--that of Candia, near the southern
extremity of the moraine--which is larger; but even this, as will be
seen by the map, is quite of subordinate importance, and whether it is
situated in a rock basin or is simply caused by a dam of moraine matter
has not yet been fully made out.
There ought also to have been another great lake, according to the
theory under consideration, in the space now occupied by the moraine of
the Dora Riparia, between Susa and Turin (see map, Figure 43). Signor
Gastaldi has shown that all the ponds in that area consist exclusively
of what M. de Mortillet has denominated morainic lakes, i.e. caused by
barriers of glacier-mud and stones.
Fifthly. In proof of the great lakes having had no existence before the
glacial period, Professor Ramsay observes that we do not find in the
Alps any freshwater strata of an age intermediate between "the close of
the Miocenic and the commencement of the glacial epoch."*
(* "Quarterly Journal of the Geological Society" volume 18
1862.)
But although such formations are scarce, they are by no means wholly
wanting; and if it can be shown that any one of the principal lakes,
that of Zurich for example, existed prior to the glacial era it will
follow that in the Alps the erosive power of ice was not required to
produce lake-basins on a large scale. The deposits alluded to on the
borders of the Lake of Zurich are those of Utznach and Durnten, situated
each about 350 feet above the present level of the lake and containing
valuable beds of lignite.
The first of them, that of Utznach, is a delta formed at the head of
the ancient and once more extensive lake. The argillaceous and
lignite-bearing strata, more than 100 feet in thickness, rest
unconformably on highly inclined and sometimes vertical Miocene molasse.
These clays are covered conformably by stratified sand and gravel 60
feet thick, partly consolidated, in which the pebbles are of rocks
belonging to the upper valleys of the Limmat and its tributaries, all
of them small and not glacially striated and wholly without admixture
of large angular stones. On the top of all repose very large erratic
blocks, affording clear evidence that the colossal glacier which once
filled the valley of the Limmat covered the old littoral deposit. The
great age of the lignite is partly indicated by the bones of Elephas
antiquus found in it.
I visited Utznach in company with M. Escher von der Linth in 1857, and
during the same year examined the lignite of Durnten, many miles farther
down on the right bank of the lake, in company with Professor Heer and
M. Marcou. The beds there are of the same age and within a few feet of
the same height above the level of the lake. They might easily have
been overlooked or confounded with the general glacial drift of the
neighbourhood, had not the bed of lignite, which is from 5 to 12 feet
thick, been worked for fuel, during which operation many organic remains
came to light. Among these are the teeth of Elephas antiquus, determined
by Dr. Falconer, and Rhinoceros leptorhinus? (R. megarhinus, Christol),
the wild bull and red deer (Bos primigenius, Boj., and Cervus elaphus,
L.), the last two determined by Professor Rutimeyer. In the same beds I
found many freshwater shells of the genera Paludina, Limnaea, etc., all
of living species. The plants named by Professor Heer are also Recent
and agree singularly with those of the Cromer buried forest, before
described.
Among them are the Scotch and spruce firs, Pinus sylvestris and Pinus
abies, and the buckbean, or Menyanthes trifoliata, etc., besides the
common birch and other European plants.
Overlying this lignite are first, as at Utznach, stratified gravel not
of glacial origin, about 30 feet thick; and secondly, highest of all,
huge angular erratic blocks clearly indicating the presence of a great
glacier posterior in date to all the organic remains above enumerated.
If any one of the existing Swiss lakes were now lowered by deepening its
outlet, or by raising the higher portion of it relatively to the lower,
we should see similar deltas of comparatively modern date exposed to
view, some of them with embedded trunks of pines of the same species
drifted down during freshets. Such deposits would be most frequent at
the upper ends of the lakes, but a few would occur on either bank not
far from the shore where torrents once entered, agreeing in geographical
position with the lignite formations of Utznach and Durnten.
There are other freshwater formations with lignite, besides those on
the Lake of Zurich, as those of Wetzikon near the Pfaffikon Lake, of
Kaltbrunnen, of Buchberg, and that of Morschweil between St. Gall and
Rorschach, but none probably older than the Durnten beds. Like the
buried forest of Cromer they are all pre-glacial, yet they by no means
represent the older nor even the newer Pliocene period, but rather the
beginning of the Pleistocene. It is therefore true, as Professor Ramsay
remarks, that, as yet, no strata "of the age of the English Crag"
have been detected in any Alpine valley. In other words, there are no
freshwater formations yet known corresponding in date to the Pliocene
beds of the upper Val d'Arno, above Florence--a fact from which we may
infer (though with diffidence, as the inference is based on negative
evidence), that, although the great Alpine valleys were eroded in
Pliocene times, the lake-basins were, nevertheless, of Pleistocene
date--some of them formed before, others during, the glacial epoch.
Sixthly. In what manner then did the great lake-basins originate if they
were not hollowed out by ice? My answer is, they are all due to unequal
movements of upheaval and subsidence. We have already seen that the
buried forest of Cromer, which by its organic contents seems clearly to
be of the same age as the lignite of Durnten, was pre-glacial and that
it has undergone a great oscillation of level (about 500 feet in both
directions) since its origin, having first sunk to that extent below
the sea and then been raised up again to the sea-level. In the countless
Post-Miocene ages which preceded the glacial period there was ample time
for the slow erosion by water of all the principal hydrographical basins
of the Alps, and the sites of all the great lakes coincide, as Professor
Ramsay truly says, with these great lines of drainage. The lake-cavities
do not lie in synclinal troughs, following the strike and foldings of
the strata, but often, as the same geologist remarks, cross them at high
angles; nor are they due to rents or gaping fissures, although these,
with other accidents connected with the disturbing movements of the
Alps, may sometimes have determined originally the direction of the
valleys. The conformity of the lake-basins to the principal watercourses
is explicable if we assume them to have resulted from inequalities in
the upward and downward movements of the whole country in Pleistocene
times, after the valleys were eroded.
We know that in Sweden the rate of the rise of the land is far from
uniform, being only a few inches in a century near Stockholm, while
north of it and beyond Gefle it amounts to as many feet in the same
number of years. Let us suppose with Charpentier that the Alps gained
in height several thousand feet at the time when the intense cold of
the glacial period was coming on. This gradual rise would be an era of
aqueous erosion and of the deepening, widening, and lengthening of the
valleys. It is very improbable that the elevation would be everywhere
identical in quantity, but if it was never in excess in the outskirts as
compared to the central region or crest of the chain, it would not give
rise to lakes. When, however, the period of upheaval was followed by
one of gradual subsidence, the movement not being everywhere strictly
uniform, lake-basins would be formed wherever the rate of depression was
in excess in the upper country. Let the region, for example, near the
head waters of the great rivers sink at the rate of from 4 to 6 feet
per century, while only half as much subsidence occurs towards the
circumference of the mountains--the rate diminishing about an inch per
mile in a distance, say of 40 miles--this might convert many of the
largest and deepest valleys at their lower ends into lakes.
We have no certainty that such movements may not now be in progress in
the Alps; for if they are as slow as we have assumed, they would be as
insensible to the inhabitants as is the upheaval of Scandinavia or the
subsidence of Greenland to the Swedes and Danes who dwell there. They
only know of the progress of such geographical revolutions because a
slight change of level becomes manifest on the margin of the sea. The
lines of elevation or depression above supposed might leave no clear
geological traces of their action on the high ridges and table-lands
separating the valleys of the principal rivers; it is only when
they cross such valleys that the disturbance caused in the course of
thousands of years in the drainage becomes apparent. If there were no
ice, the sinking of the land might not give rise to lakes. To accomplish
this in the absence of ice, it is necessary that the rate of depression
should be sufficiently fast to make it impossible for the depositing
power of the river to keep pace with it, or in other words to fill
up the incipient cavity as fast as it begins to form. Such levelling
operations once complete, the running water, aided by sand and pebbles,
will gradually cut a gorge through the newly raised rock so as to
prevent it from forming a barrier. But if a great glacier fill the
lower part of the valley all the conditions of the problem are altered.
Instead of the mud, sand, and stones drifted down from the higher
regions being left behind in the incipient basin, they all travel
onwards in the shape of moraines on the top of the ice, passing over
and beyond the new depression, so that when at the end of fifty or
a thousand centuries the glacier melts, a large and deep basin
representing the difference in the movement of two adjoining mountain
areas--namely, the central and the circumferential--is for the first
time rendered visible.
By adopting this hypothesis, we concede that there is an intimate
connection between the glacial period and a predominance of lakes, in
producing which the action of ice is threefold; first, by its direct
power in scooping out shallow basins where the rocks are of unequal
hardness; an operation which can by no means be confined to the land,
for it must extend to below the level of high water a thousand feet
and more in such fjords as have been described as filled with ice in
Greenland.
Secondly. The ice will act indirectly by preventing cavities caused by
inequalities of subsidence or elevation from becoming the receptacles
first of water and then of sediment, by which the cavities would be
levelled up and the lakes obliterated.
Thirdly. The ice is also an indirect cause of lakes, by heaping up
mounds of moraine matter and thus giving rise to ponds and even to
sheets of water several miles in diameter.
The comparative scarcity, therefore, of lakes of Pleistocene date
in tropical countries, and very generally south of the fortieth and
fiftieth parallels of latitude, may be accounted for by the absence of
glacial action in such regions.
POST-GLACIAL LAKE-DWELLING IN THE NORTH OF ITALY.
We learn from M. de Mortillet that in the peat which has filled up one
of the "morainic lakes" formed by the ancient glacier of the Ticino, M.
Moro has discovered at Mercurago the piles of a lake-dwelling like those
of Switzerland, together with various utensils and a canoe hollowed out
of the trunk of a tree. From this fact we learn that south of the
Alps as well as north of them a primitive people having similar habits
flourished after the retreat of the great glaciers.
SUCCESSIVE PHASES OF GLACIAL ACTION IN THE ALPS, AND THEIR RELATION TO
THE HUMAN PERIOD [34].
According to the geological observations of M. Morlot, the following
successive phases in the development of ice-action in the Alps are
plainly recognisable:--
First. There was a period when the ice was in its greatest excess, when
the glacier of the Rhone not only reached the Jura, but climbed to the
height of 2015 feet above the Lake of Neufchatel, and 3450 feet above
the sea, at which time the Alpine ice actually entered the French
territory at some points, penetrating by certain gorges, as through the
defile of the Fort de l'Ecluse, among others.
Second. To this succeeded a prolonged retreat of the great glaciers,
when they evacuated not only the Jura and the low country between that
chain and the Alps, but retired some way back into the Alpine valleys.
M. Morlot supposes their diminution in volume to have accompanied a
general subsidence of the country to the extent of at least 1000 feet.
The geological formations of the second period consist of stratified
masses of sand and gravel, called the "ancient alluvium" by MM. Necker
and Favre, corresponding to the "older or lower diluvium" of some
writers. Their origin is evidently due to the action of rivers, swollen
by the melting of ice, by which the materials of parts of the old
moraines were rearranged and stratified and left usually at considerable
heights above the level of the present valley plains.
Third. The glaciers again advanced and became of gigantic dimensions,
though they fell far short of those of the first period. That of the
Rhone, for example, did not again reach the Jura, though it filled the
Lake of Geneva and formed enormous moraines on its borders and in many
parts of the valley between the Alps and Jura.
Fourth. A second retreat of the glaciers took place when they gradually
shrank nearly into their present limits, accompanied by another
accumulation of stratified gravels which form in many places a series of
terraces above the level of the alluvial plains of the existing rivers.
In the gorge of the Dranse, near Thonon, M. Morlot discovered no less
than three of these glacial formations in direct superposition, namely,
at the bottom of the section, a mass of compact till or boulder-clay
(Number 1) 12 feet thick, including striated boulders of Alpine
limestone, and covered by regularly stratified ancient alluvium (Number
2) 150 feet thick, made up of rounded pebbles in horizontal beds.
This mass is in its turn overlaid by a second formation (Number 3) of
unstratified boulder clay, with erratic blocks and striated pebbles,
which constituted the left lateral moraine of the great glacier of the
Rhone when it advanced for the second time to the Lake of Geneva. At a
short distance from the above section terraces (Number 4) composed of
stratified alluvium are seen at the heights of 20, 50, 100, and 150 feet
above the Lake of Geneva, which by their position can be shown to be
posterior in date to the upper boulder-clay and therefore belong to the
fourth period, or that of the last retreat of the great glaciers. In
the deposits of this fourth period the remains of the mammoth have
been discovered, as at Morges, for example, on the Lake of Geneva. The
conical delta of the Tiniere, mentioned in Chapter 2 as containing at
different depths monuments of the Roman as well as of the antecedent
bronze and stone ages, is the work of alluvial deposition going on when
the terrace of 50 feet was in progress. This modern delta is supposed
by M. Morlot to have required 10,000 years for its accumulation. At the
height of 150 feet above the lake, following up the course of the same
torrent, we come to a more ancient delta, about ten times as large,
which is therefore supposed to be the monument of about ten times as
many centuries, or 100,000 years, all referable to the fourth period
mentioned in the preceding page, or that which followed the last retreat
of the great glaciers.*
(* Morlot, Terrain quaternaire du Bassin de Leman "Bulletin
de la Societe Vaudoise des Sciences Naturelles" Number 44.)
If the lower flattened cone of Tiniere be referred in great part to
the age of the oldest lake-dwellings, the higher one might perhaps
correspond with the Pleistocene period of St. Acheul, or the era when
Man and the Elephas primigenius flourished together; but no human
remains or works of art have as yet been found in deposits of this
age or in any alluvium containing the bones of extinct mammalia in
Switzerland.
Upon the whole, it is impossible not to be struck with an apparent
correspondence in the succession of events of the glacial period of
Switzerland and that of the British Isles before described. The time
of the first Alpine glaciers of colossal dimensions, when that chain
perhaps was several thousand feet higher than now, may have agreed with
the first continental period when Scotland was invested with a universal
crust of ice. The retreat of the first Alpine glaciers, caused partly by
a lowering of that chain, may have been synchronous with the period of
great submergence and floating ice in England. The second advance of the
glaciers may have coincided in date with the re-elevation of the Alps,
as well as of the Scotch and Welsh mountains; and lastly, the final
retreat of the Swiss and Italian glaciers may have taken place when Man
and the extinct mammalia were colonising the north-west of Europe and
beginning to inhabit areas which had formed the bed of the glacial sea
during the era of chief submergence.
But it must be confessed that in the present state of our knowledge
these attempts to compare the chronological relations of the periods
of upheaval and subsidence of areas so widely separated as are the
mountains of Scandinavia, the British Isles, and the Alps, or the times
of the advance and retreat of glaciers in those several regions and
the greater or less intensity of cold, must be looked upon as very
conjectural.
We may presume with more confidence that when the Alps were highest
and the Alpine glaciers most developed, filling all the great lakes of
northern Italy and loading the plains of Piedmont and Lombardy with
ice, the waters of the Mediterranean were chilled and of a lower average
temperature than now. Such a period of refrigeration is required by the
conchologist to account for the prevalence of northern shells in the
Sicilian seas about the close of the Pliocene or commencement of the
Pleistocene period. For such shells as Cyprina islandica, Panopoea
norvegica (= P. bivonae, Philippi), Leda pygmaea, Munst, and some
others, enumerated among the fossils of the latest Tertiary formations
of Sicily by Philippi and Edward Forbes, point unequivocally to a former
more severe climate. Dr. Hooker also in his late journey to Syria (in
the autumn of 1860) found the moraines of extinct glaciers, on which the
whole of the ancient cedars of Lebanon grow, to descend 4000 feet below
the summit of that chain. The temperature of Syria is now so much milder
that there is no longer perpetual snow even on the summit of Lebanon,
the height of which was ascertained to be 10,200 feet above the
Mediterranean.*
(* Hooker, "Natural History Review" Number 5 January 1862
page 11.)
Such monuments of a cold climate in latitudes so far south as Syria and
the north of Sicily, between 33 and 38 degrees north, may be confidently
referred to an early part of the glacial period, or to times long
anterior to those of Man and the extinct mammalia of Abbeville and
Amiens.