the widespread loss of those regions is readily accounted for. The
occurrence of irregular sheets and shreds of gravel and loam at
heights of more than a hundred feet above a valley-bottom offers no
difficulty--it is in fact precisely the kind of phenomenon we might
have expected. We are therefore not required to go out of our way to
dream about impossible volcanic action, or to call upon the winds
of heaven to help us, or upon the waters of the Deluge to float us
out of our difficulties. But while I believe the views I have now
advocated sufficiently account for the appearances presented by the
ancient valley-gravels and loams of central Europe, there are two very
considerable areas of löss which require some further explanation. The
first of these is that broad belt of löss which extends from west to
east across the plains of northern Germany, and the northern boundary
of which coincides with the limits reached by the last great ice-sheet,
from which it spreads south to the foot-hills of the Harz, and other
mountains of middle Europe. Here we have a sheet of löss which bears
no apparent relation to the valley-systems of the region in which it
occurs. But the fact of its northern boundary being coincident with the
terminal front of the last great northern ice-sheet at once suggests
its origin. It is evident that this ice-sheet must have blocked the
rivers flowing north, and dammed back their waters.[O] A wide sheet
of muddy water must therefore have extended east and west over the
very area which is now covered by the belt of löss in question. This
temporary lake would doubtless be subject to great alternations of
level--a portion draining away perhaps under the ice-sheet--but the
water would for the most part make its way westward, and eventually
escape into the English Channel. From the waters of this great lake,
fed by many large glacial rivers, abundant precipitation of loam and
silt must have taken place.
[O] The late Mr. Belt, as is well known, was of opinion that all the
rivers flowing north in Europe and Asia were dammed back by a great
Polar glacier, and that all the low-tracts in the northern portions of
the two continents were thus covered by wide inland seas of freshwater.
As I do not believe that such a Polar ice-cap existed during the
Glacial period, I cannot agree with Mr. Belt that the alluvial plains
of northern Siberia mark the sites of ice-dammed lakes.
The second and by far the most extensive sheet of löss in Europe is
the so-called "black earth," or "tchernozem," with which such enormous
tracts in southern Russia are covered. This widespread löss--for such
it really is--I have elsewhere tried to show consists of the flood-loam
and inundation-muds laid down by the water escaping along the margin
of the northern ice-sheet, which discharged its drainage in the
direction of the Black Sea, its black colour being due to the grinding
down and pulverising of the black Jurassic shales which extend over
such wide regions in middle Russia.
IV.
_The Extent of Glaciation in North America._
The various phenomena of glaciation which go to prove that a great
ice-sheet formerly covered a wide region in northern Europe are
developed on a still more extensive scale in North America. Smoothed
and striated rock-surfaces, crushed and dislocated rock-masses, and
enormous accumulations of morainic débris and fluvio-glacial detritus,
all combine to tell the same tale. The morainic accumulations of North
America have been distributed upon the same principles as the similar
deposits of our own Continent. Boulder-clay of precisely the same
character as that of Scotland and Scandinavia, of Switzerland and north
Italy, covers vast tracts in the low-grounds of the British Possessions
and the northern States of the Union, where it forms enormous sheets,
varying in thickness from 30 or 50 up to 100 feet or more. In the rough
Laurentian high-lands, however, it is more sparingly developed, and
the same is the case in the hilly regions of New England. In short,
it thickens out upon the low-grounds, and thins off upon the steeper
slopes, while it attains its greatest thickness and forms the most
continuous sheets in the country that lies south of the great lakes.
The southern limits of this deposit form a kind of rude semi-circle.
From New York the boundary-line has been followed north-west through
New Jersey and Pennsylvania to beyond the forty-second parallel,
after which it turns to the south-west, passing down through Ohio to
Cincinnati (39°); then, striking west and south-west through Indiana,
it traverses the southern portion of Illinois. Its course after
it reaches the valley of the Missouri has been only approximately
determined, but it turns at last rather abruptly to the north-west,
sweeping away in that direction through Kansas, Nebraska, Dakota, and
Montana.
The general course followed by the ice-sheet underneath which this
boulder-clay was formed has been well ascertained, partly by the
evidence of the clay and its contents, and partly by that of _roches
moutonnées_ and striated rocks. The observations of geologists in
Canada and the States leave it in no doubt that an enormous sheet
of ice flowed south over all the tracts which are now covered with
boulder-clay. During a recent visit to Canada and the States, I had
opportunities of examining the glacial deposits at various points
over a somewhat extensive area, and everywhere I found the exact
counterparts of our own accumulations. In Minnesota, Wisconsin, Iowa,
Illinois, Indiana, and Ohio, and again in New York, Connecticut, and
Massachusetts, and the low-grounds of Canada, I recognised boulder-clay
of precisely the same character as that with which we are familiar at
home. The glacial phenomena of the Hudson valley and of the lower part
of the Connecticut River were especially interesting. In those regions
the evidence of a southward flow of the ice is most conspicuous, and
the phenomena, down to the smallest details, exactly recalled those of
many parts of Europe. Professor Dana, under whose guidance I visited
the Connecticut valley, showed me, at a considerable height upon the
valley-slope, an ancient water-course, charged with gravel and shingle,
which could not possibly have been laid down under present conditions.
It was, in fact, a sub-glacial water-course, and resembled the similar
water-courses which are associated with boulder-clay in our own country.
If I met with only familiar glacial phenomena in the low-lying tracts
traversed by me, I certainly saw nothing strange or abnormal in the
hillier tracts. In passing over the dreary regions between the valley
of the Red River and Lake Superior I was constantly reminded of the
bleak tracts of Archæan gneiss in the north-west of Scotland, and
of the similar rough broken uplands in many parts of Scandinavia and
Finland. The whole of that wild land is _moutonnée_. Rough tors and
crags are smoothed off, while boulder-clay nestles on the lee-side. In
the hollows between the _roches moutonnées_ are straggling lakes and
pools and bogs innumerable. Frequently, too, one comes upon rounded
cones and smooth banks of morainic gravel and sand, and heaps of coarse
shingle and boulders, while erratics in thousands are scattered over
the whole district. If you wish to have a fair notion of the geological
aspect of the region I refer to, you will find samples of it in many
parts of the Outer Hebrides and western Ross-shire and Sutherland.
Cover those latter districts with scraggy pines, and their resemblance
to the uplands of Canada will be complete.
From descriptions given by travellers it would appear that morainic
detritus--mounds and sheets of stony clay, gravel and sand, shingle,
boulders, and erratics--are more or less plentifully sprinkled over
all the British Possessions and the islands of the Arctic Archipelago;
so that we have every reason to believe that the ice-sheet which left
its moraines at New York and Cincinnati extended northwards to the
Arctic Ocean. Nor can there be much doubt that this same _mer de glace_
became confluent in the west with the great glaciers that streamed
outwards from the Rocky Mountains; while we know for a certainty that
the southern portion of Alaska, together with British Columbia and
Vancouver Island, were buried in ice that flowed outwards into the
Pacific.
Along the eastern sea-board north of New York city there is no tract
which has not been overflowed by ice. The islands in Boston Harbour are
made up for the most part of tough boulder-clay; and boulder-clay and
striated rocks occur also in Maine, New Brunswick, Nova Scotia, and
Newfoundland.
Thus we may say that the ice-covered region of North America was
bounded on the north by the Arctic, on the west by the Pacific, and on
the east by the Atlantic Oceans. The Rocky Mountains, however, divided
the great _mer de glace_ that overflowed Canada and the States from
the ice that streamed outwards to the Pacific. Measured from the base
of the Rockies to the Atlantic, the _mer de glace_ of Canada and the
States must have exceeded 2500 miles in width, and it stretched from
north to south over 40 degrees of latitude.
Outside of this vast region and the great mountain-ranges of the
far west, there are few hilly areas in the States which reach any
considerable elevation. South of the _mers de glace_ of the north and
west, no such mountain-groups as those of middle and southern Europe
occur, and consequently we do not expect to meet with many traces
of local glaciation. Nevertheless, these have been recognised in
the Alleghany Mountains, West Virginia, and in the Unaka Mountains,
between Tennessee and North Carolina. But the glaciers of those minor
hill-ranges were of course mere pigmies in comparison with the enormous
ice-streams that flowed down the valleys of the Rocky Mountains and
the Sierra Nevada. Even as far south as the Sierra Madre of Mexico
glaciers seem formerly to have existed; and Mr. Belt has described the
occurrence of what he considered to be boulder-clays at a height of
2000 to 3000 feet in Nicaragua.
I have mentioned the fact that in Europe we have, outside of the
glaciated areas, certain accumulations (such as the Gibraltar breccias)
which could only have been formed under the influence of extreme cold.
Similar accumulations occur in North Carolina, where they have been
carefully studied by Mr. W. C. Kerr. According to Mr. Kerr, these
deposits have crept down the declivities of the ground under the
influence of successive freezings and thawings; and now that attention
has been called to such phenomena, our American friends will doubtless
detect similar appearances in many other places.
The facts which I have now briefly indicated suffice to show that
during the climax of glaciation North America must have presented
very much the same appearance as Europe. Each continent had its great
northern ice-sheet, south of which local glaciers existed in hilly
districts, many of which are now far below the limits of perennial
snow. We may note, also, that in each continent the _mers de glace_
attained their greatest development over those regions which at the
present day have the largest rainfall. Following the southern limits
of glaciation in Europe, we are led at first directly east, until we
reach central Russia, when the line we follow trends rapidly away to
the north-east. The like is the case with North America. Trace the
southern boundary of the ice-sheet west of New York, and you find,
when you reach the valley of the Missouri, that it bends away to the
north-west. Now we can hardly doubt that one principal reason for
the non-appearance of the _mer de glace_ in the far east of Europe
and the far west of America was simply a diminishing snow-fall.
Those non-glaciated regions which lay north of the latitudes reached
by the ice-sheets were dry regions in glacial times for the same
reasons that they are dry still. The only differences between glacial
Europe and America were differences due to geographical position and
physical features. The glaciation of the Urals was comparatively
unimportant, because those mountains, being flanked on either side by
vast land-areas, could have had only a limited snow-fall; while the
mountain-ranges of western North America, on the other hand, being
situated near the Pacific, could not fail to be copiously supplied. For
obvious reasons, also, the North American ice-sheet greatly exceeded
that of Europe. In all other respects the conditions were similar in
both continents.
V.
_Changes of Climate in North America during the Ice Age._
American geologists are now pretty well agreed that their "interglacial
deposits"--the existence of which is not disputed--have precisely the
same meaning as the similar deposits which occur in Europe. They tell
of great climatic changes. At present, however, there is no certain
evidence in the American deposits of more than one interglacial epoch;
but the proofs of such an epoch having obtained are overwhelming.
The occurrence again and again of fossiliferous beds intercalated
between two separate and distinct sheets of boulder-clay and morainic
accumulations, leaves us in no doubt that we are dealing with precisely
the same phenomena which confront us in Europe. No mere partial
recession and re-advance of the _mer de glace_ will account for the
facts. We have seen that during the culmination of the Glacial period
the American ice-sheet overflowed Ohio, Indiana, and Illinois. Now
interglacial deposits occur as far north as the Canadian shores of
Lakes Ontario and Superior, so that all the country to the south must
have been uncovered by ice before those interglacial deposits were
laid down. But the evidence entitles us to say much more than this.
The interglacial beds of Ohio, Indiana, Illinois, and other States,
afford abundant evidence of a great forest-growth having covered the
regions vacated by the ice of the penultimate glacial epoch. The trees
of this forest-land included sycamore, beech, hickory, red-cedar, and
others; and amongst the plants were grape vines of enormous growth,
which, according to Professor Cox, "indicate perhaps the luxuriance
of a warmer climate." At all events, the climate that nourished such
a forest-growth could not have been less genial than the present. And
such being the case, we may reasonably infer that the vast regions to
the north of the lakes were no more inhospitable then than they are now.
To this genial interglacial epoch succeeded the last glacial epoch,
when a great ice-sheet once more enveloped a wide area. In the extreme
east this latest _mer de glace_ appears to have reached as far south
as that of the earlier epoch; but as we follow its terminal moraines
westward they lead us further and further away from the southern
limits attained by the preceding ice-sheet. These great terminal
moraines form an interesting study, and the general results obtained by
American observers have been very carefully put together by Professor
Chamberlin. I traversed wide regions of those moraines in Indiana,
Illinois, Wisconsin, and Minnesota, and, so far as my observations
went, I could only confirm the conclusions arrived at by Professor
Chamberlin and others. The mounds, banks, cones, and ridges are
unquestionably moraines--of enormous dimensions, no doubt, but in all
their phenomena strictly analogous to similar gravelly moraines in our
own country and the Continent. Many of the American moraines consist
almost entirely of water-worn material--sand, gravel, shingle, and
boulders, together with large angular and sub-angular erratics. These
deposits are generally stratified, and frequently show diagonal or
false-bedding. In this and other respects they exactly reproduce--but
of course on a much larger scale--our Scottish kames, and the similar
accumulations of north Germany and Finland, and the low-grounds of
Italy opposite the mouths of the great Alpine lakes. The kames of
Wisconsin again and again reminded me of the gravelly moraines that
cover the ground for many miles round the lower end of Lake Garda.
It is this gravelly and sandy aspect of the American moraines that
is most conspicuous, water-assorted materials seeming everywhere to
form their upper and outer portions. Now and again, however, a deep
cutting discloses underneath and behind such water-worn detritus a
mass of confused materials, consisting of clay, sand, gravel, shingle,
and boulders, which are angular and sub-angular, often smoothed and
striated, and of all shapes and sizes. According to Mr. Chamberlin,
this unstratified material "is indistinguishable from true till, and
is doubtless to be regarded as till pushed up into corrugations by the
mechanical action of the ice."
This grand series of moraines stretches from the peninsula of Cape Cod
across the northern States, and passes in a north-westerly direction
into the British Possessions, over which it has been followed for some
400 miles. The disposition of the moraines, forming as they do a series
of great loops, shows that the ice-sheet terminated in a number of
lobes or gigantic tongue-like processes. Nothing seen by me suggested
any marine action; on the contrary, every appearance, as I have said,
betokened the morainic origin of the mounds; and Mr. Chamberlin assured
me that their peculiar distribution was everywhere suggestive of this
origin. No one who has traversed the regions I refer to is at all
likely to agree with Sir W. Dawson's view, that the American mounds,
etc., are the shore-accumulations of an ice-laden sea.
The morainic origin of these accumulations having been demonstrated
by American geologists, we are now able to draw another parallel
between the European and American glacial deposits. We have seen that
in Europe the ice-sheet of the latest glacial epoch was by no means
so extensive as that of the preceding glacial epoch. The same was
the case in North America. Moreover, in America, just as in Europe,
the latest occupant of the land was not the sea, but glacier-ice.
In Scotland and Scandinavia the gradual disappearance of the latest
ice-sheets was marked by a partial submergence, which in the former
country did not greatly exceed 100 feet, and in the latter 700 feet.
In America, in like manner, we find traces of a similar partial
submergence. In Connecticut this did not exceed 40 or 50 feet, but
increased to some 500 feet in the St. Lawrence, and to over 1000 feet
in the Arctic regions. If there ever was during the Glacial period a
greater submergence than this in North America it must have taken place
in earlier glacial or interglacial times, but of such a submergence
no trace has yet been recognised. In this respect the American record
differs somewhat from our own, for in Britain we have evidence of a
submergence of over 1000 feet, which supervened in times immediately
preceding the latest great extension of continental ice.[P] But nowhere
in middle Europe, and nowhere in North America, in the region south
and west of the great lakes, is there any trace of a general marine
submergence. The "Palæocrystic Sea" is as idle a dream for the northern
States of America as it is for any part of Europe.
[P] See footnote, p. 173.
VI.
_The Results of Fluvio-glacial Action in North America._
The close analogies which obtain between the glacial and interglacial
deposits of Europe and North America are equally characteristic
of the fluvio-glacial accumulations of the two continents. As in
Europe, so in America we meet with considerable sheets of gravel and
shingle, sand, fine clay, and loam, which are evidently of freshwater
origin. In the gently-undulating tracts of the northern States those
deposits often spread continuously over wide regions; in the hillier
districts, however, they are most characteristic of the valleys. They
are very well represented, for example, in the Connecticut valley,
where they have been carefully studied by Professor Dana. Like the
similar deposits of our own Continent, they have been laid down by
the torrents and swollen rivers of the Glacial period. The great
range of moraines which marks the extreme limits reached by the
latest ice-sheet is generally associated with sheets of gravel and
sand, which one can see at a glance are of contemporaneous origin,
having been spread out by the water escaping from the melting ice.
Nor can one doubt that the vast sheets of löss in the Missouri and
Mississippi valleys are strictly analogous in origin, as they are in
structure and disposition, to the löss of Europe. I have spoken of the
probable existence of a glacial lake formed by the damming back of
the Rhine and other rivers by the European ice-sheet. Now, in North
America we meet with evidence of the same phenomenon. When the last
ice-sheet of that continent attained its maximum development, all the
water escaping from its margin in the north States necessarily flowed
south into the Mississippi and Missouri rivers. But in course of time
the ice melted away beyond the drainage-area of those rivers, and
disappeared from the valley of the Red River of the north, which, it
will be remembered, empties itself northward into Lake Winnipeg. When
the ice-front had retired so far it naturally impeded the drainage of
the Red River basin, and thus formed a vast glacial lake, the limits
of which have been approximately mapped out by Mr. Upham, by whom the
ancient lake has been designated Lake Agassiz. The deposits laid down
in this lake consist of finely laminated clays, etc., which resemble in
every particular the similar unfossiliferous clays so frequently found
associated with glacial accumulations in Europe. Had the drainage of
the Red River valley been south instead of north, the clays and loams
of the far north-west would not have been arrested and spread out where
they now are, and Manitoba would have been covered for the most part
with loose shingle, gravel, and sand.
Thus the final disappearance of the American ice-sheet was marked
by the formation not only of moraines, but of flood-gravels and
torrential- and inundation-deposits of the same character as those with
which we are familiar at home. Wherever similar geographical conditions
prevailed, there similar geological results followed.
VII.
_Conclusion._
There are many other points of resemblance between the glacial and
fluvio-glacial accumulations of the two continents, but to these time
forbids any reference. Indeed, I cannot recall any signal difference.
Such differences as do occur are due simply to the varying conditions
of the two continental areas. The glacial phenomena of North America
are a repetition of those of Europe, but upon a much grander scale. The
boulder-clays of the former continent, in their composition, structure,
and distribution, exactly recall our own. Interglacial beds occur
under similar circumstances in both continents; and the same is the
case with the gravelly moraines and fluvio-glacial accumulations.
We are driven, then, to the conclusion that the physical conditions
of the Glacial period were practically the same in Europe and North
America. What those conditions were I have already indicated, and have
shown that the results arrived at by geologists are not vague dreams
and speculations, but a logical induction from well-ascertained facts.
Before we can believe that volcanic eruptions, a general deluge, or a
Palæocrystic Sea have produced the many varied phenomena of our glacial
formations, either in whole or in part, we must first shut our eyes and
then erase from our minds all knowledge of the facts which have been so
laboriously gathered by a long succession of competent observers.
[Illustration:
PLATE III
DISTRIBUTION OF ICE
PAST AND PRESENT.
POLAR VIEW
OF THE
WORLD
ON LAMBERTS EQUAL AREA PROJECTION
]
VII.
The Intercrossing of Erratics in Glacial Deposits.[Q]
[Q] _The Scottish Naturalist_, 1881.