In the autumn of 1846, six years after my visit to Great Britain in
search of glaciers, I sailed for America. When the steamer stopped at
Halifax, eager to set foot on the new continent so full of promise for
me, I sprang on shore and started at a brisk pace for the heights above
the landing. On the first undisturbed ground, after leaving the town, I
was met by the familiar signs, the polished surfaces, the furrows and
scratches, the _line-engraving_ of the glacier, so well known in the Old
World; and I became convinced of what I had already anticipated as the
logical sequence of my previous investigations, that here also this
great agent had been at work, although it was only after a long
residence in America, and repeated investigations of the glacial
phenomena in various parts of the country, that I fully understood the
universality of its action. A detailed description of these appearances
could hardly be more than a monotonous repetition of my statements
respecting their existence in other regions; but the peculiar
configuration of this continent, as compared with the more mountainous
countries of Europe and Asia, has led to some modifications of the same
phenomena here, worthy of special notice.
Thus far, the traces of ancient glaciers in America have been studied
only east of the Rocky Mountains; little is known of the glaciers still
remaining in the high mountain-ranges dividing the eastern part of the
continent from California, still less respecting any indications of
their former extension. There can be little doubt that such traces
exist, and as soon as the so-called _parks_ between Pike's Peak and Long
Peak are explored, we may hope for information on this point. Indeed,
the investigation may be spoken of as already undertaken; for among the
exploring parties now on their way to that region are some intelligent
observers who will not fail to make this point a subject of special
study. But it is well known that the usual characteristic marks of
glaciers extend over the whole surface of the land in the eastern half
of the continent, from the Atlantic shores to the States west of the
Mississippi, and from the Arctic Sea to the latitude of the Ohio, in its
middle course, while within the range of the Alleghanies they stretch as
far south as Georgia and Alabama. In no other region where these traces
have been observed do they extend over such wide tracts of country in
unbroken continuity, this being of course owing to the level character
of the land itself.
The continent of North America, east of the Rocky Mountains, is, indeed,
an immense uniform plain, intersected from east to west only by the
ranges of low hills running in the direction of the St. Lawrence and the
Canadian lakes, and from northeast to southwest by the Alleghany range
stretching from Alabama to New England, where it trends towards the
Canadian hills in the ridges known as the Green and White Mountains.
This coast-range has a short slope towards the Atlantic, and a long one
in the direction of the great Mississippi valley. With the exception of
some higher points of the Alleghany range, the surface of this whole
plain is glacier-worn from the Arctic regions to about the fortieth
degree of northern latitude, the glacier-marks trending from north to
south, with occasional slight inclinations to the east or west,
according to the minor inequalities of the surface. There is, however,
no decided modification of their general trend in consequence of the
range of hills intersecting them at right angles for nearly the whole
width of the continent between latitudes forty-six and fifty; indeed,
the Canadian, or, as they are sometimes called, the Laurentian Hills,
formed a no more powerful barrier to the onward progress of the immense
fields of ice covering the continent than did the small hummocks, or
_roches moutonnees_, in the Swiss valleys to the advance of the Alpine
glaciers. In fact, these low hills may be considered as a succession of
_roches moutonnees_, trending in a continuous ridge from east to west,
over which the masses of northern ice have moved unimpeded to the
latitude of the Ohio.
Owing to the absence of high mountain-ranges over this vast expanse of
land, the glacial phenomena of America are not grouped about special
centres of dispersion, or radiating from them, as in Europe. During the
greatest extension of the ice-fields, there were but few prominent peaks
rising above them, and dropping here and there huge boulders on their
surface, to be transported to great distances without losing their rough
angular character. And when the temperature under which these vast
frozen masses had been formed rose again, the wasting ice must have
yielded first on its southern boundary, gradually and uniformly
retreating to the Arctic regions, without breaking up into distinct
glacial regions, separated from one another, each with its local
distribution of erratic boulders and glacier-marks radiating from
circumscribed areas on higher levels, as they occur everywhere in
Europe. It is true that there are a few localities within the Alleghany
range, on the Green and White Mountains, and in parts of Maine, where it
is evident that local glaciers have had a temporary existence; but even
throughout this eastern coast-range the elevation of the mountains is so
slight, and their trend so uniform in a northeasterly and southwesterly
direction through twenty degrees of latitude, that the localization of
the phenomena is less marked than in Norway, Great Britain, or
Switzerland. In short, the ice of the great glacial period in America
moved over the continent as one continuous sheet, overriding nearly all
the inequalities of the surface. Thus the peculiar physical character of
the country gives a new aspect to the study of glacial evidences here.
The polished surfaces stretch continuously over hundreds and hundreds of
miles; the rectilinear scratches, grooves, and furrows are unbroken for
great distances; the drift spreads in one vast sheet over the whole
land, consisting of an indiscriminate medley of clays, sands, gravels,
pebbles, boulders of all dimensions, and so uniformly mixed together
that it presents hardly any difference in its composition, whether we
examine it in New England, New York, Pennsylvania, Ohio, Michigan,
Indiana, Illinois, Wisconsin, in Iowa beyond the Mississippi, in the
more northern Territories, or in Canada.
In Europe, boulders of large dimensions do not often occur within the
drift, but are usually resting above it with their sharp angles and
rough surfaces unchanged, having travelled evidently upon the glacier
and not under it. But such large boulders, polished and scratched like
the smaller pebbles, are to be found everywhere imbedded in American
drift, while the angular fragments of rock resting above these
triturated masses are comparatively rare.[H] It is evident from this
that the ice overtopped the rocky inequalities of the land, and that the
detached fragments remaining beneath the icy covering underwent the same
action from friction and pressure to which the whole mass of drift was
subjected. The distribution of the few angular boulders scattered over
the country no doubt began when some of the higher portions of the land
had emerged from the mass of snow and ice; and they are most frequent in
New England, where the mountain-elevation is greatest.
The mineralogical character of the loose materials forming the American
drift leaves no doubt that the whole movement, with the exception of a
few local modifications easily accounted for by the lay of the land, was
from north to south, all the fragments not belonging to the localities
where they occur being readily traced to rocks _in situ_ to the north of
their present resting-places. The farther one journeys from their
origin, the more extraordinary does the presence of these boulders
become. It strikes one strangely to find even in New England fragments
of rock from the shores of Lake Superior; but it is still more
impressive to meet with masses of northern rock on the prairies of
Illinois or Iowa. One may follow these boulders to the fortieth degree
of latitude, beyond which they become more and more rare, while the
finer drift alone extends farther south.
It is not only, however, by tracking the boulders back to their origin
in the North that we ascertain the starting-point of the whole mass; we
have another kind of evidence to this effect, already alluded to in the
description of the _roches moutonnees_. Wherever the natural surface of
any hill, having a steep southern slope, is exposed, the marks are
always found to be very distinct on the northern side and entirely
wanting on the southern one, showing, that, as in the case of many of
the _roches moutonnees_ in Switzerland, the mass moved up the northern
slope, forcing its way against it, grinding and furrowing the northern
face of the hill as it moved over it, but bridging the opposite side in
its descent without coming into contact with it.[I] This is true, not
only of hills, but of much slighter obstacles which presented themselves
in the path of the ice. Even pebbles imbedded in masses of
pudding-stone, but rising sometimes above the level of the general
surface, often have their northern side polished and scratched, while
the southern one remains untouched.
Moraines are not wanting to complete the chain of evidence respecting
the ancient existence of glaciers in this country, although we cannot
expect to find them here so frequently as in Europe, where the many
local glaciers in circumscribed valleys afforded special facilities for
the building up of these lateral and transverse walls. Over the broad
expanse of the United States, on the contrary, with such slight
variations of level, the disappearance of the ice at its breaking-up
would naturally be more complete and continuous than in a country
intersected by frequent mountain-chains, where the ice would linger in
the higher valleys long after it had disappeared from the plains below.
Yet it is evident that here also in certain localities the boundary-line
of the ice underwent oscillations, pausing here and there long enough to
collect mounds of the same character as those spanning the valleys of
Switzerland and Great Britain. We have several of these mounds in our
immediate vicinity. The Waverley Oaks, so well known to all lovers of
fine trees in our community, stand on an ancient moraine, and there are
others in the neighborhood of the Blue Hills. In the southeastern parts
of Maine, also, I have observed very well-defined moraines. In Vermont,
the valley of the Winooski River retains ample traces of the local
glacier by which it was formerly filled; and, indeed, throughout the
Alleghany range, in its northeastern as well as its southern extension,
we have various evidences of localized glaciers, which must have
outlived the general ice-period for a longer or shorter time.
I am unwilling to weary my readers by dwelling upon appearances
identical with those already described; but I may state, for the
guidance of those who wish to investigate these traces for themselves,
that any recently uncovered ledge of rock in our neighborhood, the
surface of which has not been altered by atmospheric agencies, presents
the glacier-worn surfaces with the characteristic _striae_ and furrows.
These marks may be traced everywhere, even to the sea-shore, not only
down to the water's edge, but beneath it, where-ever the harder rocks
have resisted the action of the tides and retain their original
character. In our granitic regions intersected by innumerable trap
dikes, as, for instance, at Nahant, the smooth surface of many of the
rocks, where sienite and trap have been evenly levelled, shows that the
same inexorable saw, cutting alike through hard and soft materials, has
passed over them. In the hills of pudding-stone in the neighborhood of
Roxbury, we have quartz pebbles cut down to the same level with the
softer paste in which they lie imbedded with pebbles of sandstone,
clay-slate, gneiss, and limestone. In the limestone regions of Western
New York and Northern Ohio, about the neighborhood of Buffalo and
Cleveland, the flat surfaces of the limestone are most uniformly
polished, furrowed, and scratched, the furrows often exhibiting that
_staccato_ grating action described in a former article. I have observed
the same traces in the vicinity of Milwaukee and Iowa City, and we know,
from the accounts given by Arctic travellers of their overland
expeditions, that these peculiar appearances of the surface are
characteristic of the rocks in those regions, wherever they are not
disintegrating under the influence of the present atmospheric agents.
Upon these surfaces, through the whole expanse of the country, rests the
drift, having everywhere the characteristic composition of
glacier-drift, and nowhere that of an aqueous stratified deposit, except
when afterwards remodelled by the action of water. But of this
stratified drift I shall have occasion to speak more in detail
hereafter. There is, however, one circumstance, of frequent occurrence
along our New-England shores, requiring special explanation, because it
is generally misunderstood. Along our sea-shore, and even within the
harbor of Boston, at the base of the harbor-islands, as well as at the
outlet of our larger Atlantic streams, numbers of boulders are found of
considerable size; and this fact is often adduced as showing the power
of water to transport massive fragments of rock to great distances, the
mineralogical character of these boulders being frequently such as to
show that they cannot have originated in the neighborhood of their
present resting-places. But a careful examination of the surrounding
country, and a comparison of the nature and level of the drift on the
mainland with those of the same deposits on the harbor-islands, (a
series of evidence to be given with more detail in a future article,)
suggest a different explanation of these phenomena. The sheet of drift
was once more continuous and extensive than it is now, and the
localities in which we find these crops of boulders are spots where the
tide has eaten into the drift, wearing away the finer materials, or the
paste in which the larger fragments were imbedded, and allowing them to
fall to the bottom, or where the same result has been produced by the
action of rivers cutting their way through the drift, and thus finding
an outlet to the sea. In short, instead of showing the power of currents
to carry along heavy fragments, these stranded boulders prove, on the
contrary, the inability of water to produce any such effect, since it is
evident that the tides washing against the shore, or the rivers rushing
down to the sea, were equally incapable of bearing off the weightier
materials, and allowed them to drop to the bottom, while they readily
swept away the lighter ones. Such localities compare with the
surrounding drift much as the bottom of a gravel-pit which has been
partially worked compares with its banks. Look into any gravel-pit, a
portion of which has already been carted away. At its bottom a number of
larger stones and boulders are usually lying, too heavy for the cart,
and therefore left upon the spot. Fragments of the same size and
character, and equally numerous, will be seen protruding at various
heights from the sides, where they are imbedded in the general mass of
the drift. As soon as the work progresses a little farther, and the
finer materials are removed, these boulders will also drop out, and lie
as thickly scattered over the surface of the ground, as they now do in
that portion of the bottom where the pit has been completely opened and
the gravel removed. We shall see hereafter how these boulders, derived
from the land-drift and scattered along the coast, may be distinguished
from those cast ashore by icebergs.
Notwithstanding the number of facts thus far collected respecting
glacial phenomena in America, certainly forming in their combination a
very strong chain of evidence, the scientific world has, nevertheless,
been slow to admit the possibility of the former existence of glaciers
over such a wide, unbroken expanse of level land. This backwardness is,
no doubt, partly due to the fact, that, as glaciers have hitherto been
studied in mountainous countries, their presence has been supposed to
imply the presence of mountains, this impression being strengthened by
the downward and onward movement of existing glaciers, so long supposed
to be exclusively due to the slopes along which all modern glaciers
advance. Were it true that glaciers move solely or mainly on account of
the sloping bottom on which they rest, and that they can advance only
on an inclined plane, all the phenomena concerning drift, polished and
furrowed surfaces, boulders, etc., in America, would hardly justify us
in assuming a moving sheet of ice as their cause. But we have seen that
the phenomena of glaciers, like those of currents, are in great part
meteorological. The Gulf-Stream does not flow toward the English shore
because the ocean-bottom slopes eastward; nor does the cold current of
Baffin's Bay run down-hill when it pours its icy waters southward upon
our northeast coast. Their course is determined by laws of temperature,
and so have we also seen that the motion of glaciers is mainly
determined by conditions of temperature, although, in this case, an
internal mechanical action is combined with external influences; and
while it is true that glaciers, as they now exist, are dependent upon
the shape of the valleys in lofty mountain-chains, yet under different
geographical conditions the same phenomena may be produced over level,
open countries.
I believe that circumstances similar to those determining the more rapid
advance of the glaciers from higher to lower levels at that point where
the alternate thawing and freezing, the infiltration of water and
consequent expansion of the ice under frost, are greatest, would also
determine the motion of a large body of ice from north to south, since
it would be along its southern limits that these conditions would
prevail; while the great reservoir of snow at the north would correspond
to the upper troughs of the present glaciers, from which their lower
ranges are constantly fed. The change of snow into ice is owing to
alterations of temperature, to partial melting and subsequent freezing,
constantly renewed,--and also to the sinking of the mass upon itself in
consequence of its own weight, the lower portions being thus forced out
by the pressure of the superincumbent ice. Upon an inclined plane the
movement consequent upon these changes will of course be downward; but
what would be the result, if a field of snow many thousand feet thick,
corresponding, except in its greater bulk, to the accumulations by which
the present glaciers are caused, were stretched over an extensive level
surface? The moisture from the upper superficial layers would permeate
the larger mass as it now does the smaller one, trickling down into its
lower portions, while the pressure from above would render the bottom
hard and compact, changing it gradually into ice. If this should take
place under climatic conditions which would keep the whole as a mass in
a frozen state, the pressure from above would force out the lower ice in
every direction beyond its original circumscription, thus enlarging the
area covered by it, while the whole would subside in its bulk. Let us
for a moment assume that such an accumulation of snow takes place around
the northern and southern poles, stretching thence over the northern and
southern hemispheres to latitude forty, and that this field of snow
acquires a thickness of from twelve to fifteen thousand feet. Such a
mass would subside upon itself in consequence of its own weight; it
would be transformed into ice with greater or less rapidity and
completeness, according to the latitude determining the surrounding
climatic influences and the amount of moisture falling upon it as rain
or dew, the alternations of temperature being of course more frequent
and greater along its outer limit. In proportion as, with the rising of
the temperature, these alternations became more general, a packing of
the mass would begin, corresponding to that observed in the glacial
valleys of Switzerland, though here the action would not be intensified
by lateral pressure; an internal movement of the whole mass would be
initiated, and the result could be no other than a uniform advance in a
southerly direction from the Arctic toward the more temperate latitudes
in Europe, Asia, and North America, and from the Antarctic toward South
America, the Cape of Good Hope, and Van Diemen's Land. But we need not
build up a theoretical case in order to form an approximate idea of the
great ice-sheet stretching over the northern part of this continent
during the glacial period. It would seem that man was intended to
decipher the past history of his home, for some remnants or traces of
all its great events are left as a key to the whole. Greenland and the
Arctic regions hold all that remains of the glacial period in North
America. Their shrunken ice-fields, formidable as they seem to us, are
to the frozen masses of that secular winter but as the patches of snow
and ice lingering on the north side of our hills after the spring has
opened; let us expand them in imagination till they extend over half the
continent, and we shall have a sufficiently vivid picture of this frozen
world. And a temperature which would bring the climate of Greenland down
to the fortieth degree of latitude would not only render the field of
ice far more extensive, but thousands of feet thicker than it is at
present. The physical configuration of Greenland also confirms the
possibility of a glacial period in America, for there we have at this
moment a wide expanse of land unbroken by mountains, over which a
uniform sheet of ice moves southward, with occasional variations of its
trend according to the undulations of the surface. The interesting
accounts of Dr. Rink show that in reality Greenland is a miniature
picture of the ice-period. The immense number of icebergs breaking off
and floating southward every summer gives us some idea of the annual
waste and renewal of the ice. How can we doubt, that, when, under the
same latitude, Norway, Sweden, Scotland, England, and Ireland were
covered by sheets of ice many thousand feet in height, the ice-fields of
Greenland must have shared in the same climatic influences, and have
been much thicker and far more extensive than they are at present?
Notwithstanding the absence of lofty mountain-chains in America, we are
not wholly without the means of measuring the thickness of the
ice-sheet, by comparing it, as in Europe, with some of our highest
elevations. The slopes of the Alleghany range, wherever they have been
examined, are glacier-worn to the very top, with the exception of a few
points; but these points are sufficient to give us data for the
comparison. Mount Washington, for instance, is over six thousand feet
high, and the rough, unpolished surface of its summit, covered with
loose fragments, just below the level of which glacier-marks come to an
end, tells us that it lifted its head alone above the desolate waste of
ice and snow. In this region, then, the thickness of the sheet cannot
have been much less than six thousand feet, and this is in keeping with
the same kind of evidence in other parts of the country; for, wherever
the mountains are much below six thousand feet, the ice seems to have
passed directly over them, while the few peaks rising to that height are
left untouched. And while we can thus sink our plummet from the summit
to the base of Mount Washington and measure the thickness of the mass of
ice, we have a no less accurate indication of its extension in the
undulating line marking the southern termination of the drift. I have
shown that the moraines mark the oscillations of the glaciers in Europe.
Where such accumulations of loose materials took place at its terminus,
there we know the glacier must have held its ground long enough to allow
time for the collection of these _debris_. In the same way we may trace
the southern border of our ancient ice-sheet on this continent by the
limit of the boulders; beyond that line it evidently did not advance as
a solid mass, since it ceased to transport the heavier materials. But as
soon as the outskirts of the ice began to yield and to flow off as
water, the lighter portions of the drift were swept onward; and hence we
find a sheet of finer drift-deposit, sand and gravel more or less
distinctly stratified, carried to greater or less distances, and fading
into the Southern States, where it mingles with the most recent
river-deposits.
One naturally asks, What was the use of this great engine set at work
ages ago to grind, furrow, and knead over, as it were, the surface of
the earth? We have our answer in the fertile soil which spreads over the
temperate regions of the globe. The glacier was God's great plough; and
when the ice vanished from the face of the land, it left it prepared for
the hand of the husbandman. The hard surface of the rocks was ground to
powder, the elements of the soil were mingled in fair proportions,
granite was carried into the lime regions, lime was mingled with the
more arid and unproductive granite districts, and a soil was prepared
fit for the agricultural uses of man. I have been asked whether this
inference was not inconsistent with the fact that a rich vegetation
preceded the ice-period,--a vegetation sufficiently abundant to sustain
the tropical animals then living throughout the temperate regions. But
the vegetation which has succeeded the ice-period is of a different
character, and one that could not have flourished on a soil that would
nourish a more tropical growth. The soil we have now over the temperate
zone is a grain-growing soil,--one especially adapted to those plants
most necessary to the higher domestic and social organizations of the
human race. Therefore I think we may believe that God did not shroud the
world He had made in snow and ice without a purpose, and that this, like
many other operations of His Providence, seemingly destructive and
chaotic in its first effects, is nevertheless a work of beneficence and
order.
* * * * *
In the next article, in order to put the reader in possession of the
glacial question as it stands at present, I shall say something of the
possible causes of this extraordinary accumulation of snow,--though all
such explanations are thus far mere suggestions,--and shall also give
some more precise estimates of the changes of temperature involved in
the history of the glacial period, before proceeding to the
consideration of the effects produced by the breaking-up of the ice, as
shown in our stratified lowland drift, and in our estuaries and
river-terraces.