I
He was a bold man who first conceived the idea of the great
continental ice-sheet which in Pleistocene times covered most of the
northern part of the continent, and played such a part in shaping
the land as we know it. That bold man was Agassiz, who, however, was
not bold enough to accept the theory of evolution as propounded by
Darwin. The idea of the great glacier did not conflict with
Agassiz's religious predilections, and the theory of evolution did.
It was a bold generalization, this of the continental ice-sheet, one
of the master-strokes of the scientific imagination. It was about
the year 1840 that Agassiz, fresh from the glaciers of the Alps,
went to Scotland looking for the tracks of the old glaciers, and he
found them at once when he landed near Glasgow. We can all find them
now on almost every walk we take to the fields and hills; but until
our eyes are opened, how blind we are to them! We are like people
who camp on the trail of an army and never suspect an army has
passed, though the ruts made by their wagons and artillery and the
ruins of their intrenchments are everywhere visible.
When I was a boy on the farm we never asked ourselves questions
about the stones and rocks that encumbered the land--whence they
came, or what the agency was that brought them. The farmers believed
the land was created just as we saw it--stones, boulders, soil,
gravel-pits, hills, mountains, and all--and doubtless wished in
their hearts that the Creator had not been so particular about the
rocks and stones, or had made an exception in favor of their own
fields. Rocks and stones were good for fences and foundations, and
for various other uses, but they were a great hindrance to the
cultivation of the soil. I once heard a farmer boast that he had
very strong land--it had to be strong to hold up such a crop of
rocks and stones. When the Eastern farmer moved west into the
prairie states, or south into the cotton-growing states, he probably
never asked himself why the Creator had not cumbered the ground with
rocks and stones in those sections, as he had in New York and New
England. South of the line that runs irregularly through middle New
Jersey, Pennsylvania, Ohio, Indiana, Illinois, Iowa, and so on to
the Rockies, he will find few loose stones scattered over the soil,
no detached boulders sitting upon the surface, no hills or mounds of
gravel and sand, no clay banks packed full of rounded stones, little
and big, no rocky floors under the soil which look as if they had
been dressed down by a huge but dulled and nicked jack-plane. The
reason is that the line I have indicated marks the limit of the old
ice-sheet which more than a hundred thousand years ago covered all
the northern part of the continent to a depth of from two to four
thousand feet, and was the chief instrument in rounding off
mountain-tops, scattering rock-fragments, little and big, over our
landscapes, grinding down and breaking off the protruding rock
strata, building up our banks of mingled clay and stone, changing
the courses of streams and rivers, deepening and widening our
valleys, transplanting boulders of one formation for hundreds of
miles, and dropping them upon the surface of another formation. When
it began to melt and retreat, it was the chief agent in building up
our river terraces, and our long, low, rounded hills of sand and
gravel and clay, called kames and drumlins. In many of our valleys
its flowing waters left long, low ridges, gentle in outline, made up
entirely of sand and gravel, or of clay. In other places it left
moraines made up of earth, gravel, and rock-fragments that make a
very rough streak through the farmer's land. All those high, level
terraces along the Hudson, such as that upon which West Point
stands, were the work of the old ice-sheet that once filled the
river valley. The melting ice was also the chief agent in building
up the enormous clay-banks that are found along the shores of the
Hudson. The clay formed in very still waters, the sand and gravel in
more active waters.
This great ice-sheet not only covered our northern farms with rocks
and stones, and packed the soil with rounded boulders, but it also
carried away much of the rock decay that goes to the making of the
soil, so that the soil is of greater depth in the non-glaciated than
in the glaciated areas of the country. The New-Englander or
New-Yorker in traveling in the Southern States may note the enormous
depth of soil as revealed by the water-courses or railroad cuts. The
ice-sheet was a huge mill that ground up the rocks in the North
probably as fast or faster than the rains and the rank vegetation
reduced them in the South, but the floods of water which it finally
let loose carried a great deal of the rock-waste into the sea.
The glacier milk which colors the streams that flow from beneath it
finally settles and makes clay. Off the great Malaspina Glacier in
Alaska the ocean is tinged by the glacier milk for nearly fifty
miles from the shores. Very few country people, even among the
educated, are ready to believe that this enormous ice-sheet ever
existed. To make them believe that it is just as much a fact in the
physical history of this continent as the war of the Revolution is a
fact in our political history is no easy matter. It certainly is a
crushing proposition. It so vastly transcends all our experience
with ice and snow, or the experience of the race since the dawn of
history, that only the scientific imagination and faith are equal to
it. The belief in it rests on indubitable evidence, its record is
written all over our landscape, but it requires, I say, the
scientific imagination to put the facts together and make a
continuous history.
Three or four hundred feet above my cabin, five or six hundred feet
above tidewater, there is a bold rocky point upon which the old
ice-sheet bore heavily. It has rubbed it down and flattened it as a
hand passing over a knob of soft putty might do. The great hand in
this case moved from the northeast and must have fairly made this
rocky prominence groan with its weight. The surface, scratched and
grooved and planed by the ice, has weathered away, leaving the rock
quite rough; its general outlines alone tell the tale of the battle
with the ice. But on the east side a huge mass of rock, that had
been planed and gouged by the glacier, was detached and toppled
over, turning topsy-turvy before it had weathered, and it lies in
such a position, upheld by two rock fragments, that its glaciated
surface, though protected from the weather, is clearly visible. You
step down two or three feet between the two upholding rocks and are
at the entrance of a little cave, and there before you, standing at
an angle of thirty or forty degrees, is this rocky page written over
with the history of the passing of the great ice plane. The surface
exposed is ten or twelve feet long, and four or five feet wide, and
it is as straight and smooth, and the scratches and grooves are as
sharp and distinct as if made yesterday. I often take the college
girls there who come to visit me, to show them, as I tell them,
where the old ice gods left their signatures. The girls take turns
in stooping down and looking along the under surface of the rock,
and feeling it with their hands, and marveling. They have read or
heard about these things, but the reading or hearing made little
impression upon their minds. When they see a concrete example, and
feel it with their hands, they are impressed. Then when I tell them
that there is not a shadow of a doubt but that the ice was at one
time two or three thousand feet thick above the place where they now
stand, and that it bore down upon Julian's Rock with a weight of
thousands of tons to the square foot, that it filled all the Hudson
River Valley, and covered the landscape for thousands of miles
around them, riding over the tops of the Catskills and of the
Adirondacks, and wearing them down and carrying fragments of rock
torn from them hundreds of miles to the south and southwest,--when
I have told them all of this, I have usually given them a mouthful
too big for them to masticate or swallow. As a sort of abstract
proposition contained in books, or heard in the classroom, they do
not mind it, but as an actual fact, here in the light of common day
on the hill above Slabsides, with the waters of the Hudson
glistening below, and the trees rustling in the wind all about us,
that is quite another matter. It sounds like a dream or a fable.
Many of the processes that have made our globe what we see it have
been so slow and on such a scale that they are quite beyond our
horizon--beyond the reach of our mental apprehension. The mind has
to approach them slowly and tentatively, and become familiar with
the idea of them, before it can give any sort of rational assent to
them. It has taken the geologist a long time to work out and clear
up and confirm this conception of the great continental glacier
which in Pleistocene times covered so large a part of the northern
hemisphere. It is now as well established as any event in the remote
past well can be. In Alaska, and in the Swiss Alps, one may see the
ice doing exactly what the Pleistocene ice-sheet did over this
country.