The other day in passing a farmer's house I saw where he had placed
a huge, roundish boulder, nearly as high as a man's head, by the
roadside and had cut upon it his own name and date, and that of his
father before him, and that of the first settler upon the farm, in
the latter part of the eighteenth century. It was an interesting
monument. I learned that the rock had been found in the bed of a
small creek near by, and that the farmer had given a hundred dollars
to have it moved to its place in front of his house. Had I seen the
old farmer I am sure I could have added to his interest and pride in
his monument by telling him that it was Adiron-dack gneiss, and had
been brought from that region on the back, or in the maw, of a
glacier, many tens of thousands of years ago. But it is highly
probable that, were he an uneducated man, he would have treated my
statement with contempt or incredulity. Education does at least this
for a man: it opens his mind and makes him less skeptical about
things not dreamed of in his philosophy.
This boulder had been rolled and worn in its long, slow ride till it
was nearly round. I have a much smaller boulder, probably from the
same quarry, which I planted at the head of my garden for a seat
when the hoe gets tired. When it was dropped here on the land that
is now my field, the bed and valley of the Hudson were occupied by
the old glacier which, during its decline and recession, built up
the terraces opposite me (where now stands a multimillionaire's copy
of an Italian palace), and which added to and uncovered the river
slopes where now my own vineyards are planted.
The flowing or the creeping of this old ice-sheet, so that it could
transport large boulders hundreds of miles, is one of the most
remarkable things about it: as slow or slower than the hour-hand of
the clock, yet an actual progression, carrying it, in the course of
thousands of years, from its apex in Labrador well down into New
Jersey, where its terminal moraine is still clearly traceable.
A river of ice, under the right conditions, flows as literally as a
river of water, fastest in the middle, and slowest along its margins
where the friction is greatest. The old ice-sheet, or ice sea,
flowed around and over mountains as a river flows around and over
rocks. Where a mountain rose above the glacier, the ice divided and
flowed round it, and reunited again beyond it. One may see all this
in Alaska at the present time. Water, of course, flows because of
its own pressure; so does ice, only the pressure has to be vastly
greater. A drop of water on the table does not flow, but, pile it
high enough, and it will. The old ice sea flowed mainly south, not
because it was downhill in that direction, but because the
accumulation of ice and snow at the North was so great. If through
any climatic changes, the snowfall were ever again to be so great
that more snow should fall in winter than could melt in summer,
after the lapse of thousands of years, we should have another ice
age.