Outlines of the Earth's History: A Popular Study in PhysiographyShaler, Nathaniel Southgate
History
Outlines of the Earth's History: A Popular Study in Physiography
Shaler, Nathaniel Southgate
Physical geography
lake of Geneva, we note successive terminal moraines which clearly
indicate stages in the retreat of the ice when for a time it ceased to
go backward, or even made a slight temporary readvance. It is easily
seen that on such occasions the stones carried to the ice front would
be accumulated in a heap, while during the time when day by day the
glacier was retreating the rock waste would be left broadcast over the
valley.
As we go up from the course of the glacial streams we note that the
successive moraines have their materials in a progressively less
decayed state. Far away from the heap now forming, and in proportion
to the distance, the stones have in a measure rotted, and the heaps
which they compose are often covered with soil and occupied by
forests. Within a few miles of the ice front the stones still have a
fresh aspect. When we arrive within, say, half a mile of the moraine
now building, we come to the part of the glacial retreat of which we
have some written or traditional account. This is in general to the
effect that the wasting of the glaciers is going on in this century as
it went on in the past. Occasionally periods of heavy snow would
refresh the ice streams, so that for a little time they pushed their
fronts farther down the valley. The writer has seen during one of
these temporary advances the interesting spectacle of ice destroying
and overturning the soil of a small field which had been planted in
grain.
It should be noted that these temporary advances of the ice are not
due to the snowfall of the winter or winters immediately preceding the
forward movement. So slow is the journey of the ice from the _neve_
field to the end of a long glacier that it may require centuries for
the store accumulated in the uplands to affect the terminal portion of
the stream. We know that the bodies of the unhappy men who have been
lost in the crevices of the glacier are borne forward at a uniform and
tolerably computable rate until they emerge at the front, where the
ice melts away. In at least one case the remains have appeared after
many years in the _debris_ which is contributed to the moraine. On
account of this slow feeding of the glacial stream, we naturally may
expect to find, as we do, in fact, that a great snowfall of many
years ago, and likewise a period when the winter's contribution has
been slight, would influence the position of the terminal point of the
ice stream at different times, according to its length. If the length
of the flow be five miles, it may require twenty or thirty years for
the effect to be evident; while if the stream be ten miles long, the
influence may not be noted in less than threescore years. Thus it
comes about that at the present time in the same glacial district some
streams may be advancing while others are receding, though, on the
whole, the ice is generally in process of shrinkage. If the present
rate of retreat should be maintained, it seems certain that at the end
Public-domain text, read in full here on John Shaqi.
Reviews
Reviews
No reviews yet
Be the first to share your thoughts on this work.
Elsewhere in the archive
Join the Discussion
Join the discussion
Sign in to leave a comment or review.
Sign InorCreate an account