Thus A Marginal
Valley Is Formed, Clear Ice On One Side, Or Nearly So, Buried Ice
On The Other.
As melting goes on, the marginal trough, or valley,
grows deeper and wider, since both sides are being melted, the land
side slower.
The dead, protected ice in melting first sheds off the
large boulders, as they are not able to lie on slopes where smaller
ones can. Then the next larger ones are rolled off, and pebbles
and sand in succession. Meanwhile this material is subjected to
torrent-action, as if it were cast into a trough. When floods come
it is carried forward and stratified, according to the force of the
current, sand, mud, or larger material. This exposes fresh surfaces
of ice and melting goes on again, until enough material has been
undermined to form a veil in front; then follows another washing and
carrying-away and depositing where the current is allowed to spread.
In melting, protected margin terraces are oftentimes formed. Perhaps
these terraces mark successive heights of the glacial surface. From
terrace to terrace the grist of stone is rolled and sifted. Some,
meeting only feeble streams, have only the fine particles carried
away and deposited in smooth beds; others, coarser, from swifter
streams, overspread the fine beds, while many of the large boulders
no doubt roll back upon the glacier to go on their travels again.
It has been cloudy mostly to-day, though sunny in the afternoon, and
my eyes are getting better.
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