In Figure 104 what characteristics of a glacier trough do you notice?  What inference do you draw as to the former thickness of the glacier?

Name all the evidences you would expect to find to prove the fact that in the recent geological past the valleys of the Alps contained far larger glaciers than at present, and that on the north of the Alps the ice streams united in a piedmont glacier which extended across the plains of Switzerland to the sides of the Jura Mountains.

The relative importance of glaciers and of rivers.  Powerful as glaciers are, and marked as are the land forms which they produce, it is easy to exaggerate their geological importance as compared with rivers.  Under present climatic conditions they are confined to lofty mountains or polar lands.  Polar ice sheets are permanent only so long as the lands remain on which they rest.  Mountain glaciers can stay only the brief time during which the ranges continue young and high.  As lofty mountains, such as the Selkirks and the Alps, are lowered by frost and glacier ice, the snowfall will decrease, the line of permanent snow will rise, and as the mountain hollows in which snow may gather are worn beneath the snow line, the glaciers must disappear.  Under present climatic conditions the work of glaciers is therefore both local and of short duration.

Even the glacial epoch, during which vast ice sheets deposited drift over northeastern North America, must have been brief as well as recent, for many lofty mountains, such as the Rockies and the Alps, still bear the marks of great glaciers which then filled their valleys.  Had the glacial epoch been long, as the earth counts time, these mountains would have been worn low by ice; had the epoch been remote, the marks of glaciation would already have been largely destroyed by other agencies.

On the other hand, rivers are well-nigh universally at work over the land surfaces of the globe, and ever since the dry land appeared they have been constantly engaged in leveling the continents and in delivering to the seas the waste which there is built into the stratified rocks.

Icebergs.  Tide glaciers, such as those of Greenland and Alaska, are able to excavate their beds to a considerable distance below sea level.  From their fronts the buoyancy of sea water raises and breaks away great masses of ice which float out to sea as icebergs.  Only about one seventh of a mass of glacier ice floats above the surface, and a berg three hundred feet high may be estimated to have been detached from a glacier not less than two thousand feet thick where it met the sea.

Icebergs transport on their long journeys whatever drift they may have carried when part of the glacier, and scatter it, as they melt, over the ocean floor.  In this way pebbles torn by the inland ice from the rocks of the interior of Greenland and glaciated during their carriage in the ground moraine are dropped at last among the oozes of the bottom of the North Atlantic.