Piedmont glaciers apparently erode but little. Beneath their lake-like expanse of sluggish or stagnant ice a broad sheet of ground moraine is probably being deposited. Cirques and glaciated valleys rapidly lose their characteristic forms after the ice has withdrawn. The weather destroys all smoothed, polished, and scored surfaces which are not protected beneath glacial deposits. The over-steepened sides of the trough are graded by landslips, by talus slopes, and by alluvial cones. Morainic heaps of drift are dissected and carried away. Hanging valleys and the irregular bed of the trough are both worn down to grade by the streams which now occupy them. The length of time since the retreat of the ice from a mountain valley may thus be estimated by the degree to which the destruction of the characteristic features of the glacier trough has been carried. 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. [Illustration: Fig. 118. Longitudinal Section of a Tide Glacier occupying a Fjord and discharging Icebergs Dotted Line, sea level] 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.