Hanging valleys are found also in river gorges where the smaller tributaries have not been able to keep pace with a strong master stream in cutting down their beds.  In this case, however, they are a mark of extreme youth; for, as the trunk stream approaches grade and its velocity and power to erode its bed decrease, the side streams soon cut back their falls and wear their beds at their mouths to a common level with that of the main river.  The Grand Canyon of the Colorado must be reckoned a young valley.  At its base it narrows to scarcely more than the width of the river, and yet its tributaries, except the very smallest, enter it at a common level.

Why could not a wide-floored valley, such as a glacier trough, with hanging valleys opening upon it, be produced in the normal development of a river valley?

The troughs of young and of mature glaciers.  The features of a glacier trough depend much on the length of time the preexisting valley was occupied with ice.  During the infancy of a glacier, we may believe, the spurs of the valley which it fills are but little blunted and its bed is but little broken by steps.  In youth the glacier develops icefalls, as a river in youth develops waterfalls, and its bed becomes terraced with great stairs.  The mature glacier, like the mature river, has effaced its falls and smoothed its bed to grade.  It has also worn back the projecting spurs of its valley, making itself a wide channel with smooth sides.  The bed of a mature glacier may form a long basin, since it abrades most in its upper and middle course, where its weight and motion are the greatest.  Near the terminus, where weight and motion are the least, it erodes least, and may instead deposit a sheet of ground moraine, much as a river builds a flood plain in the same part of its course as it approaches maturity.  The bed of a mature glacier thus tends to take the form of a long, relatively narrow basin, across whose lower end may be stretched the dam of the terminal moraine.  On the disappearance of the ice the basin is rilled with a long, narrow lake, such as Lake Chelan in Washington and many of the lakes in the Highlands of Scotland.

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Cirques and glaciated valleys rapidly lose their characteristic forms after the ice has withdrawn.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 oversteepened 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.