Geology, Vol. 1 [of 3]

by Thomas C. Chamberlin; Rollin D. Salisbury

Scale, 2+ miles per inch.

PART OF THE BIGHORN MOUNTAIN RANGE, WYOMING.

PLATE XX.

U. S. Geol. Surv.

Scale, 1+ miles per inch.

A SECTION OF THE CALIFORNIA COAST NEAR SAN MATEO, CALIFORNIA.

Fig. 257.—A glacial cirque. The lake occupies a rock basin, produced by glacier erosion. Head of Little Timber Creek, Montana.

The topographic effects of glacial erosion.—In passing through its valley, an alpine glacier deepens and widens its bottom and smooths its slopes up to the upper limit of the ice. It tends to change a V-shaped valley ([Fig. 258]) into a U-shaped one ([Fig. 259]). The change in topography at the upper limit of glaciation is often marked (Figs. [260] and [261]).