[Illustration: PLATE XX. A SECTION OF THE CALIFORNIA COAST NEAR SAN MATEO, CALIFORNIA. U. S. Geol. Surv.] [Illustration: +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). [Illustration: +Fig.+ 258.—The Valley of the American Fork. A V-shaped non-glaciated valley in the Wasatch Mountains of Utah. Compare Fig. 259. (Church.)] [Illustration: +Fig.+ 259.—U-shaped valley resulting from glaciation. Little Cottonwood Canyon, Wasatch Mountains. (Church.)] [Illustration: +Fig.+ 260.—Contrast between glaciated topography below and non-glaciated topography above. The minarets in the Sierras, Cal.] [Illustration: +Fig.+ 261.—Contrast between glaciated topography below, and non-glaciated topography above. Needles Mountains, from slope west of Hidden Lake. (Cross, U. S. Geol. Surv.)] The deepening of a valley by glacial erosion may throw its tributaries out of topographic adjustment. Thus if a main valley is lowered 100 feet by glacial erosion while its tributary is not deepened, the lower end of the latter will be 100 feet above the former when the ice disappears. Such a valley is called a _hanging valley_ (Figs. 262 and 263). Such valleys are of common occurrence in regions recently glaciated, but now ice-free. Examples are common in the western mountains of North America and elsewhere. Ice-caps which overspread the surface irrespective of valleys and hills, tend to reduce the angularities of the surface. Hills and ridges are cut down and smoothed (Figs. 264 and 265); but since valleys parallel to the direction of movement are deepened at the same time, it is doubtful if the relief of the surface is commonly reduced by the erosion of an ice-cap. [Illustration: +Fig.+ 262.—A hanging valley. East side of Lake Kootenai, B. C. All except the highest summits glaciated. (Atwood.)] =Fiords.=—A glacier descending into the head of a narrow bay may gouge out the bay to a very considerable depth, causing its head to recede. When the ice finally melts, the bay may be a fiord. Thus have arisen the glacial features of many of the fiords of high-latitude coasts, and many of the glaciers of those coasts are now making fiords (Fig. 266). Fiords also arise in other ways. Coasts indented by fiords are likely to be bordered by islands.