. Elementary physical geography;. olorado, and the Pamirs, both frequently classedamong plateaus, are examples. The latter are situated ina high mountain knot which, because of its great height,is often called the Roof of the World. Stream Valleys.—The valleys and canons formed byrunning water are due to slopes that are more or less are not a part of mountain structure, but are a resultof it. Such valleys are most common in mountainousregions and in plateaus. Most valleys show the results of stream-cutting and theweathering action of water. Shenandoah Valley, the de-pression crossin
. Elementary physical geography;. olorado, and the Pamirs, both frequently classedamong plateaus, are examples. The latter are situated ina high mountain knot which, because of its great height,is often called the Roof of the World. Stream Valleys.—The valleys and canons formed byrunning water are due to slopes that are more or less are not a part of mountain structure, but are a resultof it. Such valleys are most common in mountainousregions and in plateaus. Most valleys show the results of stream-cutting and theweathering action of water. Shenandoah Valley, the de-pression crossing Virginia, is an example. The rocks alongthe line of the valley were more easily worn away than thoseto the east and the west, and hence the valley resulted fromtheir removal. PLAINS, PLATEAUS, AND MOUNTAINS 79 The water may wear the rocks at the crest of a rangemore easily than it can remove them at other places. Inthis way canoe-shaped valleys are formed at the summit of afold. More commonly, however, the streams on opposite. ill mm CANOE VALLEYS, APPALACHIAN MOUNTAINS sides of a range wear their channels to the crest, makingdeep notches across it. Some of the passes in the SierraNevada and Rocky Mountains are examples; and so, too,are the gaps of the Delaware, Susquehanna, and HudsonRivers. If at a level with the base of the range, suchtransverse valleys are commonly called water gaps; if at aconsiderable altitude, they are passes. Among famous passes are Argentine, 13,100 feet, the highest wagonroad pass in the world; Marshall Pass, 10,900 feet, one of the highestrailway passes in the world; Alpine Pass, 13,550 feet, and MosquitoPass, 13,700 feet—all in Colorado. Simplon, St. Bernard, and Brennerare famous passes across the Alps, and for centuries they have beenhighways of commerce. A railway pass in the Andes is nearly 14,000feet above sea-level. In many instances the pass is not fully surmounted; instead of build-ing the railway over the divide, it is more economical
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