U-shaped cross sections and their hanging (tributary) valleys, but the great height and steepness of the valley walls in Yosemite are exceptional. A type of glacial erosion which is of special interest is the sculpturing of so-called "cirques" or "amphitheaters" in mountains within the region of perpetual snow. Where the main mass of snow and ice in the catchment basin or gathering ground of a valley glacier pulls away from the snow and névé on the upper slopes, the rock wall is more or less exposed in the deep crevasse. During warm days water fills the joint cracks in the rocks down in this crevasse (so-called "Bergschrund"), and during cold nights the water freezes and forces the blocks of rock apart. This is greatest toward the bottom of the crevasse and so, by this excavating or quarrying process, vertical or very steep walls are developed around a great bowlike basin or cirque. Such cirques, now free from glacial ice, with precipitous walls 500 to 2,000 feet high and one-fourth of a mile to one-half of a mile across, are common in the Sierra Nevada and Cascade Ranges and in the Rocky Mountains. What becomes of the materials eroded by the ice? An answer to this question involves at least a brief discussion of the deposition of glacial débris, this constituting an important feature of the work of ice. The débris transported by a glacier is carried either on its surface or within it, or pushed along under it. It is generally heterogeneous material ranging from the finest clay through sand and gravel, to bowlders of many tons' weight. Various types of glacial deposits are abundantly illustrated by débris left strewn over much of the northeastern United States and some reference to these will be made. Most valley glaciers carry considerable débris on their surfaces, this representing material which falls or is carried down from the valley walls upon the margins of the ice, thus forming marginal moraines. When two glaciers flow together, one marginal moraine from each will coalesce to form a medial moraine. The material carried along at the bottom of a glacier is called the ground moraine. Where it contains much very fine grained material with pebbles or bowlders scattered through its mass, it is called "till" or "bowlder clay." The pebbles or bowlders of the ground moraine are commonly facetted and striated as a result of having been rubbed against the bedrock on which the glacier moved. Ground moraine material is the most extensively developed of all glacial deposits. It is so widely scattered over the glaciated northeastern portion of the United States that most of the soils consist of it, having been left strewn over the country during the melting of the vast ice sheet.