The main reason for the dissection of the region under consideration is to be found in the fact that it is bordered on the west by high mountains where precipitation is abundant, and the streams, supplied largely by the melting of the snow in summer, flow across a comparatively rainless country. The stream channels in general have been deepened at a more rapid rate than the areas between them have been lowered by erosion. Valleys running east and west have thus been excavated, leaving the intervening spaces as uplands, which, however, in certain instances have been minutely dissected by the streams originating on them and supplied by local winter precipitation. Added to these general conditions are differences in rock texture, which have led to great variations in the details due to erosions, particularly on the valley borders. One other condition which has modified the history of the plateau region throughout, but most decidedly at the north, is the climatic change which culminated in the Glacial epoch. During the time referred to the northern portion of the Great plateaus situated in Canada and the adjacent part of the United States was invaded by glacial ice which spread an irregular sheet of detritus over the country it occupied. Decided changes occurred also in the central and southern portion owing to increased precipitation, the flooding of the rivers leading from the melting ice-front, and to movements in the earth's crust of as yet undetermined extent and amplitude. It is apparent to the geographer that much of the history of the climatic changes of glacial and post-glacial times is recorded in the relief of the interior Continental basin to the south of the limit reached by the ice and in the terraces and alluvial deposits of the valleys, but as yet for the most part this interesting story remains unread. The most deeply dissected portion of the Great plateaus occurs in western Texas, eastern New Mexico, and Oklahoma. In that region the rivers having their sources in the Rocky Mountains and flowing to the Gulf of Mexico have excavated deep and wide valleys, leaving broad intervening areas in bold relief. The Pecos River drains a large part of the mountainous region in eastern New Mexico, and flows through a valley of its own making, which is some 30 or more miles broad and its bottom about 1,200 feet below the general surface of the plateau lying to the eastward. The Canadian River has excavated a similar valley, which is some 40 miles broad throughout much of its course, and is bordered by bold rocky escarpments from 1,000 to 1,200 feet high, in which the edges of the horizontal strata underlying the adjacent plateaus are exposed. This region of large and strongly defined topographic features illustrates in a remarkable manner the nature of the work performed by streams which rise amid high mountains and flow across a dry plateau standing well above sea-level.