bility of the formation of any deep valleys. While such finished conditions may often occur over large districts, yet such is not Fig. 1.—Section showing a Hillside (d c) being washed down to the Base Plane of Erosion (c b), which is itself almost reduced to Sea Level. the character of very great regions, as Nature seldom allows the completion of these processes, for with the wearing down of one area another quarter is elevated by internal forces, until new plateaus rise in bold relief. The rains gather into streams and cut out gorges and valleys with their forms depending upon the character of the rocks and the length of time that the erosion is in progress. The gorges and valleys grow in length, like the Niagara cañon, until the slopes of the streams become so gentle that they can not deepen their channels any more. After that stage, the only work of the river is to carry away the rocks dissolved or washed from Fig. 2.—Map of a Plateau being transformed into a Valley (c c c), which is broadening out into a Plain (p p). The sluggish river is only acting as a carrier for the removal of the land washes. the sides of the valleys, which are thus widened into broad flats, and in their later stages great plains. Such a formation of base planes of erosion is illustrated in Fig. 2.
Applying this process of denudation to the lake region, it becomes evident that the land must have stood high enough above the sea for the rivers to remove the débris washed into them by the millions of little streams—that is to say, the continent was sufficiently high for the excavation of the deepest valleys now beneath the lake waters. As the sea was very distant from the lakes, much farther than now, and the upper lake basins were still farther inland, the altitude of the continent must have been even greater than the depth of the deepest lake basin below the sea level. On the other hand, the slope of the land must have been gentle, with the elevation just high enough to allow the drainage of the valleys, without the production of canons through them, and to enable the streams to widen them into broad, rolling hills and plains scores or hundreds of miles wide. (See Figs. 13 and 14.) The necessary altitude may have varied from time to time, but the duration of the proper conditions was very long. The elevation was not merely high enough to allow the reduction of