Page:Popular Science Monthly Volume 2.djvu/684

664 The occurrence of ancient beaches above the terraces on Hoosac Mountain, and among the White and Green Mountains 2,200 and 2,600 feet above the ocean, proves its former presence and the movement of its currents and waves.

At that period the site of the present rivers was the bottom of an ocean. It was during the progress of that period of continental depression and submergence that the glacial drift was modified and redistributed, forming enormous deposits, filling old basins and river-valleys, so that when the land emerged from the waters it was comparatively level, a few mountain-peaks rising above the plain.

It is at this point, as we have seen, that the present river-system with its terraced valleys begins, and the phenomena may occur in the following order:

 1. Elevated beaches, indicating ancient sea-shores. 2. The highest river-terraces. 3. Continuous excavation of river-valleys, and formation of flats, or flood-plains. 4. Elevation of those plains above the overflow of the river, forming terraces.

The process of formation we have already described; nor does it appear that any dynamic agent was then in operation which is not in operation now. The work has been continuous.

The superposition of terraces early suggested the idea that their origin was due to a succession of sudden elevations of the land rather than to a continuous movement, and such, indeed, may have been the case in some instances. But their usual want of uniformity through long distances and of correspondence on opposite sides of the valley induces the conclusion that their immediate distribution is controlled by local circumstances, while the general cause has been a continuous and gradual elevation of the land, and the equally continuous action of running water. Currents of rivers are thrown from side to side by ice-borne bowlders and accumulations of débris, and pebbles will become so adjusted in the river's bed as to resist erosion, as shown in Fig. 3.



This is one of the causes of the sinuosities of rivers. The water, as Sir Charles Lyell observes, is thus frequently forced to cut new channels, by which means new terrace flats may be formed by redistribution of materials.

The transporting power of running water depends on its velocity. Hopkins, cited by Prof. Dana, says that its force varies as the sixth