Page:Popular Science Monthly Volume 53.djvu/503

Rh feet above the surfaces of the lakes, situated on their depressed borders. A view of one of these interesting lakes is given in Fig. 1.

To the northwest of the basins holding lakes, just referred to, there is a series of ridges and hills inclosing undrained basins, which extends about two miles from the base of the main escarpment and gradually decreases in height at the same time that the minor features in their relief become more and more subdued. This belt of ridges and basins finally merges by insensible gradations into a tract of undulating, prairielike land, two to three miles broad. The edges of the more recent of the fallen blocks stand out as sharp-crested ridges, with gentle slopes toward the great cliffs from which they fell, but present precipitous escarpments of bare rock toward the valley. As one descends the series of ridges and hills, the cliffs become less and less sharply defined and soon give place to rounded swells. Old lake basins change to swampy areas, and at a still greater distance become grassy dells.

As already stated, there is a gradual transition from the still hilly region to the undulating plain, at the northwest base of Lookout Mountain, where the relief has been smoothed out and only gentle, flowing outlines attract the eye. On the margin of the plain adjacent to the lower hills there are obscure ridges, on which there are many rounded and much-weathered bowlders of basalt, but a mile farther westward the soil is exceedingly fine and homogeneous, and scarcely a stone can be found. Such pebbles as do occur are of basalt, rounded by decay. A characteristic feature of the plain, now cleared of the scattered groves of pine that formerly covered it, and sown with wheat, is the presence of shallow, undrained basins, with low, gently swelling hills between them. This tract of country, eight or ten square miles in area, lies between Teanaway River and Swank Creek, but is entirely without stream channels. The scanty rain is absorbed by the deep, porous soil.

The undulating, prairielike lands just described have resulted from the slow disintegration and decay of blocks of basalt which fell as landslides during the slow recession of the thick lava sheet and of the soft volcanic tuff beneath, which once covered the region. The undulating surface of the wheat lands, with undrained basins, illustrates the old age of landslide topography. A view of this undulating plain is reproduced in Fig. 2. The hills seen in the distance owe their origin to another sheet of Columbia lava, the lowest of the series, which slopes toward the observer, and breaks off in steep slopes to the northward.

An ideal section through the margin of Lookout Mountain is