TOPOGRAPHIC FEATURES DUE TO LANDSLIDES. 485 



dred 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 de- 

 scends 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 for- 

 merly covered it, and sown with wheat, is the presence of shal- 

 low, 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 with- 

 out 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 



