TOPOGRAPHIC FEATURES DUE TO LANDSLIDES. 483 



the slow recession of inward-facing cliU's, due principally to their 

 having fallen from time to time in landslides, and the gradual decay 

 and removal of the fallen blocks by streams and percolating waters. 

 This process is till in progress, and a series of topographic changes 

 from fresh landslides to rolling, prairielike lands with deep, rich soils 

 and features characteristic of old land surfaces, can be easily traced. 

 The reader must not infer, however, that the entire area from which 

 the successive sheets of Columbia lava have been removed has a low 

 relief. The streams have cut deeply into the rocks beneath the lowest 

 lava sheet, and produced a markedly diiferent series of land forms, 

 in which sharp ridges and deep, narrow valleys are conspicuous 

 elements. A belt of country marked by landslide topography which 

 was gradually smoothed out, owing to the decay and erosion of the 

 fallen blocks of basalt, receded with the slow retreat of the encircling 

 cliffs and was replaced by exceedingly rugged topography. 



The nature of the changes produced by landslides and the subse- 

 quent decay of the fallen masses and their melting down, as it were, 

 into an undulating plain with undrained basins, is graphically dis- 

 played at many localities adjacent to the still receding escarpments. 

 Favorable localities for this study are furnished by Table and 

 Lookout Mountain, to the northward of Ellensburg, Washington, 

 or on the southeastern margin of the truncated Wenatchee dome. 

 Standing on Lookout Mountain, for instance, one beholds toward the 

 southeast a gently sloping table-land which rises toward his station. 

 The surface of this inclined table is formed of a sheet of Columbia 

 lava, but not the older sheet, which dips southeast at an angle of four 

 or five degrees. On its northwestern margin this table-land breaks off 

 so as to form a precipice from a thousand to twelve hundred feet high. 

 In many places this escarpment is vertical, but its lower slopes are 

 masked by talus. Below this palisadelike escarpment are many 

 others of a similar character, but of less height and seldom over 

 half a mile in length. The lower escarpments are formed of the 

 edges of blocks of lava which have broken away from the main 

 escarpment from time to time and plowed their way down into 

 the valley. The fallen blocks are inclined at angles of ten to fifteen 

 degrees toward the cliffs from which they fell. At the base of the 

 main escarpment there is a series of irregular depressions or basins, 

 which connect one with another more or less perfectly, and are 

 bounded on their northwest margins by the backward-sloping blocks. 

 In three of these basins at the present time there are small lakes 

 without visible outlets. Each lake is four or five acres in area, and, 

 except where the basins have become partially filled with talus from 

 the overshadowing cliffs to the eastward, are deepest on that side. 

 The western edges of the fallen blocks rise some two or three hun- 



