Page:Popular Science Monthly Volume 53.djvu/501

Rh the slow recession of inward-facing cliffs, 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 different 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 subsequent decay of the fallen masses and their melting down, as it were, into an undulating plain with undrained basins, is graphically displayed 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