Page:Popular Science Monthly Volume 53.djvu/498

498

T is well known that masses of rock sometimes break away from steep slopes and descend as avalanches or landslides into adjacent depressions.

The topography of a region where landslides occur is changed in two ways—i. e., by the removal and by the accumulation of material. The contours of a mountain, plateau, or other land form that stands in relief, are altered by the removal of material; as, for example, when a portion of the border of a plateau falls away, a re-entering angle or curve is produced; or, when a rock avalanche occurs on a mountain side, a high-grade gorge or depression may result. The material composing a landslide comes to rest in ridges and piles which have certain characteristic shapes. The most noticeable feature in such instances is the backward slope of the surface of the displaced material after it comes to rest. The surface of a landslide, whether composed mainly of a single block or of a heterogeneous mass of loose material, slopes toward the cliff from which it came. This backward slope tends to the formation of basins in which water accumulates, and lakes and swamps result. The backward slope referred to appears to be due to friction between the moving mass and the rocks beneath, which retards the progress of the material at the bottom and in front, so as to allow the material which comes later and at a higher level to slide over it. In a heterogeneous mass of fallen rocks there appear to be several planes of shear along which differential motion has taken place.

The changes produced by landslides are usually considered, even by geologists, I believe, as of a local character and of minor importance in the topography of most regions. Recent studies by the writer of the geography of the lava-covered region of Oregon, Washington, and Idaho, however, have shown that the phenomena referred to furnish an explanation of the origin of a class of topographic forms which occur not only on mountain sides and plateau margins, and among the hills at the bases of such elevations, but in certain instances in the minor features of broad and approximately level plains.

The landslides or avalanches which sometimes rush down the sides of mountains are frequently, and probably in most instances, composed of loose rock and soil. The most frequent condition leading to such catastrophes is the saturation of the material with water. Landslides usually follow heavy rains. The famous Willey land