[Illustration: +Fig.+ 208.—Landslide topography. The protruding mass on the right has slumped down from the mountain to the left. South face of Landslip Mountain, Colo., seen from the west; Rico quadrangle. (Cross, U. S. Geol. Surv.)] Under certain circumstances caves may give rise to striking features of another sort. If for any reason the roof is destroyed at the two ends of a cave, remaining intact over the middle, the latter part constitutes a _natural bridge_. Natural bridges also originate in other ways (pp. 151, 153). [Illustration: +Fig.+ 209.—Landslide topography on Badger Mountain, Washington. The slumping material in this case is basalt.] =Creep, slumps, and landslides.=—When the soil and subsoil on a slope become charged with water they tend to move downward. When the movement is too slow to be sensible it is called _creep_. The common downward inclination of trees growing in such situations, the result of the more rapid creep of the surface as compared with the deeper part of the soil, is both an expression of the movement and of its slowness. Other factors besides ground-water are involved in creep (see p. 112). When the movement is rapid enough to be sensible the material is said to _slump_ or _slide_. This may happen when the slope on which water-charged mantle rock lies is steep (Fig. 207). Great landslides of this sort have been recorded, and some of them have done great damage. Where a stream’s banks are high, and of unindurated material, such as clay, considerable masses sometimes slump from the bank or bluff into the river, or settle away slowly from their former positions. This is a common phenomenon along streams which have cut valleys in drift, and along shores on which waves are encroaching. The same phenomenon is common on a larger scale on the slopes of steep mountains.[107] Considerable terraces are sometimes developed on their slopes in this way, but they are usually irregular and discontinuous (Figs. 208, and 209). The loose débris on steep slopes sometimes assumes a sort of flowing motion and descends the slope with some such form and at some such rate as a glacier. Such bodies of débris are sometimes called “talus glaciers” (Fig. 210). In many such cases, snow and ice have had some part in their development. In creep and in landslides gravity is the force involved, and the ground-water only a condition which makes gravity effective. Gravity alone accomplishes similar results, as illustrated by Fig. 211. _Summary._