Deposition often takes place in caves after they are formed (Figs. [204] and [205]). It may even go on at the same time that the cave is being excavated. Here are formed the well-known stalactites and stalagmites. A stalactite may start from a drop of water leaking through the roof of the cave. Evaporation, or the escape of some of the carbonic gas in solution, results in the deposition of some of the lime carbonate about the margin of the drop, in the form of a ring. Successive drops make successive deposits on the lower edge of the ring, which grows downward into a hollow tube through which descending water passes, making its chief deposits at the end. Deposition in the tube may ultimately close it, while deposition on the outside, due to water trickling down in that position, may greatly enlarge it.
Fig. 206.—A limestone sink-hole, east-northeast of Cambria, Wyo., exceptional for its steep sides. Minnekahta limestone. (Darton, U. S. Geol. Surv.)
Underground caves sometimes give rise to topographic features which are of local importance. When the solution of material in a cavern has gone so far that its roof becomes thin and weak, it may collapse, giving rise to a sink or depression in the surface over the site of the original cave. This is so common that regions of limestone caves are often affected by frequent sinks formed in this way. They are a conspicuous feature of the landscape in the cave region of Kentucky, and are well known in many other limestone districts. They are known as limestone sinks. ([Fig. 206] and [Fig. 2, Pl. XVII].)
Fig. 207.—A fresh landslide near Medicine Lake, Mont. The bare space shows the position from which the slide started. (Whitney.)
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]).
