=Caverns.=[106]—Where local solution is very great results of another sort may be effected. In formations like limestone, which are relatively soluble, considerable quantities of material are frequently dissolved from a given place. Instead of making the rock porous, in the usual sense of the term, large caverns may be developed (Fig. 202). In their production, solution may be abetted by the mechanical action of the water passing through the openings which solution has developed. Considerable caves are found chiefly in limestone. They were probably developed when the surface relief was slight, and surface drainage therefore poor. Regions where caves were developed under these conditions may subsequently acquire relief, so that caves are not now confined to flat regions. One of the best known regions of caves is in the basin of the Ohio in Kentucky and southern Indiana, where the number of caves is large, and the size of some of them, such as Mammoth and Wyandotte, very great. A ground-plan of Wyandotte (Ind.) Cave is shown in Fig. 203. The aggregate length of the passageways is about 23½ miles. [Illustration: +Fig.+ 204.—Deposits in Wyandotte (Ind.) Cave. (Hains.)] [Illustration: +Fig.+ 205.—Deposits in Wyandotte Cave. (Hains.)] 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. [Illustration: +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.) [Illustration: +Fig.+ 207.—A fresh landslide near Medicine Lake, Mont. The bare space shows the position from which the slide started. (Whitney.)]