208 



GEOLOGY 



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. 168. The aggregate length of its passage- 

 ways is a number of miles. 



Deposition often takes place in caves after they are formed 

 (Fig. 169), or it may even go on at the same time that the cave is 

 being excavated. Stalactites and stalagmites are common forms 

 of cave deposits. A stalactite may start from a drop of water leak- 

 ing through the roof of the cave. Evaporation, or the escape of 



Fig. 168. Ground-plan of Wyandotte Cave. The unshaded areas repre- 

 sent the passageways. (21st Ann. Kept., Ind. Geol. Surv.) 



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 deposit > 

 at the end. Deposition in the tube may ultimately close it, while 

 deposition on the outside, due to the water trickling down in that 

 position, may greatly enlarge it. 



Limestone sinks. Underground caves sometimes give rise to 

 topographic features of local importance. When the roof of a 

 cavern 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 numerous sinks formed in this way. They are conspicuous in 

 the cave region of Kentucky, and are well known in many other 

 limestone districts, and are known as limestone sinks (Fig. 170 and 

 Fig. 166, and PI. XV). 



