THE WORK OF GROUND WATER 71 



" galleries," occur (Fig. 47) ; as, for example, in Mammoth Cave, 

 Kentucky. These galleries are the result (1) of the presence of layers 

 of relatively insoluble rock upon which the underground streams flow 

 until they dissolve and erode out a wide passage. If this layer is 

 worn through after a time, or a joint is enlarged, permitting the water 

 to reach a lower soluble layer, it may descend until a second relatively 



Fig. 47. — Diagram showing the formation of the galleries of limestone caves by 

 the lowering of the valley (indicated by dotted lines) to which the underground water 

 dissolving them flowed. 



insoluble layer is encountered. If this process is repeated, several 

 galleries will result. The lowest level at which caves may be formed 

 is that of the lowest surface stream into which the underground water 

 is discharged. (2) Migration from one level to another may also 

 result from the intermittent lowering of the valleys (Fig. 47) of the 

 surface streams into which the underground waters of the caverns 

 flow. The galleries of caves may divide and reunite, forming a 

 network of channels at the different levels. It has been estimated 

 that in Kentucky alone there are 100,000 miles of underground 

 passages. 



Natural Bridges may be formed by the partial caving in of the roofs 

 of caverns, or by the enlarging of two swallow holes opening to the 

 same underground stream. Natural bridges are also formed in other 

 ways (pp. 91, 112). 



Cave Deposits. — After a cave has been abandoned by the stream 

 which formed it, the water entering is confined chiefly to small seepage. 

 At this stage much of the water is removed by evaporation so that 

 solution gives place to deposition. The deposits in caves are usually 

 in the form of stalactites and stalagmites. The former begin as a thin 

 film of lime around the outside of a drop of water which evaporates 

 on the roof of a cavern. Upon this additional lime is left by other 

 drops until a stalactite, resembling an icicle, is suspended from the 

 roof of the cavern. The accumulations of lime which form where the 



