THE TRAVELS OF THE UNDERGROUND WATER 183 



their bottoms the insoluble clay which was contained in the impure 

 limestone as well as a certain amount of slope wash from the sur- 



Fia. 192. Plan of a portion of Mammoth Cave*, Kentucky (after H. C. Hovey). 



face. Inasmuch as the clays are impervious to water, the bottoms 



of these swallow holes are better supplied with moisture than the 



surrounding rock surfaces, and 



by nourishing a more vigorous 



plant growth are strongly im- 



pressed upon the landscape 



(Fig. 193). 



Certain of the depressions 

 above caverns are, however, 

 less regular in outline, and their 



bottoms are Occupied by a 



mass of limestone rubble. In 



some instances, at least, these photograph by H. T. A. de L. Hus). 



FlG - 193. Trees and shrubs growing 



