83 



smaller the percentage of pore space, and the thinner the beds the greater 

 is the t-^ndency to form sinks and caves and the more sharply angular are 

 the subterranean drainage courses. This is excellently illustrated in the 

 cave region of Indiana. The Mitchell limestone is very dense, thin bedded 

 and impervious for a limestone and is broken into small joint blocks. Here 

 the subterranean flow is largely contiued ro the joints and bedding planes, 

 thus concentrating the solution effected by the water to the immediate 

 channels through which it flows. In tliis w.iy channels are produced and 

 enlarged with maxinunn rapidity. 



On the other hand we may contrast this condition with that of tin- 

 Sak'm limestone lying immediately beneath the Mitchell limestone. Th.> 

 Salem is nearly devoid of bedding planes, a rather Sdlt and quite porous 

 limestone, through which tlie water percolates with relitive ease The re- 

 sult is that cave;, in the Salem limestone are very rare. When thev occur. 



Fig. 1. Diasi-ammatic illirstration of incipient sulitoiTanean drainage. Tlie 

 main stream is entrenclied and tlio tributaries out of adjustment pitch over rapids 

 to join it. Underground drainage lias started throu.^h the joints. The vertical 

 dotted line a !j npi-csents the original unbalanced static water head which started 

 the circulation. 



as at Mays cave, they may be foimed by a cave jiassing down from the 

 ^litchtdl limestone into it to reach the surface nearer rhe drainage level. 

 The lack of frequent bedding planes is a strong contributory factor to this 

 condition. Aside from its structure the ofiportunities for the formation of 

 subterranean chaiuiels are as good as in the ;\Iitchell limestone. 



Again, the Ilarrodsliurg liniestdiie, lying immediately l>eliiw the Salem 

 limestone, is hai'der. less porous, more highly jointed and thinner Ijedded 

 than the Salem and shows a correspondingly greater tendency to deveh)]) 

 underground water cliiniiicls. The Mitchell limestone jxissesses the ex- 

 treme (jf these (■ondili(Jiis and llie extreme develo[)ment of underground 

 tliannels. 



nil-; SI i;ii;i;i;.\XL'.\\ duai naok cyci-K. 



In either a cuastai plain or an interioi- region which has been thor- 

 oughly baseleveled and I'eelevated, what drainage there is to begin with js 



