82 



Clear crwk, were effected by glacial waters wiiirii wvie one of the potent 

 factors in producing the beautiful terraces of Beauhlossoiu. Salt, Richland 

 and Coon creeks. However, it ''s with the subterranean drainage that we 

 wish to deal at this time. 



GENERAL COXSIOERATIOXS. 



Before discussing the details of the underground drainage of the 

 Bloomington region it is nece'-sary to discuss somo of the general features 

 of the development of subterranean drainage under various conditions. 

 Underground drainage is develojied in two ways : 



1. In a region of very soft, porous rocks, where jointing and bedding 

 may play a somewhat minor role, the channels are determined to some ex- 

 tent by the varying degree of porosity of the rocks through whicli the water 

 percolaies. Under such conditions the caves are apt to i)e less regular in 

 their forms and their coiu'ses less angular than would otherwise be the 

 case. This also has a marked effect upon the origiu of the sink-holes and 

 o.-ive oix^nings. I'nder these conditioi^s the sinks may be formed where 

 tlie rock is somewhat more porous or where there was a slight depression 

 originally. These factors are modified b.^' the proximity of channels l>e- 

 iie.itii the surface. In such cases, as has been pointed out by Sellards", the 

 sinks first appear as "cave-ins" of. the soil and rock structure, the sink 

 r>eing first a hole of greater or less size, sometimes being larger below than 

 at the surface. That is. the hole may be conical or ■•jug-shaped," as sug- 

 gested by Eig'^nmann'. The caves of Cafias, Cuba, are of this type. Sinks 

 of this kind are foruied most abundantly where tiie surface of the region 

 is but little elevated ab(jve tide or general drainage level and the caves or 

 channels are close to the rock surface so that it is easily undermined. 

 In cases where the caves are far beneath the surface the sinks will be de- 

 termined — in the absence of surface irregularities — by the location of the 

 more porous spots in the rocks near the subterranean channels and will be 

 developed by solution from the top downward. It may be remarked here 

 that the joints in some of the Cuban caves are inconsiiicuous. 



2. The other condition under which ca^•es are formed and free under- 

 ground drainage developed is in the firmer limestones, usually well above 

 ..sea level and the major drainage lines. The denser the limestone the 



=> Science, XXVI. p. 417. lOoT. More fully. Bull. I. Fla. (!enl. Surv., pp. 49-.j7 

 t008. 



^ Bull. r. S. iMsb Comni., 1002. pp. 211-230. 



