82 
Clear Creek, were effected by glacial waters which were one of the potent 
factors in producing the beautiful terraces of Beanblossom, Salt, Richland 
and Coon creeks. However, it is with the subterranean drainage that we 
wish to deal at this time. 
GENERAL CONSIDERATIONS. 
Before discussing the details of the underground drainage of the 
Bloomington region it is necessary to discuss some of the general features 
of the development of subterranean drainage under various conditions. 
Underground drainage is developed in two ways: 
1. In a region of very soft, povous 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 which the water 
percolates. Under such conditions the caves are apt to be less regular in 
their forms and their courses less angular than would otherwise be the 
ease. This also has a marked effect upon the origin of the sink-holes and 
eave openings. Under these conditions the sinks may be formed where 
the rock is somewhat more porous or where tere was a slight depression 
originally. These factors are modified by the proximity of channels be- 
aeath the surface. In such cases, as has beeu pointed out by Sellards’, the 
sinks first appear as “cave-ins” of the soil and rock structure, the sink 
Leing first a hole of greater or less size, sometimes being larger below than 
9 
at the surface. That is, the hole may be conical or “jug-shaped,”’ as sug- 
gested by Eigenmanin’. The caves of Canhas, Cuba, are of this type. Sinks 
ot this kind are formed most abundantly where the surface of the region 
is but little elevated above 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—iu 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 cayes are inconspicuous. 
2. The other condition under which caves 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 
3 Science, XXVI, p. 417, 1907. More fully, Bull. I, Fla. Geol. Surv., pp. 49-57, 
1908, 
4 Bull. U. S. Fish Comm., 1902, pp. 211-236. 
