180 



bend or curve developed at the soluble place. (Fig. 5.) In old caves these 

 factors, together with that of collapse from lateral erosion after base level 

 has been reached, change the shape so that a straight line or right-angled 

 bend is seldom seen. 



SPECIAL PHASES — PITS AND DOMES. 



Many caves are characterized by pits and domes. The former may 

 be formed in two ways. Where there is a particularly soft or soluble 

 place in the floor of a cave, the hard, angular fragments of chert will con- 

 gregate, and by a whirlpool-like abrasion and solution, a pot-hole will be 

 produced. These sometimes reach large dimensions, as in Wyandotte, 

 where pits twenty or thirty feet deep have been formed. In one particular 

 passage of Wyandotte, the downward erosion has been very rapid, so that 

 the stream has cut down to a lower level, leaving several natural bridges 

 of solid rock. 



The second type of pit and the domes are related. Often where two 

 sets of vertical joint-planes cross, the water trickling down will dissolve 

 out an erosion dome. In Mammoth Cave of Kentucky, these domes often 

 reach a height of one hundred feet or more. They may be formed down to 

 the level of the passage along one of the joints, in which case they are 

 simply domes, or they may continue eroding after one passage has been 

 deserted by the stream and continue to erode to a lower level occupied by 

 another stream, thus forming a pit or dome according to the level from 

 which they are viewed. (Figs-. 6 and 7.) 



CAVE ENTRANCES. 



Gave entrances may be formed in four principal ways. A sink-hole 

 may become large enough to serve as an entrance, either by corrosion and 

 solution, or by subterranean solution of the dome-forming type. Ropes, 

 ladders, or steps are generally needed in this type of an entrance^ The 

 entrances to Little Wyandotte and Marengo caves of Crawford County are 

 of this type. 



Another and common type of entrance is that by way of the mouth 

 of the out-flowing cave stream. In a young cave this is apt to be on the 

 horizontal ; but when one mouth is abandoned for another at a lower 

 level, weathering produces a curious change. The rocks above the cave 

 mouth will weather and fall to the floor, thus causing the entrance to pro- 

 gress up the slope and a great pile of debris to collect on the original floor 

 of the cave. (Fig. 8.) The entrances to Wyandotte and Saltpeter caves 



