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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. 
Cave 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 cayes 
