89 



surface or subterranean, oucl the smaller ones farther from them, as is 

 illustrated in the plain southwest of Bloomington. When this stage has 

 been reached — with sinlvs well developed ovei- most of the region and col- 

 lapse has begun at the exits of the cave streams — a region may be re- 

 garded as in its maturity. It is only after the mature stage of the cycle 

 has been reached that sinlvS, due to the collapse of cave roofs, begin to ap- 

 pear in considerable numbers, and natural bridges, due to collapse of the 

 cave roofs above and below a given point, begin to be developed. Solution 



Fig. 8. Spring at Leonards Mill (house in dnep gulcli soutli of Leonards 

 school), showing similar features as preceding. Note water escaping all around the 

 foreground. A portion of the water from the main spring is shown in the extreme 

 lower left corner of the picture. The outlet for the sinks south and northwest of 

 Leonards school. 



sinks that happ-eu to be located above caverns may be, and frequently are, 

 transformed into collapse siiilcs in the latest stages of subterranean erosion. 



When these features of collapse become prominent and much of th(> 

 drainage has been brought to the surface again and collapse sinks are 

 numerous, old age has been reached. 



The valleys produced by the collapse of caverns and the transfcn-nia- 

 tion of subterranean drainage to surface drainage have a characteristic 

 form that at once distinguishes them from ordinary drainage valleys. 

 They are rather sharply U-shaped, with steep sides like a youug valley 



