89 
surface or subterranean, and the smaller ones farther from them, as is 
illustrated in the piain scuthwest of Bloomington. When this stage has 
been reached—with sinks well developed over 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 sinks, 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 deep gulch south 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. F 
sinks that happen to pe located above cavers may be, and frequently are, 
transformed into collapse sinks in the latest stages of subterranean erosion. 
When these features of collapse become prominent and much of the 
drainage has been Lrought to the surface again and collapse sinks are 
humerous, old age has been reached. 
The valleys produced by the collapse of caverns and the transforma- 
tion of subterranean drainage io 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 young valley 
