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IOWA ACADEMY OF SCIENCE Vol. XXVI, 1911) 



water is low the underground passage is so large and open that one 

 can hear the water falling over the stones underground. A short dis- 

 tance downstream is a long shallow basin filled with loose limestone. 

 After a rain, the stream rises and, flowing beyond the sink hole, as 

 shown in figure 18, enters this basin. If the rain is not too great all 



Fig. 22. — Showing fossils found in limestone in wall of quarry. These are 

 harder than surrounding stone, are not so easily dissolved and so help to sup- 

 port layers forming underground passage for water. 



Fig. 23. — Limestone ledge just below uppermost quarry showing harder ledge 

 below. 



