SUEFACE FEATUEES. 79 



liaving the shape of an inverted hollow cone, with a depth at 

 the apex, sometimes reaching twenty or thirty feet from the 

 surface, and even more. Water accumulates in these sinkholes 

 from every hard rain, but it soon drains out again through 

 their subterraneous passages — hence their name of sinkholes. 



Sometimes they are larger than has just been indicated, and 

 have miniature ravines leading into them through which little 

 rills of water flow in time of rain. Just eastward from the 

 town of Decorah, in Winneshiek county, a good sized creek 

 empties into the south side of the Upper Iowa river. About 

 a mile up from the mouth of this creek, it is seen to issue from 

 the abrupt end of its valley in the form of a huge spring, and 

 is immediately set to work by an enterprising citizen to fur- 

 nish power for a woolen factory. Going back from this point 

 upon the undulating uplands, we come upon the creek valley 

 again within about half a mile, and see the creek disappear 

 beneath the surface in a depression, which is really a sinkhole 

 on a large scale, with a constant stream of water running into 

 it. With the exception of this half a mile of subterraneous 

 passage, however, the creek is in all respects an ordinary 

 one. It is the peculiar fissured and laminated character and 

 great thickness of the strata of the age of the Trenton lime- 

 stone, which underlies the whole region, and forms the valley- 

 sides of its streams, that has produced this interesting 

 phenomenon, and which also gives origin to numerous 

 sinkholes along the upland borders of the Upper Iowa 

 valley. From the same cause, also springs of large size are 

 very numerous in that valley, a number of which, since they 

 have so great a fall, give sufficient water-power for small 

 mills. 



Springs, of course, issue from all formations and from the 

 sides of almost all valleys, but they are more numerous and 

 important when the underlying strata are of such a character 

 as to facilitate the percolation of the surface-water down 

 to a certain level there to be arrested, so that it will flow out 

 upon the surface below. 



No mineral springs, properly so-called, have yet been 



