118 



INDIANA UNIVERSITY STUDIES 



The geologic history of Indiana begins with an old sea which 

 graduall}^ retreated to the southwest as the region to the northeast 

 was raised. The general dip of the rocks is to the southwest, at 

 the rate of twenty to thirty feet to the mile. In some places the 

 dip is much more, at times, being as much as a hundred feet to the 

 mile. Beginning with the Ordovician formations which are the 

 oldest rocks found in Indiana, the other formations are exposed 

 as one goes from east to west across the State, until the Merom 

 sandstone is reached at the extreme western part of the State. 

 In each case, the older passes under the younger, and each is ex- 

 posed at the surface for a distance dependent upon the thickness 

 of the formation and the amount of stream erosion. 



Ordovician. The Ordovician rocks are the oldest rocks ex- 

 posed in Indiana. They consist of a series of hardened clays and 

 thin bedded limestones, commonly designated as the Cincinnati 

 group. This region includes a strip from fifteen to twenty-five 

 miles in width extending from the Ohio River northward to the 

 northern part of Wayne County. The entire territory is drained 

 by Whitewater River, and other streams that flow into the Ohio 

 River. Since shales are easily eroded, the relief is rather pro- 

 nounced, being as much as four hundred feet. The limestones of 

 the region are very thin, rarely more than a few^ inches in thickness, 

 thus affording very little protection to the shales. The region 

 in general is one of the physiographic divisions of the State and 

 may be called the Eastern Highland, the elevation above the sea 

 being from 700 to 1,200 feet. 



Silurian. The Clinton and the Niagara limestones of Silurian 

 age succeed the Ordovician rocks. They are the surface rocks 

 along the Ohio River, extending in a narrow strip northward through 

 the eastern part of Clark County, the middle of Jefferson County, 

 the eastern part of Jennings County, thence with the western limit 

 near Greensburg and Rushville, northwest past Nobles ville, as 

 indicated on Chart No. 1. From Rushville south the outcrop 

 will average fifteen miles in width, except at the extreme southern 

 part. The Clinton limestone, which is basal Silurian in Indiana, 

 is a rather thin bed, varying from a few inches to about seven feet 

 in thickness. The Niagara group, which overlies the Clinton 

 limestone, is composed of several divisions of limestone and shales, 

 aggregating in all about one hundred and twenty-five feet, in the 

 southern part of the State. To the north where the Niagara passes 

 under the glacial drift, it reaches a thickness of four hundred 

 feet. The topography of the Niagara limestone outcrop is rather 



