47 



At the beginning of the Lower or Sul)-Carboniferons Era. which 

 followed the Devonian in regnlar sequence, we find more than half 

 of Indiana above the level of the sea. By the deposition and subsequent 

 raising of the rocks of tlie Corniferous and Genesee epochs, the gap 

 between the large area of Niagai'a limestone in the eastern part of 

 the State and the mainland to the northwestward had been tilled and 

 that portion of the future Indiana became for the first time a part of 

 the' slowly growing North American continent. The roclvs which were 

 afterward added on its western side were deposited on the sloping floor 

 of the Central Interior sea which stretched far away to the southwest, 

 and they consequently have a notable dip in tliat direction. 



The lowermost stratum of the Sub-Carboniferous rocks in Indiana is 

 a thin but very persistent bed of greenish limestone, known as the Rock- 

 ford Goniatite limestone. It is but about two feet in thickness at its 

 most notable outcrops, and hence forms but a very narrow area of 

 the surface rocks of the State. It serves well, however, as a line of 

 demarcation separating the Upper Devonian shales from the thick beds 

 of Knobstone which represent one of the early and important epochs 

 of Lower Carboniferous time. 



These Knobstone rocks consist at the base of a series of soft, bluish 

 shales, which gradiially become more arenaceous or sandy, until toward 

 their western horizon they merge into massive beds of impure grayish 

 sandstone. The formation ranges in known thickness from 440 to G50 

 feet. The name "Knobstone" was first given it by that eminent geol- 

 ogist, David Dale Owen, because its siliceous strata weather into those 

 peculiar conical "knobs" or hills which are so prominent a feature 

 of the topography in the southern unglaciated portion of its area. By 

 the deposition and upraising of the Knobstone a strip of territory, 3 

 to 38 miles in width, extending from the Ohio River southwest of 

 New Albany north and northwesterly to a point a few miles south 

 of the present site of Rensselaer, Jasper County, was added to the 

 existing land of the future State. Deep bores have also shown the 

 Knobstone to immediately underlie the drift in a strip of varying width 

 along the extreme northern border of the State. By its deposition 

 and subsequent upraising over this area, all of the northeastern portion 

 of the State became for the first time dry land, and the waters of the 

 Eastern Interior Sea were forever banished from the future Indiana. 



