238 GEOLOGY OF ILLINOIS. 



border near the Spring creek valley east of Onarga, and its western at Chats- 

 worth, in Livingston county. Passing on to the southward and westward, 

 Urbana was within its limits, though probably near its eastern border ; and 

 Bloomington appears to have been near its center. Further westward, its loca- 

 tion has not been determined. This valley was doubtless first excavated by 

 the glacier which dug out the basin of Lake Michigan ; and, as this gradually 

 melted and retired, the material of the " terminarmoraine " partially filled the 

 channel, while the river formed from the melting ice, still occupied a part of it- 

 The partially-filled bottom, as the glacier retired to Lake Michigan and beyond) 

 became overgrown with vegetation, the remnants of which we find both in dis- 

 tinct beds, such as have been encountered in boring and shafting at Chats- 

 worth, Urbana and Bloomington, and in the sand and gravet beds which 

 afterward accumulated and filled the valley. 



Rock Fo rmations . 



No outcrop of rock is known within the county, and we are obliged to rely 

 wholly upon bores and shafts for our knowledge of the underlying beds. The 

 southeastern part of the county is probably underlaid by Coal Measure rocks ; 

 but the only point at which this is known to be the case, is between Gilman 

 and Watseka, where coal is said to have been found recently at a depth of one 

 hundred and five feet. Reported thickness of seam, eight feet. No details 

 known, at the present writing. 



Limestone was reported as existing on the bank of the Iroquois, in section 

 14, township 27 north, range 13 west, but no outcrop was found ; and all evi- 

 dence favors the supposition that loose fragments found there were remnants of 

 loads of rocks formerly brought in flat-boats from Sugar Island just below the 

 county line, in Kaukakee county. These rocks are the uppermost beds of the 

 Silurian, and may be referred to either the Onondaga Salt group, or the top of 

 the Niagara group. 



A boring at Onarga encountered its first rock at about three hundred feet in 

 a bed of calcareous shale, which should probably be referred to the upper part 

 of the Cincinnati group, the overlying Niagara and Coal Measure rocks having 

 been removed from their original position here, during the excavation of the 

 glacier valley. 



It is not improbable that some thin continuations of the Devonian and Sub- 

 carboniferous rocks of Indiana might be found in place, between the Niagara 

 limestone and the Coal Measures, in the eastern part of the county j but it is 

 not probable that they reach to the western portion of it. 



