GEOLOGY OF IvA SALLE COUNTY. 21 



The Trenton as a surface rock covers but a small 

 area in the county, and no\Yhere shows beds more than 

 seventy-five feet thick, perhaps as thick as this at the 

 entrance to Deer Park, but at Streator a carefully 

 made boring- shows it to be no less than 203 feet. A 

 boring- made many years ag-oat Lowell reports it at that 

 place as 170 feet, and there are thirty feet of strata 

 still before the St. Peters is reached. We have not 

 seen a singde place where these rocks are exposed 

 where there is anything- between them, and we do not 

 believe there is at Ivowell, and we therefore consider 

 the whole 200 feet as Trenton, for there has been little 

 denudation of this rock at that place. 



The scattered exposures, as we have remarked, 

 that on Covell creek and west of Ottawa — that on the 

 north side of the river, seem to lie in troughs or chan- 

 nels cut in the St. Peters, and seem to be in most cases 

 the upper beds of the area, but that at Troy Grove — 

 formerly Homer — cannot be of that character. The 

 beds there dip to the northeast, and we venture to say 

 are only one of several ridg-es, one of which is seen to 

 the northeast of Milling-ton, bring-ing" the St. Peters to 

 the surface at that point, and now the site of a g-reat 

 sand quarr\"ing- and washing- industry. 



It seems to us that the explanation is a very sim- 

 ple one. Whoever has observed the calciferous be- 

 tween Utica and the Pequanisaug-g-in must have noticed 

 that it lies in rolls, or constitutes a series of small 

 s^^nclinals and anticlinals. We believe the whole rock 

 mass from the tunnel east to be made up of such rolls, 

 but perhaps larg-er ones, the tops of which have been 

 removed by g-lacial action, and the Troy Grove Tren- 

 ton is a part of what was once a continuous sheet 

 of Trenton strata, that on the top of the rolls having- 

 been broken up and carried away. 



