WILL COUNTY. 209 



and drop the burdens of rock which they had brought from more northern 

 regions. 



Of the Drift proper, we see but little, since the alluvium covers so large a 

 portion of the surface. The boulder clay, however, with occasionally a patch 

 of conglomerated sand and pebbles, shows along the Kankakee for two or 

 three miles below Wilmington, and the same beds are often met with in deep 

 wells in the eastern part of the county, and also in the northwestern townships 

 above the DuPage. 



The gravel bed above the boulder-clay is, at gome points, more or less com- 

 pacted by a ferruginous cement, so as to form quite a solid conglomerate. The 

 most notable instance of this is at " Knowlton Mound," about a mile east of 

 Joliet. along the Cut-off- railroad, where huge masses of the conglomerate lie 

 about in every direction, the looser and finer underlying beds having been 

 shipped to Chicago for street improvements. Traces of iron, in the water 

 which leaches through the overlying soil and clay, give to the gravel a cement- 

 ing quality, so that it packs very finely in the roadway, and, after a few months' 

 use, can hardly be broken up with a pick. C. Knowlton, Esq., of Joliet, the 

 owner of the mound, informs me that between 30,000 and 40,000 yards of this 

 gravel were delivered in Chicago during the season of 1869, at a cost of over 

 $70,000. 



The rock formations of the county are confined to the Coal Measures, the 

 Niagara limestone and the Cincinnati group. 



Coal Measures. The rocks of the Coal Measures cover something less than 

 two townships in the southwestern corner of the county. They consist mainly 

 of fine grained sandstones, clay shales and fire-clays, accompanied by one, or, 

 possibly, two seams of coal. 



The outcrop enters the county near the mouth of the DesPlaines river, in- 

 cludes a few sections about the junction of the DesPlaines and the Kankakee, 

 passes south of the latter river below the feeder-dam, crosses the center of sec- 

 tion 8, the west half of section 17, the northwest corner of section 20, the east 

 halves of sections 19 and 30, the north halves of sections 32 and 33, and the 

 west half of section 34, of township 33 north, range 9 east, and through the 

 west half of section 3, the east half of section 9, the west half of section 16, 

 the east halves of sections 20, 29 and 32, of township 32 north, range 9 east ? 

 to the southern line of the county. 



Two seams of coal appear to exist in this county, viz. that worked at the 

 Schoonmaker shaft, in section 7 of Wilmington township, and the main seam 

 of all this region, which is worked at all the other mines, and is the equivalent 

 of No. 2 of the general section of the coals of the Illinois valley.* 



*See Geology of Illinois, vol. Hi., p. 5. 

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