OGLE COUNTY. 107 



the river rises in bold outcrops of from twenty-five to two hundred feet 

 in thickness. The only other outcrops of this interesting formation in 

 the Northern part of the State is at Starved Kock and Deer Park, on 

 the Illinois river, and a few other points in LaSalle county. 



The following section will show the measured outcrops. They are all 

 thicker, perhaps, than these figures indicate, except the St. Peter's sand- 

 stone. The bottom of that formation and its full outcrop, we think, 

 was reached. The floor of Rock river, three or four miles below Oregon, 

 where its thickest development is reached, is the top of the Calciferous 

 sandstone or Lower Magnesian limestone : 



Section of Ogle county formations. 

 Usual surface deposits, consisting of sands, clays, soils and gravel beds, aggregate, perhaps. .. 125 feet. 



Cincinnati group, green and blue shales 25 



Galena limestone 35 



Blue Trenton limestone 44 



Buff Trenton limestone 36 



St. I Yrt-i s sandstone, -white, soft 200 



Lower Magnesiau limestone 



The above figures indicate the maximum thickness of the surface de- 

 posits, the St. Peter's sandstone, and perhaps the Buff limestone. The 

 other members of the section I think are thicker than the above 

 measurements indicate. Nowhere could I find exposures where the full 

 thickness could be determined. Commencing at the top, we will describe 

 these formations in their descending order. 



Surface Geology. 



The usual " river bottoms " exist along the streams to a limited ex- 

 tent. This, together with the common prairie soil, a vegetable mold, 

 covering most of the county, comprises the extent of the alluvial de- 

 posits. The drift formation is much more heavily developed. Over the 

 southern and eastern portion of the county, and along the lower Kock 

 river bluffs, it thins out to a considerable extent ; but over the northern 

 and northwestern parts of the county the true drift, in the form of drift- 

 hills and coarse gravel-beds, is very heavily deposited, reaching a thick- 

 ness, as we have above indicated, of one hundred and twenty-five feet. 

 Over the parts first mentioned fine-grained clay, sometimes marly and 

 sometmes sandy, cover the nether rocks. These clays are almost uni- 

 formly of a light yellowish color. Few gravel beds and little coarse 

 gravel can be noticed in passing over them. Boulders are of rarer oc- 

 currence than in any other portion of this part of the State. It is not 

 a driftless region, but the drift forces have acted peacefully here, and 

 nothing but the finer sediments and precipitates of the water were here 

 deposited or accumulated under the action of chemical, atmospheric and 



