136 GEOLOGY OF ILLINOIS. 



spoken of them at some length, because they are very marked features 

 in the surface geology of part of this and the adjoining counties, and are 

 known as remarkable places over all this part of the State. 



T he Drift proper. This county is covered with the usual drift clays 

 of this part of the State. If these superficial deposits were stripped 

 off, the surface of the underlaying rocky formations would probably pre- 

 sent quite as level an appearance as the present face of the county. The 

 depth of these drift clays is hard to ascertain, being quite variable. Over 

 the township of Palmyra wells are put down from thirty to fifty feet 

 before striking the rock. One of these wells gave the following sec- 

 tion, as given to me by the person who dug it : 



1. Bl ack mold and subsoil 6 fei-t. 



2. Finely comminuted buff-yellow clay 12 " 



3. Blue, compact, laminated clay 10 " 



4. Black, oozy, marly mud, full of sticks, etc 5 " 



At this point an abundance of rather brackish and not very sweet 

 and pure water was struck, and the well up to the present time is never 

 failing, and the water growing sweeter and purer. At other localities 

 in this township wells are put down to the rock, and then drilled fifty 

 or sixty feet in the Galena formation below, before water is found. 



Over the southern part of the county the drift clays are probably 

 thicker than in the vicinity of Rock river. Where thickest, the blue 

 clay is' usually much the heaviest deposit, and is often underlaid by the 

 black mud of the above section, No. 5, or by a bed of gravel and dirt 

 of variable thickness. In the eastern and central portions of the county 

 beds of sand often cover the surface and alternate with the clays below 

 the surface. 



This blue clay and the black deposit containing the decayed remains 

 of timber, and the gravel beds on which the blue clay often rests, lies 

 at and near the base of the true drift in this part of the State. Clay 

 deposits covered the Silurian rocks before the drift forces acted. These 

 deposits were then undoubtedly very much thinner than now, and were 

 derived from the slow decomposition of the underlaying rocks and par- 

 took of their characters. The ice and waters of the drift period, the 

 transporting, grinding and abraiding agencies then acting with so much 

 power, increased these deposits very greatly ; mingled them up ; as- 

 sorted them, and left them in their present forms as beds of sand, dif- 

 ferent colored clays, gravel and boulder beds, and other deposits as we 

 now find them, modified somewhat by subsequent surface influences. 

 Since the drift epoch there has been a constant struggle, with varying 

 results, between the ravines and the level lands. Rains and water cur- 

 rents constantly struggle to cut out ravines in the crumbling clays, 

 Rains and other atmospheric agencies constantly struggle to fill up these 



