22 DRIFT DEPOSITS OF ILLINOIS. 



The scratching and polishing of the hard rock surfaces be- 

 neath the drift, as well as the striation of some of the bould- 

 ers, may be readily explained by floating ice, with embedded 

 angular fragments of still harder material, that would act as 

 gravers upon whatever solid substances with which they were 

 brought in contact. 



The "iceberg" theory requires the assumption of no extraor- 

 dinary conditions to explain the drift phenomena, and it is a 

 generally accepted principle in geological science that no resort 

 to extraordinary suppositions is justifiable, where phenomena 

 can be explained by the operation of well known causes. 



The gradual change of level in the relations of land and water, 

 is a dynamical fact universally admitted, and a general sub- 

 mergence of the surface after the disappearance of the conti- 

 nental glacier, or ice sheet, is admitted by the advocates of the 

 glacial theory; and, if the date of this submergence should be 

 carried back to the period immediately following the formation 

 of the ancient surface soil, beneath the boulder drift, we obtain 

 the essential conditions required for the accumulation of the 

 succeeding deposits of the drift period. 



From the facts presented in the foregoing pages, we may as- 

 sume that the drift period represents a vast epoch in geologi- 

 cal time, during which a considerable portion of the Mesozoic 

 formations were in process of deposition over such areas as 

 were still submerged beneath the ocean; and hence the term 

 Post Pliocene can only be applied to the latest formed of these 

 deposits, the loess, which contains the oldest fossil mammalia 

 hitherto found in the superficial deposits of Illinois. 



The succession of events transpiring in the northwestern por- 

 tion of the United States during the drift epoch, we may assume 

 to be somewhat as follows: First, the gradual elevation of the 

 surface above the ocean level at the close of the carboniferous 

 period, followed by the extensive denudation of the palaeozoic 

 rocks, and the excavation of extensive valleys, which seems to 

 have been the prevailing condition in the region now under 

 consideration during the Triassic and Jurassic periods. 



The next occurrence in the order of succeeding events was the 

 partial filling of the valleys with clay, sand and gravel, and 

 the formation of the lowest bed of ancient soil beneath the 

 boulder clay. This was followed by a partial or entire submer- 



