6 DRIFT DEPOSITS OF ILLINOIS. 



In the report a.bove cited, Volume 2, page 212, Dr. White 

 sums up his conclusions as to the probable extent of the Cre- 

 taceous strata in Iowa before they were subjected to denuding 

 forces, as follows: 



"How far the Cretaceous strata which a-ppear upon the south- 

 western margin of the region, extend over the remainder of it 

 unseen beneath the surface now covered with the drift, is not 

 now known, because no exposures of them are found there. AW 

 have, however, many reasons to believe that the last named 

 strata once covered nearly, if not quite, the whole region, and 

 a large part of them, being all very friable, were removed by 

 erosion during the Tertiary age, and much more swept away 

 by glacial action during the drift or glacial epoch, and the re- 

 mainder are now mostly covered by the drift. Besides more or 

 less indistinct indications of the correctness of this view, galh- 

 ered from observations within the region itself, we find traces 

 of Cretaceous strata at several points within the State of Minne- 

 sota, extending as far as, or farther to the eastward than the 

 eastern border of the region under discussion. Frequent expo- 

 sures of strata of Cretaceous age exist to the southward of it, 

 and also to a point in Gut.hrie county as far eastward as the 

 eastern border of this region ; and, although no exposure of 

 these strata now appear at the surface in the intermediate 

 space, judging from the uniformity of the surface, and from our 

 knowledge of the position that the underlying strata must 

 necessarily assume, it is inferred that the whole intermediate 

 region originally received deposits of Cretaceous strata continu- 

 ous with those that now appear. 



These Cretaceous strata everywhere, both in Minnesota and 

 Iowa, show evidence of extensive glacial and other denudation. 

 This has been accomplished with facility in consequence of the 

 great softness and friability of all the strata of the age. The 

 denudation of these strata have been so great that their re- 

 mains now exist in some parts only as scattered outlines rest- 

 ing upon older rocks." 



The facts observed in Illinois would seem to fully sustain Dr. 

 White's views in regard to the former extent of the Cretaceous 

 formation, and it is highly probable that the local patches of 

 Cretaceous strata may have been deposited as far eastward as 



