286 GENEKAL GEOLOGY. 



lie was the first to announce the existence of Cretaceous strata 

 in Iowa at any point farther south than the immediate vicinity 

 of Sioux City,* which he represents as extending as far south- 

 ward as the mouth of Boyer river. During the labors of the 

 present Geological Survey, rocks of this age have been recog- 

 nized still further southward, in Montgomery county, and as 

 far eastward as the eastern part of Guthrie county, more than 

 eighty miles eastward from the Missouri river. 



Although the actual exposures of Cretaceous rocks are few 

 in Iowa, there is reason to believe that nearly all the western 

 half of the State was originally occupied by them, but all the 

 strata being very friable they have been removed to a great 

 extent by subsequent denudation; This denudation has 

 doubtless taken place during two separate periods, and is the 

 result of two separate causes. The first was the natural sur- 

 face denudation that is constantly taking place upon all land 

 surfaces, and was accomplished both during its elevation from 

 the Cretaceous sea, and during the long Tertiary age that 

 passed between the time of that elevation and the commence- 

 ment of the Glacial epoch. Secondly, they suffered still 

 greater erosion during the last named epoch by the action of 

 ice ; amounting even to their entire removal over considerable 

 areas, so that the drift now rests directly upon strata that 

 were once covered by Cretaceous deposits, the latter having 

 added largely to the materials of the drift. Thus every 

 exposure of these rocks is found to show a more or less 

 incomplete series of strata, by having had their upper portions 

 either disturbed or removed, and they are not unfrequently 

 parts of outliers of greater or less extent, which have become 

 detached from the principal deposit by the erosion before 

 mentioned. 



There are two distinct causes why there are so few expos- 

 ures of the Cretaceous rocks in the region within which they 

 are known to occupy the surface, or immediately beneath the 



*See (Bulletin de la Societe Geologique de France, 2d serie, t. xxiv, p. 56, seance du 19 

 Novombre 1866,) an article entitled, "Le terrain cretace des environs de Sioux City, 

 etc., par M. Jules Marcou." 



