IOWA ACADEMY OF SCIENCES. 39 



SOUTHERN EXTENSION OF THE CRETACEOUS IN IOWA. 



BY E. H. LONSDALE. 



The Cretaceous deposits of Iowa, from time to time, have received the 

 attention of a number of geologists. The most important researches were 

 made by Mai'cou, Meek, Heer, White and Calvin. Their investigations were 

 carried on chiefly in the vicinity of Sioux City. The formation elsewhere in 

 the State has, with a few exceptions, received no consideration. Its exact 

 extent is yet to be determined; its vertical thickness is yet unknown; the 

 aelative ages of some of its beds remain to be established. 



Over western Iowa, in fact, over practically the whole State, resting 

 upon the pre-tertiary beds, whatever these beds maybe, is a mantle of debris 

 collected and carried by the great glaciers as they advanced and receded, 

 then and in the end depositing that material which is now recognized as drift 

 clays, sands, gravels and boulders. 



This drift material, as a whole, commonly so extensive in vertical thick- 

 ness, so persistent in its occurrence, and so readily yielding to the weathering 

 agencies, has almost completely concealed the older rocks upon which it 

 traveled and deposited itself. There are, however, occasional exposures of 

 these rocks standing out more or less precipitously along preglacial streams 

 which were of such magnitude or position, as the case may be, to withstand 

 the attack made by the glaciers, and thereby continue their existence; along 

 postglacial water courses which have cut through the drift and upper strata 

 of the underlying formations thus developing a narrow or broad channel 

 and growing new exposures along its way. These few outcroppings afford 

 about the only source from which reliable geological results can be gathered. 



The Cretaceous, made up as it is of soft layers, such as sandstones, 

 whose particles are commonly loosely or not cemented together, and beds 

 of clay shales, would naturally suffer to a greater extent from the effects of 

 the glaciers and weathering than would the limestones and other hard rocks 

 of older formations. It would consequently be expected that the limits of 

 the former would not now be even approximately the same as the original 

 restriction of the Cretaceous in Iowa, nor, as nearly the same as are the 

 boundaries of the earlier formations. Again, on account of the texture of 

 the Cretaceous the exposures soon became covered with debris, even though 

 at the close of the glacial period they were yet bare. Therefore, only rarely 

 will faces of rocKs be left to view. This is the case not only inland but 

 along the bluff's of large and small streams. 



White has probably given more attention to the inland exposures of 

 Cretaceous than any one else. In addition to the Sioux City region he 



