110 IOWA STUDIES IN NATURAL HISTORY 



in Jo Daviess County, Illinois^ in the Sparta district- and 

 along the Wisconsin river" in Wisconsin, and in many of 

 the valleys of Iowa, notably in the valley of Village Creek. 

 These terraces are formed by resistant layers of rock at 

 various stratigraphic horizons, as for instance, certain re- 

 sistant sandstone layers in the Potsdam formation, the 

 Mendota limestone member, the cherty member of the Ga- 

 lena formation, and the calcareous beds in the upper Ma- 

 quoketa. They are purely structural, occur at different 

 levels, cannot be correlated with either the Dodgeville or 

 the Lancaster plain, and cannot be used as evidence of more 

 than one cycle of erosion in the Driftless Area. 



In several of the main tributaries of the Wisconsin river, 

 notably in the valley of the Kickapoo river and the valley 

 of Pine creek, there are distinct and almost continuous ter- 

 races which slope gently downstream. They consist of non- 

 resistant rock, and bevel the layers of the Potsdam sand- 

 stone. The valleys have a double appearance, there being 

 a narrow, rock-bound valley within a much wider, older 

 one. These terraces are also due indirectly to structure. 

 The Wisconsin river flows west and south with a gradient 

 considerably less than the slope of the strata in that direc- 

 tion, so that its bed is on progressively younger strata to- 

 wards its mouth. Although it has now penetrated the re- 

 sistant Prairie du Chien formation where it joins the Mis- 

 sissippi, there was a time when its lower course was in this 

 resistant formation and its upper course and its tributaries 

 were on the Cambrian sandstone. Under these conditions 

 the degradation was so much slower on the Prairie du Chien 

 formation than was possible upstream on the sandstone that 

 a temporary grade was established and maintained on the 

 sandstone. There the main and tributary streams developed 

 broad, open valleys with flat bottoms. When the resistant 

 dolomite at the mouth of the river was finally cut through 

 the sandstone beneath it was excavated rapidly and the 

 streams above the resistant rock were allowed to intrench 



1. Trowbridffe. A. C. and Shaw, E. W., Bull. No. 26, III. Gcol. Surv., pp. 144-5. 



2. Martin, Lawrence, Bull. Gcol. Soc. Am., Vol. 28, pp. 148-149. 



3. MacClintock, Paul, The Wisconsin River beticecn Prairie du Sac and Prairie du 

 Chien, manuscript so far unpublished. 



