EROSION AL HISTORY OF DRIFTLESS AREA 89 



forms gently rolling intermediate surfaces, so extensive and 

 so nearly flat as to be known locally as "prairies." Areas 

 representing the plain here are known south of Preston and 

 in other portions of Fillmore county, in Houston county 

 south of Root river near Caledonia, in nearly all parts of 

 Winona county and in the southeastern portion of Wabasha 

 county. Though the areas are most extensive away from 

 the Mississippi river, representatives of it are known right 

 to the edge of the river gorge. 



As in the case of the Dodgeville plain, the Lancaster 

 plain lies on different rock formations at different places. 

 North of the Prairie du Chien cuesta in Wisconsin it lies 

 on the Cambrian sandstone, in Minnesota on Platteville, St. 

 Peter and Prairie du Chien, south of the Prairie du Chien 

 cuesta on the Prairie du Chien and St. Peter formations, 

 south of the Platteville-Galena cuesta on the Galena and 

 Maquoketa formations. In general, the farther north a 

 portion of the plain is the older the formation, and the low- 

 er the part of the formation on which it lies. Progressively 

 younger rocks are bevelled by the plain toward the south. 



The Lancaster plain slopes in a general southerly direc- 

 tion at an angle less than the angle of dip of the strata. 

 The details in the relations of the plain and the structure 

 south of the anticlinal axis are shown in the table on page 

 90. 



The computations, the results of which appear in the 

 table, show that there is a marked parallelism of plain and 

 strata in several of the individual districts, such as the 

 Richland Center, Sparta and Mineral Point quadrangles, 

 but that in other districts and where greater distances are 

 involved, this parallelism fails. It is notable that all the 

 local estimates show the intermediate surface sloping in 

 directions west of south, which is the direction of dip of 

 the strata and that those estimates including more widely 

 separated points on the plain show a general slope east of 

 south. Considering only the local districts the plain appears 

 to have an average slope of 10.9 feet per mile in the direc- 

 tion S 17 W and the strata dip S 28 W at an angle of 16.6 

 feet per mile. Over the larger areas the average direction 



