130 IOWA ACADEMY OF SCIENCE 



THE LOWER PLAIN. 



There are considerable areas of fl.a1 land throughout the region 

 which lie at distinctly lower levels than the flats referred to the 

 upper plain (see the lower of the two straight lines in figure 11). 

 In the Baraboo district west of Devils Lake, the lower plain is 

 extensively developed at 1200 feet altitude, or 200 feet lower than 

 the upper plain in the same locality. In the Richland. Center 

 quadrangle there is a remarkably flat area, in places almost un- 

 touched by stream work, which is seven miles long and as much 

 as two and one-half miles wide in places. South of the Wiscon- 

 sin river the lower plain is well developed at Lancaster where 

 it has an altitude of 1100 feet, and still farther south it is ideally 

 represented in the Elizabeth quadrangle at 975 to 900 feet. This 

 plain has been traced in Iowa by Trowbridge and Williams from 

 the Minnesota line to Dubuque. In the correlation of these va- 

 rious patches of the lower plain and in assigning their origin to 

 peneplaination, the following facts are taken into consideration : 



(1) The plain has numerous erosional remnants above it, for 

 wherever the remnants of the upper plain occur in the forni of 

 mounds or ridges they rise above the lower plain surface, in 

 most places as much as 200 feet or even more. 



(2) This plain is in no way influenced by the strata of resistant 

 rock, since it cuts across formations dipping at varying angles, 

 the slope of the plain being remarkably uniform. In the Bara- 

 boo district the flat at 1200 feet above sea level cuts across the 

 Baraboo quartzite formation which has dips of 15° or more. In 

 the adjacent area to the west at the same elevation, the plain 

 lies upon Prairie du Chien dolomite. South of the Baraboo dis- 

 trict, the plain next cuts across the Galena dolomite at Lancaster 

 at an elevation of 1100 feet, and finally in the Elizabeth quad- 

 rangle it is found upon five to fifteen feet of soft Maquoketa 

 shale. In Iowa the plain cuts from the Prairie du Chien forma- 

 tion at the Minnesota line, across the St. Peter, Platteville, 

 Decorah, and Galena formations, to the Maquoketa formation at 

 Dubuque. 



Over the area studied this plain dips at the rate of about four 

 feet per mile in a direction 26° west of south. Thus it is seen 

 that it dips at an angle smaller than that of the upper plain ; 

 accordingly the two plains draw T more nearly together as they 

 are projected to the southwest, Moreover, both plains have 



