170 
The American Geologist. 
September, 1894 
less area, modified but slightly by glacial action. The ice 
seems to have passed over the country without abrading tin- 
rock surface, except oil the crests of the highest hills. The 
amount of till or boulder clay deposited in the region is in- 
significant, but the chief obstacle to an examination of the 
stratified rocks is presented by a mantle of loess, which, 
though rarely exceeding 8 or 10 feet in thickness, is continu- 
Fig. 1. Map of the Elk Horn creek area of St. Peter sandstone. 
ous over hilltop and valley bottom alike. Here and there, 
however, modern erosion has removed this loess on steep hill- 
sides, especially on the slopes underlain by the sandstone. 
The geographical extent of the outcrop of each formation, 
especially of the sandstone, can be largely determined by the 
topography alone. Beginning with the Galena limestone rim 
of the basin (resembling a basin when viewed from surround- 
ing hights), we find it characterized by an undulating topog- 
raphy with comparatively gentle slopes. This has been 
effected by glacial action. Descending to the Trenton lime- 
stone we find the ravines narrower, with steeper slopes, and 
outcropping rocks more numerous. Still lower, we can readily 
determine the situation of the sandstone by comparatively 
narrow, very steep-sided valleys, with all the angles beauti- 
fully rounded off. instead of being sometimes sharp and an- 
gular as on the surrounding limestone areas. In short, the 
