PARTLY DISSECTED PLAINS IN JO DAVIESS COUNTY 741 
rather closely, and seems to disappear where that formation dips 
beneath the surface. However, close study of stratigraphy in 
connection with this flat shows the coincidence of flat and struc- 
ture to be not so striking after all. In southwestern Wisconsin, 
Grant and Burchard" find evidence of a dissected peneplain on the 
upper part of the Galena formation, which now has an altitude of 
1,000-1,100 feet. In the northern part of Jo Daviess County this 
surface is above a few feet of Maquoketa, and the shale formation 
gets slightly thicker under the flat to the south. That is, to a 
small degree the flat does lie across the beveled edges of different 
formations. These relationships are shown in Fig. 1. The Galena 
plain is believed by the writer to have been a true but only partly de- 
veloped peneplain, but one somewhat controlled by rock structure. 
The feature which seems at first to militate against the pene- 
plain interpretation is the disappearance of the flat where the 
Galena dolomite dips below the surface. However, this may be 
explained. If the peneplain was developed on the Galena and 
Maquoketa formations as shown in the diagram (Fig. 1), the 
Maquoketa shale was thickest under the southern part of the plain. 
When the plain was uplifted and the streams were rejuvenated, the 
shale was attacked most vigorously downstream. ‘The shale being 
soft, its dissection resulted in speedy destruction of the plain where 
the shale was thick; where lacking, the plain was preserved longer. 
Even where there was a little shale under the plain the main streams 
cut through it rapidly, and then were held to slow cutting by the 
hard Galena dolomite. Working in the hard rock, they did not 
send out tributaries as readily as they did in the soft shale farther 
south, and the flat was protected. It is believed then that the 
plain is a partly developed peneplain, the southern part of which, 
after uplift, has been destroyed because the shale which underlay 
this part was exposed and not resistant to erosion. The plain is 
more distinct in the central part of the district, and in the north- 
ern part it is very well preserved, because of the hard dolomite 
near or directly under it. Possibly also the dolomite had some 
influence in the development of the plain as well as in its modifi- 
cation. 
t Lancaster-Mineral Point Folio, U.S. Geol. Surv., p. 2. 
