742 ARTHUR C. TROWBRIDGE 
CONCLUSIONS 
The mounds and ridges in Jo Daviess County, Illinois, are rem- 
nants of a plain which is either an old peneplain or a structural 
plain, and in the mind of the writer the latter interpretation is 
more likely to be correct than the former. It is probable that the 
district remained under the sea for some time following the deposi- 
tion of the Niagaran rocks now found here, and that emergence 
took place some time after the Niagaran epoch. The region was 
then subjected to erosion, and a broad plain was developed on the 
hard Niagara dolomite. Finally this formation was cut through 
by the major streams, and the Maquoketa shale underlying was 
rapidly removed. The streams reached grade at the level now 
represented by the Galena plain, which in this district happened to 
lie in the upper part of the Galena formation and the lower part of 
the Maquoketa shale. Before the mounds and ridges on this plain 
had been brought low, probably in Tertiary time, an uplift occurred 
and the streams were again allowed to deepen their valleys in the 
old plain. 
It is realized that a close study of these plains over wide areas 
will be necessary before a complete understanding of them is 
obtained. The whole driftless area seems to be the field of work. 
The writer hopes to do something more toward the solution of the 
problem at a later date. | 
