U2 
ARTHUR C. TROWBRIDGE 
The upper 
Fic. 1.—A diagram showing the relations of the Niagara and Galena plains to each other and to the rock structure of the district. 
The uppermost rock formation is Niagara dolomite and the 
Between the two is the Maquoketa shale, represented by dashes. 
dotted line represents the Niagara plain, and the lower dotted line the Galena plain. 
lowermost is Galena dolomite. 
extensive and definite plain midway 
between the skyline and the valley 
bottoms. Its surface is broken by 
mounds and ridges above it, and by 
many sharp valleys below it. For 
convenience this may be called the 
intermediate plain. The mounds and 
ridges which stand on the inter- 
mediate plain and whose tops form 
the upland come up to a somewhat 
common level, in such a way that 
if the low land were filled up to their 
tops the region would be a nearly 
flat plain. The upper dissected plain 
lies everywhere on the Niagara: 
dolomite, and the intermediate plain 
follows more or less closely the 
surface of the Galena dolomite; 
hence the upper plain is here called 
the Niagara plain and the inter- 
mediate surface the Galena plain. 
These two plains, the first now 
almost entirely dissected and the 
latter somewhat but less so, con- 
stitute the problems of this paper. 
Their general relationships are shown 
qn bles te 
The Niagara plain was known to 
Hershey’ as “‘ Peneplain No. 1,” and 
he considered it to be a true pene- 
plain partly dissected, of Cretaceous 
age. Bain? mentions the plain and 
seems to agree with Hershey con- 
cerning it. The upper plain was 
also known to Grant and Burchard 
who state that “‘the facts observed 
t Am. Geologist, XVIII, 75-78. 
2 Bull. U.S. Geol. Surv. No. 204, p. 15. 
3 Lancaster-Mineral Point Folio, U.S. Geol. 
Surv., p. 2. 
