THE PRAIRIE DU CHIEN -ST. PETER UNCON- 
FORMITY IN IOWA. 
ARTHUR C. TROWBRIDGE. 
Unconformable relations between the Prairie dn Chien and 
St. Peter formations have long been known in Minnesota, Wis- 
consin, Illinois and Missouri. The existence of this unconfor- 
mity in Iowa, though long suspected, has never been demon- 
strated, or if it has been known to exist here, the fact has not 
been recorded. There is nothing in the reports of the State 
Geological Survey suggesting anything but conformable rela- 
tions between the two formations. 
The purpose of this paper is to place on record various evi- 
dences of unconformity at this stratigraphic horizon within the 
boundaries of the state. The data here presented have been 
gathered within the last few years during the progress of field 
work in that part of the state commonly known as the Drift- 
less Area. 
The Prairie du Chien and St. Peter formations outcrop 
abundantly in the rough topography of Allamakee, Clayton and 
Dubuque counties, There are hundreds of exposures of each 
formation in this part of the state. In spite of this fa n t, the 
writer does not know of a point where the exact contact between 
the two formations can be seen. The dolomite is resistant and 
the overlying sandstone is so non-resistant that it washes down 
over the contact and obscures it. This doubtless explains why 
the discovery of the unconformity was so long delayed. How- 
ever, it is possible to get the altitude of the contact approxi- 
mately in a great many places. 
There are several evidences of unconformable relations be- 
tween the Prairie du Chien and St. Peter formations in Iowa. 
(1) The contact between the two formations is irregular. 
The top of the St. Peter and the base of the Prairie du Chien 
are parallel and dip almost uniformly in a south by southwest 
direction at an amount of about seventeen feet to the mile. But 
between these two horizons the interformation contact is found 
at all stratigraphic positions from just above the base of the 
Prairie du Chien to just below the top of the St, Peter. The 
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