PRAIRIE DU CHIEN-ST. PETER UNCONFORMITY IN IOWA 181 
stone which occurs down in the valleys, below the general sur- 
face of the Prairie du Chien, is soft, friable, highly and va- 
riously colored. It breaks to pieces in the fingers and can be 
excavated easily with the chisel-edge hammer. Some of it is 
massive, but most of it is so bedded as to weather out in small, 
thin, wavy scales. The upland phase of the formation, on the 
other hand, is massive, firmly cemented, and gray. These dif- 
ferences within the formation are due doubtless; to the different 
conditions which existed in the valleys and on the divides during 
the early part of the St, Peter stage. 
This unconformity also explains the discontinuity of the New 
Richmond member of the Prairie du Chien formation. In Min- 
nesota the formation is subdivided into the Oneota dolomite at 
the base, the New Richmond sandstone above that, and the 
Fig. 19 — Diagram explaining the discontinuity of the New Richmond 
sandstone in Iowa. 
Shakopee dolomite at the top. The basis for this division is 
the New Richmond sandstone separating the two dolomites. It 
has been a source of worry to some workers in Iowa, because 
this sandstone, although it occurs at many places, is clearly 
wanting in the section in other places. The pre-St, Peter ero- 
sion period resulted in the removal of Shakopee, New Richmond 
and part of the Oneota, where the main valleys were, so that the 
St. Peter was deposited here on Oneota and there on Shakopee. 
The New Richmond is missing where the Prairie du Chien for- 
mation is thin, and present where the formation is thick. At 
the Clayton sand pit the New Richmond is thirty-four feet from 
the top of the formation where it is thickest. Two and one-, 
half miles west of McGregor the sandstone is fifty feet below 
the top of the formation. This explanation of the irregular 
occurrence of the New Richmond is made clear in figure 19. 
