632 W. H. TWENHOFEL AND F. T. THWAITES 



quite commonly being present about 30 feet above the base. In 

 fully half of the occurrences they are composed of chert. Other 

 common fossils are small cephalopods, low-spired gastropods, and 

 shells of a chiton-like organism. These are generally poorly pre- 

 served, casts and molds being the only forms which have been 

 observed, and the writers — the chiton-like form excepted — -have 

 found them only in the chert. The writers have found no fossils 

 composed of dolomite except those of Cryptozoa. 



Conditions of origin. — The strata of the Oneota formation are 

 wholly of marine origin. They are not the deposits of deep water, 

 but in large part appear to have been deposited below wave-base. 



THE ST. PETER FORMATION 



The St. Peter sandstone occurs only as small patches which fill 

 erosion depressions in the Oneota. None has been discovered over 

 the northern half of the area. The most extensive occurrence is in 

 the southeastern corner of the Sparta and the adjacent part of the 

 Tomah quadrangle. Residual float from the St. Peter is quite 

 abundantly present over those parts of the uplands which are 

 underlain by the Oneota. 



Relations to underlying formations.— The most striking feature 

 of the St. Peter formation is the relief of the disconformity which 

 marks its base. The stratigraphic position of the St. Peter in this 

 region is above the Shakopee formation; but that formation, if 

 deposited over these two quadrangles, was removed before St. Peter 

 deposition, so that the St. Peter rests on the Oneota, except in one 

 exposure, where it was found to rest on the Jordan, thus giving to 

 the surface of disconformity a stratigraphic relief of fully 200 feet. 



Characteristics. — The St. Peter formation consists of loose, 

 friable, yellow to brown, fine- to medium-grained sandstone with a 

 variable thickness of red and green non-calcareous shales and fine- 

 grained shaley yellow sandstones in the basal portions. At every 

 place where the base of the stratified St. Peter was observed it 

 rests on a residuum of red clay and chert which is altogether without 

 stratification. This residuum was derived from the weathering of 

 the Oneota and possibly higher formations. In this article this 

 unstratified material is assigned to the St. Peter, although it 

 developed during the time of erosion which intervened in this area 



