66 The Saint Peter Sandstone. 



limit of the formation that must have existed at the time of 

 its deposition. It probabh^ extended a considerable dis- 

 tance northward. On the other hand the general southern 

 extent is so deeply buried below the earth's surface that the 

 limits to the southeast, south and southwest are undis- 

 covered. Our knowledge is in fact confined to a relatively 

 small fraction of the Saint Peter. The surface exposures as 

 well as the area in which these exposures are found are 

 narrow, from reasons that will be given below. 



GEOLOGICAL DISTRIBUTION. 



So far as we know, the Saint Peter was prlmarilj^ coex- 

 tensive with the overlying Trenton (Galena) and the underly- 

 ing Shakopee. The now existing difference in extent is due 

 to erosion, which of course has worked upon the uppermost 

 strata to the greatest degree and it has caused the under- 

 most — Shakopee — to have a few miles widest extent north- 

 ward, as seen upon maps of the region. 



The inequality in extent of these three Lower Silurian 

 (Ordovician) formations is due very often to erosion that is 

 still going on, very much also to glacial and preglacial 

 erosion. In the latter case the once denuded area may be 

 entirely covered with drift. How long ago the preglacial 

 erosion took place is uncertain. An important and interest- 

 ing addition to our knowledge is afforded, further, by the 

 existence of a small area of Cretaceous clays (28, pi. 3) in. 

 Goodhue county, Minnesota, which lie upon the Saint Peter 

 and Shakopee, the Trenton and other formations having 

 been removed before the Cretaceous was deposited. 



The situation of this small Cretaceous area at a high 

 elevation, on top of the ridge near deep river valleys suggest 

 that the valleys have been eroded mainly since Cretaceous 

 time, and also its position upon an already eroded surface 

 many miles within the present outer limits of the Saint 

 Peter sandstone proves that the latter formation had been 

 greatly eroded even before Cretaceous time. We learn from 

 Worthen (12) that in a few exposures in La Salle county, 

 Illinois, the Coal Measures rest immediately upon the Saint 

 Peter sandstone ; from this it is to be inferred that the over- 

 lying Trenton had been removed during the Devonian or, 

 more probably, the early Carboniferous period. 



