SOURCE OF SUPPLY OF MATERIAL 243 



rock; but for such a deposit all the sources in this case are marginal, ex- 

 cept perhaps locally in Missouri ; and of all the margins only the northern 

 one seems to fulfill the conditions of a coastal supply. Along this north- 

 ern coast there was at all times a broad and thick accumulation of sancU 

 from the waste of the continent. It lapped against the retreating shore 

 in such position and in such condition that any considerable diastrophic 

 movement elevating the land would subject great quantities of this loose 

 material to the outwash of the sea and other transporting agencies. Any 

 differential movement along this region might also subject underlying 

 strata to erosion along the margin to the extent of destroying them, as 

 is represented by Eominger in northern Michigan. Likewise any oro- 

 graphic movement or displacement by faulting, resulting in the elevation 

 of any considerable portion of the subcoastai margin, would still further 

 augment the supply of these sands for seaward transportation. 



In this connection it is recalled that there is extensive displacement by 

 faulting within the area of the basal sandstones — Eastern and Western 

 sandstones — of the Lake Superior margin. These movements were at 

 least subsequent to the deposition of the chief mass of these formations, 

 since the beds still remaining are much affected along this fault zone.* 



It is not clear what time this faulting took place, but certain field rela- 

 tionships suggest that the displacement may have begun before the Lake 

 Superior sandstones, as we now have them, were completed. At any 

 rate, the raised blocks must have been capable of furnishing considerable 

 excess of sand over regular erosion sources and must have resulted in 

 some extension of the sand sediments seaward. The Saint Peter is the 

 last of these sand extensions and certainly owes its existence to one or 

 another of these dynamic factors, perhaps to all of them. 



In the areas of no faulting in eastern Wisconsin the "Potsdam" thins 

 out northeastward, according to the Wisconsin geologists, not by general 

 thinning of all the beds, but by suppression of the lower members. It is 

 worth while to note that this would not be true under normal conditions, 

 except so far as overlap prevailed, and in case of retreat and advance with 

 destruction of underlying beds. 



To this phase of the problem the present condition of the underlying 

 Shakopee dolomite adds evidence. The Saint Peter lies on the billowy 

 surface of the Magnesian limestone, filling up its troughs and in most 

 cases surmounting its prominences. In given instances 54 and 82 and 

 100 feet of sandstone was observed in these troughs, while adjacent knolls 

 of magnesian dolomite had scarcely more than a film of sand to mark 



* U. S. Grant : Bulletin vi, Geol. Survey, Wisconsin, p. 17. 



