300 
PETER SANDSTONE 
It seems quite probable that the low, flat condition of the north¬ 
ern land-mass at the close of the Peter deposition allowed the 
Lowville-Black River sea to lap far up on the old crystalline 
shield. When Decorah deposition set in there appears to have 
been some disturbance or slight uplift to the northwest, but not 
in an area that exposed any of the older sandstones to erosion. 
In any event, the source was such as did not yield sand-grains; 
and the Decorah terrane thickens appreciably to the northwest, 
to the point where it was cut off by Jurassic or Comanchan 
erosion. The top of the Decorah shale, where thickest in the 
vicinity of Minneapolis, may be the equivalent of the basal 
Galena dolomite farther south, the shale grading into limestone 
as the distance from the land source increased. 
Following Decorah deposition there seems to have been an 
even wider submergence during which the sea probably covered 
much of the Canadian shield from Wisconsin north to Hudson 
Bay and from Lake Temiscaming west beyond Winnipeg. This 
submergence, also, it is believed, entirely overspread the Ozark 
region, or if not, left only a very low island of limestone, not 
deeply eroded. The absence of any sandy layers such as would 
surely have resulted from erosion of the Peter sandstone makes 
it certain that that formation was not then exposed. 
Finally the area of the Mississippi Valley again emerged, and 
remained land during most of the time when the Eden and Mays- 
ville beds were being deposited. Wide submergence resulted in 
the laying down of a thin limestone, the Fern vale formation, 
over much if not all of the Ozark region and far to the south¬ 
west; while in the north, beginning simultaneously, or possibly a 
very little later, came the great influx of calcareous muds that 
produced the Maquoketan shales. This formation is more limey 
to the north, as represented by the Wykoff beds, and more sandy 
to the south, as indicated by the Thebes sandstone, and suggests 
a change in the direction of the source of sediments. Mid Ordo- 
vicic submergence seems to have closed a portion of geologic his¬ 
tory when the Canadian shield was an active source of important 
Paleozoic sediments. 
