298 
PETER SANDSTONE 
were probably buried beneath overlapping limestones, and the 
land-mass, now well worn down, could contribute but little sand 
to the Canadian seas of the Mississippi Valley region, and that 
little even more completely sorted than before. The emergence 
in post-Cambrian time first exposed a surface capable of yielding 
but little sand, but as the northern fringe of the Canadian or 
Beekmantown beds was gradually removed, and supplied calcare¬ 
ous material to form the Everton formations, a broad belt of 
Potsdam sandstone was likely ultimately to be uncovered. This 
seems actually to have occurred, since, as already pointed out pre- 
Peterian erosion in one area in central Wisconsin even now shows 
the Peter sandstone resting on the Cambric Jordan sandstone. 
With such a belt of Potsdam sandstone exposed along the 
border of the crystalline mass as now fringes the pre-Cambrian 
shield in central Wisconsin, but farther to the north of course, 
conditions would be particularly favorable for maximum wind 
action. While some sand would continue to be contributed from 
the crystallines to the north, much would now be derived from the 
newly-exposed Cambric beds. This material, already once sorted 
over under very favorable conditions for rounding and removal 
of clay, would now undergo a second rounding and sorting of the 
same character, and when laid down as Peter sands might be 
expected to show a remarkable degree of purity. 
It is believed that the sea, in which the Peter beds were laid 
down, entirely submerged the Ozark region, and that the Peter 
sandstone once entirely covered that area. There are two chief 
lines of argument to substantiate this view. The first is that 
Peter formation maintains its high degree of purity everywhere 
around the present Ozark border. Had there been large areas 
of Canadian beds exposed to erosion on an Ozark island, the 
bordering Peter sands would surely have carried much clay and 
considerable cherty gravel, of the sort that the Ozark streams are 
now transporting in such abundance from the surface of the 
former. The second line of evidence is the wide distribution 
over the Ozark upland of patches of sandstone very similar in 
grain to the Peter formation. In fact, much of it was called 
Peter sandstone until the discovery of pockets of coal associated 
with it, and locally of Mississippian fossiliferous chert boulders 
beneath it, since which date it has been classed as Coal Measures. 
