294 
PETER SANDSTONE 
out of the sea large areas of already deposited marine sands to the 
north of the limestone facies, allowing them to drift and become 
further rounded and sorted. The chief rounding was not pro¬ 
duced in this way, since the formation consists of well-rounded 
grains quite to its base. This emergence appears to have been 
followed by a tilting which submerged the area to the south and 
uplifted the source of supply somewhat, and it was following 
this episode that the great bulk of the Peter sandstone was laid 
down. 
In a formation of the character of the Peter sandstone, it is 
difficult, if not quite impossible, to say with certainty just where 
the shore was at any given time. That the Peterian sands often 
encroached from their supply ground upon the surface of the 
old Canadian or Beekmantown tracts before the sea covered the 
entire area, is certainly to be expected; and this was probably ac¬ 
complished both by streams and by dune-drifting. Such local 
dune areas may never have been completely worked over by the 
waves. Twenhofel describes cross-bedding which he thinks is 
probably of eolian origin. Cross-bedding is perhaps more highly 
developed in Wisconsin than in Missouri. This would be true, 
even though the formation was entirely marine, because in the 
near-shore zone of more active wave-work, cross-bedding would 
be the more to be expected. Nevertheless it would not be sur¬ 
prising, even though the Peter sands were dominantly marine, 
to find some dune-structures in this northern area. It is already 
pointed out that along the trunk streams, dune-drifting may have 
gone some distance on either side. Elsewhere than along main 
drainage lines it was probably of only very local extent. Unless 
we assume a profound modification of the earth’s axis of rota¬ 
tion, the area under discussion must have been then, as now, 
under the prevailing westerly winds. Within this area, at the 
present time, there is little evidence that sands would drift south¬ 
ward in large quantity. In fact, in discussing the origin of the 
Sylvania of Michigan, it has been postulated that it was derived 
from the Peter, and older, sandstones of Wisconsin, driven by 
the wind from west to east. If the prevailing direction of the 
wind in Monroan time was from the west, surely no logical 
reasons can be advanced to show why it was any different when 
the Peter sands were deposited. 
The objection has been raised that a sea encroaching over 
