THE ORIGIN OF THE ST. PETER SANDSTONE 
173 
On the other hand, sand could and would be so deposited if a 
sandy sea existed over the surface for a long time. 
The variation in thickness of eolian sand is due to the ir- 
regular piling up of the sand into dunes. It is most commonly 
the surface rather than the base of the deposit which is irregu- 
lar. Save for a slight structural dip the surface of the St. 
Peter formation is horizontal. Its variable thickness is due to 
unequal altitudes of its base rather than of its upper surface. 
Such variability could be obtained more easily under marine 
than under eolian conditions. 
The St. Peter sandstone is conformable with the Platteville 
limestone above. Between the sandstone and the limestone 
there is the Glenwood shale. The contacts between sandstone 
and shale and between shale and limestone are parallel with 
the general dip of all the strata and there is no evidence of 
erosion or other break in deposition on either contact. The 
change from sand to shale and from shale to limestone is nor- 
mal as a result of a gradually deepening or advancing sea. It 
is not clear that an eolian deposit could grade conformably up- 
ward into marine deposits. The Glenwood and Platteville are 
known to be marine. 
The stratification of the sandstone, as an evidence of its 
origin, is inconclusive. Indeed it is doubtful if sand deposited 
by the wind can ever be certainly distinguished from marine 
surf deposits by the means of stratification alone, Eolian sand 
is deposited on the lee slopes of sand dunes and assumes its 
angles of rest. These, slopes may be oriented in any direction. 
Similarly sand is dumped over the fronts and sides of deltas, 
bars, spits, hooks and barriers along irregular coast lines, and 
takes certain angles of rest. These slopes also are oriented 
irregularly. The only difference is that in the one case the 
sand is dry and in the other case it is wet. This difference 
would give rise to slight differences in the degree of dip in 
cross bedding. But this dip is influenced by so many other fac- 
tors, such as the sizes, shapes and specific gravities of the grains, 
and perhaps by the strength of air or water currents, that the 
presence or absence of water at the time of deposition might, 
well be obscured. For the most part the St. Peter sandstone' 
is massive and devoid of stratification lines. In a few places,, 
irregular stratification appears, but the writer has not beeir 
