THE ORIGIN OF THE ST. PETER SANDSTONE. 
ARTHUR C. TROWBRIDGE. 
Originally, all sedimentary rocks were thought to he marine. 
When the St. Peter sandstone was first recognized as a distinct 
formation, it was assumed to have had a marine origin. More 
recently, however, the marine origin of many sediments has been 
doubted, and the criteria for distinguishing various sorts of sedi- 
mentary rocks have been worked out. As early as 1907 evidences 
were presented for the eolian origin of the St. Peter sandstone, 
although there are those who have never accepted the evidence as 
conclusive. In the literature of the subject, the matter is not 
settled. 
In connection with field work in the Driftless Area during the 
last twelve years, the writer has had opportunity to study the 
formation in many places and to collect evidence bearing on the 
problem of its origin. The conclusions arrived at are here re- 
corded. 
The characteristics of marine sediments deposited in agitated 
water and of eolian deposits have been listed. 1 Reference to these 
lists will help to render the present argument clear. 
The St, Peter sandstone certainly has some of the characteris- 
tics of eolian deposits. The material is sand of uniform texture 
and of a size which is commonly transported and deposited by the 
wind. The formation contains so few fossils that many geologists 
believe that it contains none. No wind-deposited sand contains 
abundant fossils. The thickness of the sandstone formation 
varies greatly within short distances, as is true of all eolian de- 
posits. There are places where an irregular stratification ap- 
pears in the sand, which suggests eolian stratification. The 
shapes of the sand grains, when seen under the compound 
microscope, are not notably different from the shapes of sand 
grains taken from existing sand dunes. 
There are, however, other features of the sandstone and other 
interpretations of the above-mentioned points, which are in har- 
mony with the marine rathei tlian with the eolian theory. These 
points are discussed in separate paragraphs. 
1 Trowbridge, A. C., Jour. Geol. Vol. XXII. pp. 422-3, 432, 435. 
