THE 
AMERICAN GEOLOGIST. 
Vol. XVII. APRIL, 1896. No. 4. 
APPARENT ANOMALIES OF STRATIFICATION IN 
THE POSTVILLE WELL. 
By Samuel Calvin, Iowa City, Iowa. 
A well recently bored at Postville, Iowa, reveals some inter¬ 
esting geological phenomena that may not be unworthy of 
consideration by those who have occasion to interpret the 
records preserved by drillers of deep wells. The well in ques¬ 
tion was bored to supply the incorporated town of Postville 
with water. Postville is located in the southwest corner of 
Allamakee county, near the margin of the Driftless Area. 
There is some drift in the neighborhood; but the superficial 
deposits within the limits of the town are such as were laid 
down along the free margins of the Iowan ice sheet, and con¬ 
sist of loess and sub-loessial sands, which here overlie residual 
clays and cherts derived from the preglacial secular decay of 
the Galena limestone. Beneath Postville the indurated rocks 
are, first, Galena limestone; second, Trenton limestone; third, 
Saint Peter sandstone; fourth, the different members of the 
Oneota limestone, embracing the equivalents of the Shakopee 
limestone, New Richmond sandstone and Main Body of lime¬ 
stone of the Minnesota geologists; and fifth, the Saint Croix 
sandstone, the upper part of which includes the Jordan sand¬ 
stone and St. Lawrence limestone as described in the geological 
reports of Minnesota. The formations named are all exposed 
in the bluffs along the Yellow river. The stream flows in a 
deep, canon-like valley, which may be reached about four 
