268 
IOWA ACADEMY OF SCIENCE 
KEMARKABLE PRAIRIE SYNCLINORIUM. 
CHARLES KEYES. 
Geotectonics of great plains regions seldom offer very much 
attraction for structural studies. Large plains are almost uni- 
versally tracts especially notable for their deep soils. Rock- 
exposures are few in number and unimportant. There are no 
marked contrasts of relief. These lowlands are frequently true 
peneplains the surfaces of which are either still lying near sea 
level or are hut recently raised only slightly above it. 
The upper Mississippi basin is just such a region as that 
postulated. Its surface is .so deeply covered by glacial debris, 
wind-borne loess, and fine soils that the bedrock of entire coun- 
ties is hardly map able even approximately. The superficial in- 
equalities are so small, the slopes of the streams so slight, and 
much of the substructure so soft that rock outcrops are infre- 
quent and give little clue to the attitude of the bedded terranes 
beneath. Beyond the expressed belief that the strata of the sub- 
structure are fiat-lying or only slightly inclined mention is sel- 
dom made of the regional tectonic features. 
Recent years witness a great relief to the difficulties of in- 
terpretation of the geologic structures of this region. Numerous 
deep-wells put down in quest of good water supplies sufficient 
for municipal purposes indicate clearly the larger features of 
tectonics. Many of these borings go down distances of 2,000 
feet or more. Inasmuch as the principal aquifer of the Iowa 
region is the St. Peter sandstone deep-borings endeavor to sink 
to this horizon. By connecting the various boring records along 
different lines the formations between the St. Peter sandstone 
and other well known layers are also more or less readily and 
accurately determined. The resulting sections disclose the fact 
that there is well-defined flexing that is far from being so simple 
and so slight as is commonly supposed. 
As elucidating some of the broader tectonic features of the 
prairie region certain of these geologic cross sections are par- 
ticularly instructive and suggestive. The line of one section 
connecting two insular outcrops of very old rocks, or pre- 
Cambric formations, passes entirely across the western part of 
