PRAIRIE SYNCLINORIUM 
269 
our state. It extends from Sioux Falls, South Dakota, south- 
ward or southeastward to the crest of the Ozark dome, in south- 
east Missouri. This is a section about 600 miles lon^. From the 
north end to a point east of Kansas City the section coincides 
closely with the course of the Missouri river, affording the best 
surface exposures in all the region. Along this line are an un- 
usual number of deep-wells having good records of the forma- 
tions penetrated. 
This cross section presents the form of a great trough, end- 
ing at each extremity in an old mountain ridge. Something 
of an adequate conception of the grand proportions of this 
structure is gained from the statement that the middle of the 
syncline is depressed a distance of more than two miles below 
the ends. Each of the raised extremities of the section is a part 
of a notable mountain ridge which in early Mesozoic times 
trended northeast and southwest across the Upper Mississippi 
Valley region. ^ 
The northern one of these ranges is now designated as the 
Siouan mountains. Its features are figured forth in a recent 
paper read before this Academy. The geologic history of this 
remarkable erogenic elevation is, briefiy, this: Since all the 
Paleozoic formations take part in the arching while the Cretacic 
rocks do not, it is quite evident that the main movement or up- 
rising occurred in early Mesozoic time. At the beginning of 
Comanchan deposition (Early Cretacic), when this portion of 
the continent was a land area, the country was again completely 
base-leveled, the Siouan arch as well as the lower lands. Upon 
this even plain, worn out on the bevelled edges of the ancient 
strata, which was then gradually carried beneath sea-level, 
sediments were laid down during Mid Cretacic times. These 
are the deposits which cover the northwestern portion of our 
state and out of which peeps the crestal remnant of the old 
arch, called by us the Sioux Quartzite area. 
The Siouan mountains were rapid in formation and rapid in 
their decline. At the time of their highest stage their crests 
probably stood 3,000 to 4,000 feet above the surrounding coun- 
try. They were greatly diversified. In the Black Hills, the 
Ozarks, and the Appalachians of today we find their nearest 
counterparts. 
A similar history obtains for the Ozarks. In early Mesozoic 
times they too were notable positive features of landscape. As 
