116 
IOWA ACADEMY OF SCIENCE Vor. XXIX, 1922 
of its valley exposures of Fort Dodge beds, Des Moines shales 
and St. Louis and Ste. Genevieve beds alternate along the walls. 
It is in this valley that the gypsum may be seen at its lowest known 
levels. While it is found nowhere in actual contact with the 
Mississippian beds it lies so low in several places and its relations 
to the Mississippian limestones are such as to make it very probable 
that there is little if any vertical interval at these low points be- 
tween gypsum and limestone. The sections in Plate I wilDshow 
this situation quite clearly. Lizard creek empties into Des Moines 
river from the west about a third of a mile above the mouth of 
Soldier creek. Limestones, sandstones and marls of Missis- 
sippian age rise thirty to sixty feet above the water at various 
places along the banks of the creek and in several localities are 
seen to be overlain by Coal Measures shales. Above these again, 
at an old clay pit on the north side of the valley, gypsum may be 
seen in large blocks beneath the glacial drift. 
Now this region about the junction of these two creeks with 
the main stream is a critical area in connection with the structure 
of the Fort Dodge beds and the fact that the Mississippian strata 
rise above water level at so many points and are known to be just 
under the valley filling at several others is perhaps the most sig- 
nificant fact of the local stratigraphy. It shows that erosion of 
the Mississippian surface had proceeded so far in pre-Pennsyl- 
vanian time that irregularities of contour amounting to seventy- 
five feet and perhaps as much as a hundred feet had been devel- 
oped but no faulting of the strata occurred , during either this or 
subsequent periods of time. 
As has been stated already the Fort Dodge beds lie in a broad 
shallow basin eroded in the underlying strata. But into this basin 
there seems to have been cut a fairly deep and steep-walled valley 
which probably extended from northeast to southwest. After this 
valley and the larger depression were filled with gypsum and the 
overlying sandy shale and sandstone the whole region was planed 
down by erosion in the long post-Permian-pre-Pleistocene inter- 
val, with possibly an incursion of marine conditions during Upper 
Cretaceous time. During the development of the present drainage 
system Soldier creek has cut its valley irregularly across the old 
gypsum- and shale-filled basin in such a way as to expose the 
filling, at some points high up in its walls and underlain by Des 
Moines beds, ad other points low down, in the deep valley, and 
presumably lying on Mississippian beds. This accounts for the 
