IOWA ACADEMY OF SCIENCES 
185 
2. In the Report on Plymouth county two points need modification. 
The first is the mistaking the chalky stratum of the Benton, now named 
Greenhorn for the Niobrara. Dr. Bain, in making the statement simply 
followed the interpretation which had been first made by Hayden, who 
named the Benton and Niobrara, and very naturally followed by all ob- 
servers after him, with the exception of White and St. John, who wisely 
used other names. 
The mistake was virtually corrected by Iowa geologists finding Ben- 
ton fossils above the limestone in Sioux county. Iowa Geol. Surv. X. p. 
114. 
The other mistake grew out of the first, viz; the top of the chalkstone 
is taken for the bottom of the till or Pleistocene (la. G. S. VIII, p. 331). 
The thinness of the chalkstone was ascribed to glacial erosion conse- 
quently Cretaceous shales 50-60 feet thick in some places, and forming 
a distinct bench along the face of the bluffs in section 13, T. 91, R. 49, 
was called till. This is somewhat excusable because the clay was weath- 
ered to a structureless mass near the surface and it was well sprinkled 
with pebbles and boulders from the drift beds above which are compara- 
tively thin and much stratified above. The Cretaceous clays are usually 
readily recognized by striking a pick into them. The clay a few inches 
from the surface is gritless and waxy. 
3. Another variant interpretation concerns the exposure at the old 
site of Otis’s mill on the Dakota side of the Big Sioux below Hawarden. 
The differences of reading are difficult to account for except by consider- 
able changes in the exposure. 
The Section given in Iowa Geol. Survey, Vol. VIII, p. 334, is as follows: 
G. Loess, sandy with many lime concretions 20 feet. 
5. Drift, new, fresh pebbles, yellow color, and with traces of soil at 
top 30 
4. Old soil, black to drab, sandy 2 
3. Sands, white to lemon yellow, very fine above, coarser and 
orange, below, 10 
2. Niobrara, principally thin shelly limestone, with few chalky 
streaks 35 
1. Shale, black to drab, with poorly developed laminations 20 117 
The writer has more recently studied the locality with the following 
results; 
Table of Exposures at Otis’ Mill. 
Top of hill back about 135 feet above river. 
Grassy slope mostly loess though pebbles may show as high as the top of 
the exposure. 
8. Loess, apparently in situ, though probably slipped 8 8 
7. Till, yellowish, though bluish in places 22 30 
G. Slope with loess and yellow till mingled 15 45 
5. Dark fossiliferous soil or mud, Planordis large and small, Pisid- 
ium Gyrena, Limnea 2 or 3 species, Yalvata, Unio^ large jaw 
of horse, rather larger than domestic, with three molars, 
thicker to south 7-3 48 
4. Yellowish loam and sand coarser below passing into gravel 
thicker northward 2-10 38 
3. Dark waxy shaly ciay mingled with till above, shown at ex- 
treme south 4-0 58 
2. Bluish chalk, with hard shaly limestone interstratified, softer 
above and below passing into shale 18 76 
1. Shale, laminated, black and blue 12 88 
Level of Big Sioux at medium stage. 
