234 
IOWA ACADEMY OF SCIENCE 
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The Nebraskan drift of the region to the southwest along the Missouri 
river has been described 2 as a dark bluish-black, tough joint clay con- 
taining a few pebbles and boulders and which when dry breaks up with 
a starch-like fracture into small angular fragments. The Nebraskan 
drift in this region along the Little Sioux differs somewhat from that of 
the region to the southwest. Here the color is gray, rather than black, 
and tints of various colors, chocolate, brown, purple and blue modify this 
gray color. The most usual surface colors are a chocolate-gray or a pur- 
plish-gray and generally the color darkens as you dig beneath the surface 
to the fresher material. The drift is almost free from pebbles or sand 
grains and is so fine-grained that usually very little grit is detected when 
a small piece is bitten between the teeth. Calcareous material is very 
abundant in the till, and often occurs as concretions, which resemble 
pebbles. These concretions range in size from small grains to masses 
8 or 10 inches across, and are particularly abundant in the weathered 
portion of the Nebraskan. The till is compact and moderately tough 
when very fresh, but in the ordinary surface .exposure it is weathered 
and can be dug easily with the hammer. 
At the contact horizon of Kansan over Nebraskan there is usually a 
transition zone, of 4 to 6 feet or more* grading upward into the Kansan 
drift and downward into the Nebraskan. This zone represents the 
Nebraskan material plowed up by the Kansan ice, and mixed with the 
Kansan drift, but which did not lose all of its Nebraskan characters. 
At a few places, however, the contact is a definite horizon so that hand 
specimens may be taken which will show both Nebraskan and Kansan 
drifts. This is true at the exposure noted above in southwest 4 of 
Cherokee township. 
In a few of the exposures studied the Nebraskan is "overlaid by a 
horizon of grayish -black material very similar to the drift, being usually 
very fine-grained and without grit. It contains calcareous concretions, 
even more abundantly than the Nebraskan, and at several places con- 
tains shells of small pond snails. An excellent exposure of this material 
may be seen in the southeast part of section 3, Cherokee township. This 
horizon is apparently of Aftonian age. It is overlaid by the Kansan 
either directly, or through a transition zone, composed of a mixture of 
the silt horizon and the Kansan drift. In the dozen or more exposures 
studied which include both Nebraskan and Kansan drifts, no gravel 
was found between the drifts and there is apparently no Aftonian gravel 
in the region, a; point which is noteworthy in view of the great develop- 
ment of gravel material in the Aftonian farther southwest where it has. 
2 Shimek, B., Science, vol. 31, p. 75. Also Iowa Geol. Survey, vol. 20, p. 307. 
