LOSS Wik HORIZONTAL, SHEARING PICANES, 
WHILE engaged in field work on Pottawattamie county for 
the Iowa Geological Survey in the summer of 1900, a peculiar 
structure in the loess came under my observation. This appeared 
to me so unique as to merit special notice. 
The loess in Pottawattamie county is of the usual western 
type. It is lighter in color, slightly coarser, and of a more open 
texture than the loess in the eastern part of Iowa and in Illinois. 
It is also heavier, and contains a greater number of fossils than 
the latter. In the east bluffs of the Missouri river it averages 
not far from a hundred feet in thickness in this county, and 
occasionally this measure is exceeded. It frequently contains 
fossils. In the north part of the county, and sometimes also in 
the south, it rests on a somewhat darker and more ferruginous 
deposit. This is similar to loess in appearance, but is less cal- 
careous, in places sparsely pebbly, and much less pervious to 
water, owing to the presence among its particles of an exceed- 
ingly fine and silty ochreous ingredient. It resembles in gen- 
eral the ‘““gumbo” which has been described by Leverett? as 
occurring under the loess in some parts of Iowa, Missouri, and 
Illinois. This gumbo changes downward into the old leached 
and weathered upper part of the underlying till. Its upper sur- 
face is sometimes marked by an old soil horizon. 
The peculiar structure noted in the loess involves the horizon 
where the two formations just described come into contact ; the 
level where the gumbo changes, gradually or abruptly, upward 
into loess. Perhaps it would, be more accurate to say that it 
affects the top of the gumbo as well as the base of the loess. 
Examining closely one day the lower part of the loess in an 
excavation southeast from the Pierce Street School in Council 
Bluffs (near the corner of Voorhees street and Franklin avenue), 
the lower part of the embankment was seen to be laminated, and 
tU.S. Geol. Surv., Mon. XXX VIII, p. 28. 
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