20 
IOWA ACADEMY OF SCIENCES. 
INTERLOESSIAL TILL NEAR SIOUX CITY, IOWA. 
BY J. E. TODD AND H. FOSTER BAIN. 
Till and loess are well known deposits and need not be 
defined. It is a peculiar and anomalous relation between the 
two which it is proposed to describe in this paper. It is well 
known that the till almost universally occupies a position below 
the loess v/herever the two are found in the same section. A 
few cases only have been noted where the reverse is true. Such 
an instance was described by McGee and Call* in an account of 
the loess near Des Moines. Similar occurrences have been 
reported by other observers from central and eastern Iowa. 
These may all be readily explained by an advance of the ice 
sheet over the loess already deposited around its margin. 
The senior author of this paper has, during several years of 
observation, noted only three or four instances which could by 
any use of language be mentioned as illustrations of the subject; 
of these the one illustrated in the plate 1 is the only clear 
case. 
The first instance of interloessial till noted was in 1889 at 
Riverside Park, near Sioux City. This may still be seen in a 
gravel pit facing the Big Sioux river near its mouth. The notes 
taken at that time are as follows: 
4. Loess, thickening back from bluff — to 100 feet or more; above a few 
fresh water shells, including Lymnea and Cyclas; below one or two speci- 
mens of Heliy hirsuta. 
3. Till, brownish; with northern pebbles; disappearing a few rods far- 
ther north: containing fine sand blending with the loess. 
2. Compact, whitish, silt-like loess, containing Succinea similar to shells 
still living on the bluff above. The upper portion containing carbonaceous 
streaks and marks, suggesting marsh grass. 
1. Gravel, coarse: obliquely stratified, with occasional northern bould- 
ers. 
The most probable explanation of this occurrence seemed to 
be that number 1 marks a stage when the river was larger and 
*Am. Jour. Sci., (3), XXIV, £02-223, 1882. 
