234 
IOWA ACADEMY OP SCIENCE 
Topography taken from the following named sheets. 
Des Moines, Iowa. Milo, Iowa. 
Pella, Iowa. Savanna, lowa-Illinois. 
Fig. 6. — Topographic maps showing river terraces. 
larly related terrace is very evident;^ found not only along the 
river itself but also along the tributaries. 
THE TERRACE DEPOSITS. 
Near Avon the surface deposits of the terrace are very sandy; 
but farther up the valleys of the tributaries the surface of the 
terrace is composed of a dark material, frequently without evi- 
dent stratification, and always without pebbles. Evidently it 
has been washed from the upland and deposited along the' val- 
leys. Indeed the process is very evident in the spring time 
where the drainage has been disturbed by the winter’s ice. This 
gumbo” is like material in the upland, but should not be con- 
fused with it. The undisturbed deposit in the upland is a 
2Note the location of Sabula. See also, S. Calvin, “Geology of Winneshiek 
county,” Iowa Geological Survey, Vol. X,VI (1906), pp. 55-66. The depth of 
sediment in the valleys of northeastern Iowa is stated by James H. Lees in 
the Proceedings of the Iowa Academy of Science, Vol. XXI (1914), p. 176, 
and by A. C. Trowbridge in the same volume, p. 209. 
