974 Glacial Drift of the Basin of Iowa. 
As belonging to the later Glacial epoch, we recognize three divi- 
sions: the Upland, or (for the most part) unstratified drift ; Valley, 
or stratified drift; and Loess. The Upland drift constitutes by far 
the greater bulk of the superficial covering of the region. Its 
material is mainly true till, with local deposits or pockets of coarse 
gravel and sand, which is sometimes slightly cemented together by 
calcareous cement, and large numbers of transported erratics. 
There also occur local deposits of stiff red clay; for example, 
near Floyd, Nora Springs, and Rockford, in Floyd county. 
These deposits of clay are usually quite free from erratics, gravel 
or sand, has a soapy feel, and becomes very hard upon short expo- 
sure to the-atmosphere. The gravel deposits of this division are 
not restricted to any definite horizon, but are irregularly distributed 
through it. The material is usually well rounded pebbles of several 
varieties of greenstone, quartz (much of it probably of Cretaceous ori- 
gin), granite, Devonian limestone, jasper, etc. There also some- 
times occur specimens of coal, native copper and iron,! as well as 
* Devonian and Trenton fossils, Juarge numbers of deangulated 
granite and greenstone boulders, and more rarely those of quartz, 
are seen distributed over the surface and mixed with the till. The 
boulders vary in weight from two pounds to ten or twenty tons; 
of the latter, fine examples are observed near Portland, Osage, 
Nashua, and other places. i 
n some parts of the area, angular fragments of Cretaceous 
conglomerate are not uncommon in the drift. This material 
was probably derived, at least in part, from the southwest por- 
tion of Floyd county, where rocks of this age are known to occur 
in place.? The material of this division is usually unstratified, but 
at times it is observed to be obliquely and discordantly stratified at 
high points, distant from waterways. 
The region of most numerous erratics is that of the eastern portion 
of the area. A strip of country from one-half to four miles in width 
is occupied by great numbers of boulders, mostly of granite and green- 
1 The occurrence of these ores, ete., in the drift of this region, was 
noted by the writer in a paper on “The Glacial Flow in Iowa,” which 
appeared in the August number of this Journal for 1887. 
? A description of this rock has been given by us in a paper oD “A 
Description of the Rockford Shales of Iowa,” which appears in Vol. VI. 
of the Proceedings of the Davenport Academy of Science. 
