Glacial Drift of the Basin of Iowa. 975 
stone, This boulder line has been traced by us from a point about 
one mile northeast from Charles City, in Floyd county, to a little 
east of Waterloo, a distance of about fifty miles. Below this point 
we have not traced it; neither have personal observations of it been 
made beyond Charles City. It has been reported to me, however, 
that from Charles City the boulder line extends in an easterly direc- 
tion to a point near Fort Atkinson, in Winneshiek county, and 
there turns and pursues a more northerly course. From Charles 
City this boulder line takes a general southeast course, passing about 
two miles east of Nashua, in Chickasaw county, where it attains its 
greatest observed width, four miles. From this point it assumes a 
somewhat more easterly direction, and gradually diminishes in width. 
At a point midway between Richland Centre and Frederica, it 
again turns and pursues a more southerly course to Waterloo; the 
direction corresponding in the main with the general trend of the 
Wapsipinecan and Cedar rivers. In some localities, for example, 
Richland Centre, Frederica, ete., breaks in this line occur, from 
one-fourth to one-half mile in extent, where the erratics are rela- 
tively rare, but which soon appear again in strong force, An 
interesting and instructive feature of these boulders is, that they are 
all (so far as noticed) deangulated, smooth, and very hard, while 
large numbers of them are flattened and striated on one side by gla- 
cial action. This feature is shared in to no such extent by the 
erratics of other portions of the area. The second recognized divi- 
sion of the later Glacial epoch is the Valley, or stratified drift. 
Of the river valleys of this region, only one is known to 
contain true Valley drift; and this the valley of the Shellrock. 
Although at different places along the course of some of the 
streams, local deposits of more or less distinctly stratified drift occur, 
still, as a rule, this feature is no more pronounced than at some 
points in the Upland drift. The Shelirock heads near Lake Albert 
Lea, Minnesota, and flows into this sheet of water at its upper 
extremity, and makes its exit at its lower of southern extremity. 
This sheet of water occupies a depression in the Inter-lobular 
moraine which enters Iowa. The Shellrock flows upon this 
heterogeneous drift accumulation for a distance of ten miles, and 
' Owing to the lack of other rock here for building purposes, the 
boulders are extensively used for underpinning, well and cellar walls, 
bridge piers, etc., for which they answer a most excellent purpose. 
