The Buchanan Gravels. — Calvin. 77 
Iowan drift sheet was spread over northeastern Iowa after the 
gravels were in place. 
These sands and gravels are now so incoherent that they 
may be excavated easily with the shovel, and yet there is no 
evidence that the glaciers that transported the overlying boul- 
ders and distributed the Iowan drift cut into them, or dis- 
turbed them, to any appreciable extent. The Iowan ice sheet 
was probably thin, and all the loose surface materials in front 
of its advancing edge were frozen solid. The thickness of the 
gravels is somewhat variable owing to the uneven floor upon 
which they were deposited, but it ranges from fifteen to 
twenty feet. The beds have been worked out in places down 
to the blue clay of the Kansan drift. 
Throughout the gravel bed, but more particularly in the 
lower portion of it. there are numerous boulders that range in 
diameter up to ten or twelve inches. These boulders are all 
of the Kansan type. Fine grained greenstones predominate. 
Proportionally large numbers of them are planed and scored 
on one or two sides. Those that are too large to be used as 
ballast are thrown aside on the bottom of the excavation, and 
in the course of a few seasons many of the granites and other 
species crumble into sand. The contrast between the decayed 
granites of the Kansan stage and the fresh, hard, undecayed 
Iowan boulders in the drift sheet above the gravels is very 
striking. Many of the boulders from the gravels are coated 
more or less with a secondary calcareous deposit, a feature 
not uncommon among boulders taken directly from the Kan- 
san drift sheet in other parts of Iowa. 
As to their origin, the Buchanan gravels are made up of 
materials derived from the Kansan drift. As to age, they 
must have been laid down in a body of water immediately be- 
hind the retreating edge of the Kansan ice. There are reasons 
for believing that the Kansan ice was vastly thicker than the 
Iowan. but the temperature was milder, and so when the 
period of melting came enormous volumes of water were set 
\'v>-i-. That strong currents were developed is evidenced by 
the coarse character of the material deposited as well as by 
the conspicuous cross-bedding that characterizes the whole 
formation. Some of the larger boulders found at various lev 
els throughout the beds were probably not directly trans 
