IOWA ACADEMY OP SCIENCES. 
59 
granite boulders of massive size. Some of these boulders may 
be seen perched on the very margin of the pit, and some have 
been undermined in taking out the gravel and have fallen to the 
bottom. The surface of the whole surrounding region is thickly 
strewn with Iowan boulders. It is evident that the 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 boulders and 
distributed the Iowan drift cut into them, or disturbed 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 some- 
what 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 cJay of the 
Kansan drift. 
Throughout the gravel bed, but more particular; y 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. Pine grained greenstones predominate. Pro- 
portionally 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 directiy from the Kansan 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 behind 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 free. That 
strong currents were developed is evidenced by the coarse char- 
acter of the material deposited as well as by the conspicuous 
