BRIDGING THE AGES OF ICE 
353 
This spot on Capitol Hill, where first were obtained the de- 
positional proofs of the complexity of the Glacial Period, is for 
several reasons exceptionally instructive. It seems to be the 
first locality ever recorded in which the stratigraphical relations 
of two drift-sheets were unmistakable. It is also this section 
which later gave first intimation of the eolian origin of American 
loess loams. It is here- that was disclosed first clue to that 
wonderful interlocking of the continuous south-western loess 
and adobe deposits with the northeastern Glacial tills. This site 
bids fair long to remain one of the classic geological localities 
of the continent. 
At this time and at this distance there are few of us who can 
have any adequate appreciation of the almost unsurmountable 
difficulties which this novel problem once presented, albeit now 
it seems all so simple. Still fewer of us there are who can gather 
directly from experience what it really means actively and de¬ 
terminedly to contend on the skirmish-line of the unknown. By 
our distinguished/ colleague, the late W J McGee, than whom 
no one was in better position to know intimately the marvelous 
intricacies of the attempt to decipher the Glacial puzzles of that 
day, the procedure, so far as it concerns Iowa, is thus graphically 
portrayed : * * * “In the solution of the problem it is nec¬ 
essary to do more than assume the existence and action of a great 
sheet of ice hundreds or thousands of feet in thickness and hund¬ 
reds or thousands of miles in extent. In order to explain the 
sum of the phenomena it is necessary to picture the great ice- 
sheet not only in its general form and extent, but in its local 
features, its thickness, its direction, and its rate of movement 
over each square league, the inclination of its surface both at 
top and bottom, and the relations of these slopes to the sub¬ 
jacent surface of earth and rock; and all this without a single 
glacial stria, or inch of ice polish, save in one small spot, in the 
whole tract of 16,500 square miles. It is necessary to con¬ 
ceive not only the mode of melting of the ice at each league of 
its retreat, but also every considerable brook, every river, and 
every lake or pond formed by the melting, both at its under 
surface and on its upper surface; it is necessary not only to 
restore not only the margin of the mer de glace under each minute 
