^fan and the Glacial Period. 197 
cial. Prof. Chamberlin is strongly of the opinion that it is inter- 
glacial, and that this immense erosion marks the separation in 
time between the first and the second glacial periods. Some idea 
of how great the lapse of time would be may be obtained b}' com- 
paring the generally acknowledged postglacial gorges, like those 
below the falls of Niagara and the falls of St. Anthony, which 
are onl}' a little over seven miles long, with this supposed inter- 
glacial gorge, which is deeper than the others mentioned, and 
from 150 to 200 times as long. 
The theor}- which I, with many others, have maintained is, that 
this gorge of the Ohio is preglacial, having been worn with con- 
siderable rapidity during the continental uplift which culminated 
contemporaneousl}' with the climax of the glacial period, when 
the general elevation was considerably more than now; that then, 
during the Champlain subsidence, which increased northward, the 
gorge was filled with water and to a considerable extent with 
glacial d('ljris, when these deposits were made upon the rocky 
shelf of the old base-level. Mj' theory of the glacial dam at 
Cincinnati was at first thought liy the Penns^'lvania geologists to 
give much assistance in simplifj'ing the problem and in account- 
ing for the facts, and while doubtless it was at first worked for 
rather more than it was worth, it is by no means clear that it has 
been entirely disproved or rendered entirely useless. 
The most important crucial test which 1 have heretofore pre- 
sented was that of Beech fiats, in the northwestern part of Pike 
county, Ohio, at the head of the Ohio Brush creek. The facts 
respecting this with a map were detailed in ni}- Bulletin (oS) of 
the United States Geological Survc}', pp. !I2-*J(), and repeated in 
m}- volume on "The Ice Age in North America,'' pp. 382-3:55. 
These flats lie in front of the glacial boundar}', where it passes 
over into the headwaters of the Ohio Brush creek. It is from 900 
to 1,000 feet above tide, and from 4(1(1 to 500 feet above the 
Ohio river at the nearest point. It is evidently a deposit from 
glacial streams when; they entered still water. Otherwise Ohio 
Brush creek would have been lined with glacial terraces through- 
out its extent, as all other streams similarh' situated are. But 
there is no such distriltution of gravel down the Ohio Brush 
creek, as ordinarily characterizes the streams which flow south- 
ward from the glaciated area. Evidently from some cause there 
was here a still water level which was maintained luitil the ice 
