Fig. 74.— Quartz implement, found by Miss P. E. Babbitt, 1878, at Little Falls,
Minnesota, in modified drift, fifteen feet below surface : a, face view ; 6,
profile view. The black represented on the cut ie the matrix of the quartz
vein (No. 31, ,323) (Putnam).
But that it was ice-laden water which flooded the banks is
shown by the frequent occurrence of large blocks of stone
in the deposits, such as could have been transported only



256 MAN AND THE GLACIAL PERIOD.
in connection with floating ice. Tlie boulders in the
Columbia formation clearly belong to the individual river
valleys in which they are found, and doubtless are to be
connected with the flooded condition of those valleys
when, by means of a northerly subsidence, the gradient of
the streams was considerably less than now.
There is some difference of opinion in respect to the
extent of this subsidence, and, indeed, respecting the
height attained by the Philadelphia brick-clay, or McGee's
Columbia deposit. Professor Lewis (whose residence was
at Philadelphia, and who had devoted much time to fleld
observations) insisted that the deposit could not be found
higher than from 180 to 200 feet above the immediate
flood-plain of the river valleys where they occur. But,
without entering upon this disputed question, it is sufiB-
cient to consider the bearing of the facts that are accepted
by all — namely, that towards the close of the Glacial period
there was a marked subsidence of the land on the eastern
coast of North America, increasing towards the north.
