Modified Drip and the Champlai/t Epoch. — Upliam, 321 
This term was used by the same author four years later, 
in the report on the Geology of Vermont (vol. i., p. 56). The 
definitions in each of these works imply that the deposition of 
the glacial drift antedated that of the modified drift. The ori- 
gin of both through glacial action, the former directly, the lat- 
ter indirectly, and their essentially contemporaneous deposi- 
tion, were not yet well understood; but the imperfection of 
knowledge, at that time, concerning the processes by which 
the drift was produced, should not invalidate the judicious and 
well defined classification and terminology. To me it seems 
just as suitable to preserve this name, modified drift, as if the 
origin of the drift through the agency of land ice, and the con- 
ditions of aqueous modification of large parts of it, had been 
completely ascertained when the name was first proposed. It 
has a priority of forty-two years, in comparison with the term, 
"wash group," which is proposed by Woodworth for the same 
grand class of drift formations. 
In my report on the '"'Modified Drift in New Hampshire," 
the derivation of this water-laid drift is attributed to the ice- 
sheet; and its subsequent sculpturing from broad and thick 
flood-plains into terraces in the large river valleys is shown to 
have been in progress from the closing part of the Ice age to 
the present time, although it was doubtless mostly done soon 
after the ice retreated. The characteristic development of the 
modified drift, as seen in the Connecticut, Merrimack, Andros- 
cogin, Saco, and other valleys is described as follows, with ref- 
erence to the change of interpretation of the conditions of its 
origin : 
The deposits included under this title are the vvaterworn and strati- 
tied gravel, sand, and clay or silt, which occur abundantly in almost 
every valley in the state. These river-lands comprise the intervals, 
which are annually overflowed at the high water of spring, and succes- 
sive terraces which rise in steps upon the side of the valley, the highest 
often forming extensive plains. 
The origin and distribution of these materials present many interest- 
ing questions. When the term was first employed, it was the prevailing 
opinion that modified drift was gradually formed. from the unmodified 
glacial drift by the ordinary action of rain and streams It it evident, 
however, that the high terraces and wide plains bordering our rivers 
were formed by much greater floods than those of the present time, 
laden with vast quantities of alluvium. Both the materials and the 
water for sweeping them into the valleys appear to have been supplied by 
