no.lsre] 270 [Marrh 21, 
valleys, as for instance those in the CoiinectuMit valley just to 
the soutli of lirattleboro, Vermont, shows just such a variation 
in the size of the ])avticles exposed in sections of terraces. 
It is often found in the t(M'races of the Connecticut valley 
that the material suddenly changes from fine to coarse as one 
ascends the terrace escarpment. This seems to me to be well 
explained if we imagine that the terrace deposits were formed 
in many cnses in temporary lake basins, caused by a glacial or 
accidental barrier down stream. If th:it barrier should suddenly 
be removed, as well might be the case, the rapidity of the 
stream would at once l>e increased and it would begin to drop 
coarser fragments than had hithei'to been deposited at that 
idace. The same fact has been stated by Mr. Robert Chalmers of 
the Canadian geological survey when he says, "the materials are 
water worn gravels and sand with a few small boulders, and 
usually become coarser toward the summit. The lower terraces 
nre usually capped with loam of various depths. Boulder clay 
often underlies them especially in the upper slopes of vnlleys.''^ 
The theory given above for accounting for glacial terraces is a 
m.odification of the plan proposed by Mr. Chalmers, which is as 
follows : — 
"Tlie generally accepted theory that river terraces have been formed at 
the close of the glacial period and subsequently denuded in part, as the 
volume of the water decre.ased and became confined to narrower channels, 
thus cutting down into their original deposits, seerns to be only partially 
correct and to require modification and extension. Moreover this theory 
fails also to take into account the fact that these valleys, in certain 
places, were partially or wholly filled with masses of glacial drift during 
the ice age, sufficient to block them up nearly to the general level of the 
country on both sides. This is evidenced by the accumulations still 
occupying them in places, causing the diversion of the rivers from their 
preglacial channels. A more reasonable solution of the problem seems 
therefore to be, that on the final retreat of the ice sheet of the glacial 
period, the river valleys, or such portions of them as were then open 
between the embankments of glacial drift referred to, would be re- 
occupied by the precipitation of the river basin. These waters in portions 
of the river valleys form lakes or chains of lakes all tending to over- 
flow by the lowest passage which in most cases would be aloug the 
course of the preglacial valley. ' Erosion and the tilling up of these lake- 
* Report on the surface geology of southern New Brunswick. Geol. aiitl naf. hist. 
surv. Can. Ann. rept., new ser., vol, 4, for 1888-89, p. 60, N. 
