N.S. Shaler—Fluviatile Swamps of New England. 217 
which emerged from beneath the ice while its base lay in the 
sea. As the sub-glacial streams normally had their points of 
discharge at the lowest parts of the ice front, the greater part 
of the kame gravels were deposited in the depressions of the 
sea floor which have since become river valleys. But a share 
of this debris found its way to the higher lying districts and 
occasionally occupied positions on the divides between adjacent 
valleys.* 
When the surface of this district rose above the level of the 
sea the movement took place suddenly as is shown by the pre- 
servation of the delicate features of the kames, especially along 
the sea shore as well as by the absence of old benches of levels 
above the plane of the present shore. The elevation probably 
brought the level of the land to a position considerably above 
its present altitude, as is shown by the submerged forests 
along the shore of Massachusetts Bay and on Nantucket, where 
there has been a down sinking of ten to twenty feet in depth 
since the post-glacial elevation. 
During this period of post-glacial elevation, when this part 
of the continent was somewhat higher than at present, the val- 
leys of all the streams were, to a greater or less extent, cleared 
of their incumbrance of drift. It is during this period that the 
valleys of the rivers which flow from south to north were ex- 
posed to the measure of erosion which we find indicated in 
them. This condition of the surface must have endured for 
some thousands of years in order to have permitted the removal 
of such extensive masses of debris as have been cleared from 
these valleys. As before remarked, the erosion of these valleys 
as far as it is measured by the abrasion of the original drift de- 
posits, the kame terraces and other glacial materials, is nearly if 
not quite as extensive in the valleys whose streams flow to the 
‘ northward as in those that decline to the south. It is evident 
that the elevated condition of the continent endured long enough 
to permit the complete re-establishment of the drainage in the 
drift-encumbered valleys of this region, a process which could 
not have taken place with the present declivity of the streams, 
When the change in the position of the land came about it led 
to the relative lowering of the southern part of New England 
and a corresponding relative increase in the height of the north- 
ern part of this section of the shore. It is not perfectly clear 
whether the movement actually lifted the northern districts 
above their previous level or no, but it seems likely there was 
a positive sinking of the southern section. 
It is probable that the submerged forests and swamps of 
southern New England before and after, referred to, were 
depressed beneath the sea level at the time of this movement, 
* Compare on the Origin of Kames, by N.S. Shaler, Proceedings Boston Soc, 
Nat. Hist., vol. xxiii, pp. 30-44. i 
