REVIEW OF THE CHAMPLAIN AND TERRACE PERIODS. 175 
the Champlain period, our rivers were swollen as we see them now in 
the floods of spring, but they were laden with far greater amounts of 
alluvium. The valleys were thus gradually filled with modified drift, 
which took the same slope with the descending current. In some in- 
stances (pp. 65, and 116-119) there is evidence that the ice-sheet in its 
departure was for some time a barrier, holding back lakes where there 
are now empty valleys, sloping to the north or west. If the principal 
lines of drainage throughout the state had been in these directions, such 
lakes must have been frequently formed during the retreat of the ice 
towards the north-west. With these exceptions, the deposition of our 
valley drift appears to have taken place in the same manner that addi- 
tions are made to bottom-lands by high floods at the present day. 
The modified drift, though extensive, constitutes only a part of the 
material which was contained in the ice-sheet. Probably more escaped 
erosion than was carried away by the glacial streams. When the ice 
was wholly melted, the part remaining fell upon the striated ledges and 
ground-moraine, being the loose upper till, with numerous angular boul- 
ders, which forms the surface generally throughout the state. Outcrop- 
ping ledges, however, are frequent ; and both the kames and valley drift 
rest upon the upper till. 
The modified drift deposits show that the ice retreated slowly, and 
with varying rates of progress (pp. 36, 39, 121, 122, and 149). Evidence 
of its reddvance has been found in only a few places, near the coast (pp. 
163 and 164). The ice-sheet appears to have continued uninterruptedly 
through a very long period (pp. 7-9). Doubtless it resisted the influence 
of the warmer climate and changed conditions before which it disap- 
peared, continuing late like the snow in spring. Its departure at the 
last was correspondingly rapid; and the hardier forms of vegetable and 
animal life were soon established near its retreating margin. 
During the recent or terrace period the work of deposition by the 
streams has not been equal to that of erosion, and they have excavated 
deep and wide channels in the Champlain deposits (pp. 15 and 16). In 
this process terraces have been formed, sloping with the stream. Neither 
the deposition nor terracing of the modified drift requires any submer- 
gence, as by lakes or the sea. These deposits have the form which they 
must naturally take, in being rapidly brought into the valley by floods, 
