MODIFIED DRIFT IN NEW HAMPSHIRE. 15 
Plains and Terraces. The extensive level plains and high terraces 
which border our rivers, constituting the most conspicuous and by far 
the largest portion of our modified drift, were also deposited in the 
Champlain period. The open valleys became gradually filled with great 
depths of horizontally stratified gravel, sand, and clay, which were brought 
down by the glacial rivers from the melting ice-sheet, or washed from 
the till after the ice had retreated, and which were deposited in the same 
way as by high floods at the present time. The departing ice-sheet was 
the principal source both of the vast amount of material and of water for 
transporting it into the valleys, which appear in most cases to have been 
filled to the level of the highest terraces or plains. The prevailing hori- 
zontal stratification of these deposits shows that they were spread. over 
large areas by the current of the floods which held them in suspension. 
The modified drift thus increased in depth in the principal valleys through 
a long period, which may have continued until the last of the ice at the 
head of the valley and of its tributaries had disappeared. 
Tue TERRACE PERIOD. 
During the recent or terrace period the rivers have been at work exca- 
vating deep and wide channels in this alluvium. The terraces mark 
heights at which in this work of erosion they have left portions of their 
successive flood-plains. As soon as the supply of material became insuf- 
ficient to fill the place of that excavated by the river, a deep channel was 
gradually formed in the broad flood-plain. The process was very slow, 
allowing the river to continue for a long time at nearly the same level, 
undermining and wearing away its bank on one side, and depositing the 
material on the opposite side, till a wide and nearly level lower flood-plain 
would be formed, bordered on both sides by steep terraces. When the 
current became turned to wear away the bank in the opposite direction, a 
large portion of this new flood-plain would be undermined and re-depos- 
ited at a lower level; but the direction of the current’s wear might be 
again reversed in season to leave a narrow strip, which would then form 
a lower terrace. In this way the Connecticut river, along the greater 
part of its course on the west border of New Hampshire, has excavated 
its ancient high flood-plain of the Champlain period to a depth of from 
one hundred and fifty to two hundred feet for a width varying from one 
