1877.] Surface Geology of the Merrimack Valley. | 539 
both of the vast amount of material and of water for sweeping . 
it into the valleys, which appear in most cases to have been thus 
filled to the level of their highest terraces. The prevailing hor- 
izontal 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 con- 
tinued till the last of the ice at the heads of the valley and of its 
tributaries had disappeared. 
During the recent or terrace period the rivers have been at 
work excavating 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 insufficient to fill the place of that 
excavated by the river, a deep channel was gradually formed 
in the broad flood-plain. This 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 de- 
positing 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 redeposited 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 Merrimack River through New 
Hampshire has excavated its ancient high flood-plain of the 
Champlain period to a depth of seventy-five to one hundred and 
fifty feet, for a width varying from an eighth of a mile to one 
mile. In Canterbury and Concord we see the highest plain is 
aing now undermined by the wear of the current, forming steep 
als e i 
The very fine character of the materials which compose the 
lowest terraces and the interval or present flood-plain is due to 
is wearing away and redeposition by the river, which have been 
many times repeated, till what may have been at first gravel be- 
comes very fine sand or silt. By each removal this alluvium is 
made one degree finer, and is deposited at a lower level and 
farther down the stream. The end of its slow journey is the 
Ocean, where it will help to make the sedimentary rocks of this 
epoch. It has completed a great cycle of changes, ending where 
the upheayed and contorted ledges from which it was derived had 
their remote beginning. 
