MODIFIED DRIFT ALONG MERRIMACK RIVER. 



79 



below this normal line, which is here shown on the west side in the plains 

 north and south of Fisherville. 



Boscawen village is built on the south end of a similarly sloping terrace 

 three miles long, in which distance it falls 30 feet, and we find 30 feet 

 more fall of the same terrace in less than a mile along the village street. 

 The whole of this terrace is below the normal height, showing a defi- 

 ciency of 15 feet at its beginning, and of 40 feet at the north end of Bos- 

 cawen village. It appears as if the supply of alluvium was insufficient, 

 and the direction of the current at first caused it .^ 



to be deposited in greatest amount at one side, 

 without filling the valley. South of Boscawen the 

 supply of material became still more inadequate, 

 and the lower portion of the sloping plains east 

 of the river was probably 60 feet below the surface 

 of water, which was held back by the extensive 

 plains of Concord, derived in large part from the 

 Contoocook and Soucook valleys. 



Although the plains in Concord were obviously 

 brought in from tributary sources, they belong to 

 the ancient flood-plains of the Merrimack, since 

 they form a portion of the series of high terraces 

 and plains which extends with a slightly varying 

 but unbroken slope along this whole valley. Even 

 if no modified drift were supplied, except from the 

 upper part of the main valley, irregularities of ^ '^ 

 slope, as in Boscawen and Canterbury, with in- 

 creased height below, as in Concord, would still 

 be produced by an irregular rate of retreat of the 

 ice-sheet, allowing long and abundant deposition 

 in some portions, but much less in other portions 

 of the same valley. In this way we must explain < g 

 the sudden and permanent increase in height of f^" 

 the upper terrace of Connecticut river at North Thetford (p. 36). Prob- 

 ably this cause was combined with the aid of tributaries to produce the 

 high plains in Concord and southward. 



Between Plsherville and West Concord these plains have a large 



