100 SURFACE GEOLOGY. 



found in Massachusetts is shown near the head of Nashua river, along 

 the railroad between Fitchburg and South Ashburnham. These kames 

 lie in north-west to south-east ridges parallel with the valley. When 

 they were being formed we must suppose that the ice had gone from the 

 lower east and north-east portions of the river's course, and that the 

 floods of water supplied from the melting ice-sheet at its source were 

 then completing the deposition of these extensive plains at its mouth. 



At the same time floods were here poured into the Merrimack from 

 the north-west, where no stream now exists. A continuous belt of allu- 

 vium, upon which the Wilton Railroad is built, extends six miles from 

 the Souhegan river in Amherst to the plains of the Nashua river. Its 

 narrowest place, three miles from the city, is a third of a mile wide, 

 while its widest portions, in the north-west corner of Nashua and south 

 part of Amherst, are a mile and a half wide. These plains show a grad- 

 ual descent from north-west to south-east, amounting to 75 feet in the six 

 miles. They consist of levelly stratified sand and gravel, and in general 

 have a very regular surface; but several ponds, often with no outlets, 

 fill depressions upon their widest portions, as Stearns pond in Amherst, 

 Pennichuck pond near South Merrimack, and Round pond in Nashua. 

 Deposition probably took place very rapidly from floods which brought 

 down the material from the melting ice-sheet. In some cases masses of 

 ice may have remained where we now find these ponds, or they may be 

 due to an unequal supply of material and varying currents. The waters 

 of the Souhegan valley at this period found their way to the Merrimack 

 by three routes. One was along the present course of this river, which, 

 below its extensive plains in Amherst, is narrowly enclosed at two points 

 by high land of till or ledges ; a second, similar to the first, was along 

 Pennichuck brook; while the third, which differs from the others in its 

 ample width and direct course, brought the greater part of these floods 

 to the same mouth with the Nashua river. In this way the flood-plains 

 of the last route appear to have become slightly higher than along the 

 present Souhegan river and Pennichuck brook, which therefore became 

 the channels of drainage after the Champlain period. 



A half mile below the mouth of Salmon brook, hills approach nearly to 

 the river, beyond which is a plain of similar height with that south of 

 Nashua river. In the remaining three miles to Tyngsborough the allu- 



