526 Surface Geology of the Merrimack Valley. (September, 
different degrees of coarseness. Southward, banks and terraces 
of sand begin to appear, but gravel still predominates for a long 
distance below. ‘The stream here frequently occupies a broad, 
shallow channel paved with pebbles of all sizes up to two feet in 
diameter, with little admixture of fine gravel or sand, which ac- 
cumulates only in deep or sheltered places. 
For ten miles south from the mouth of East Branch, or nearly 
to the south line of Thornton, a high terrace of gravel or sand is 
commonly well shown on both sides of the river, and has a uni- 
form, continuous slope of fifteen feet to the mile. This slope is 
nearly the same as the descent of the river, which has evidently 
swept away this deposit to a depth of from seventy to one hun- 
dred feet over the area occupied by its channel and bordering 
bottom-land or interval. Nowhere else in New Hampshire is 
_ the erosion of the modified drift, by which it has been shaped in 
terraces, so clearly displayed. Here it seems certain that a former 
flood-plain, ten miles long, has been terraced as we see it by the 
excavation of the river. 
In Campton the Pemigewasset receives two considerable trib- 
utaries from the east, Mad and Beebe rivers, which drain basins 
on the northwest and southeast of the mountain range that cul- 
minates in Sandwich Dome. South of the Beebe River the upper 
terrace, increased in height by ‘alluvium from the tributary, 
forms a pine-covered plain, one mile long and a half mile wide. 
These ‘pine plains,” appearing in a few places on the Pemige- 
wasset and commonly along the Merrimack, form one of the char- 
acteristic features of this valley. 
In Plymouth and Holderness both the high plain and interval 
are finely shown, and the extent of the alluvial area, at one point 
a mile and a half wide, is greater than at any other place on 
Pemigewasset River. 
Dunes. Inthe north part of New Hampton and in many 
places for thirty miles southward to the north line of Concord, 
we find numerous dunes or sand-drifts lying at various heights 
on the east side of the valley up to three hundred feet above the 
highest terraces. These dunes appear in large amount and f 
their greatest height near their beginning, two miles south 0 
Ashland, Here the sand-drifts, one to five feet deep, are strewn 
in a pathway ten to twenty rods wide, which extends a quarter 
of a mile along the hill-side, with a northwest-southeast cours® 
rising three hundred feet above the ordinary modified drift, or to 
a height about eight hundred and fifty feet above the sea- 
