MODIFIED DRIFT IN NEW HAMPSHIRE. 17 
vary in size, the largest sometimes covering an acre or more, with their 
thickest portions from ten to fifteen feet in depth. These dunes appear 
to have been swept up from the broad plains of the Champlain period, 
before forests had fully covered the land, by the strong north-west winds, 
which we may suppose prevailed the same then as now. That this is a 
true explanation of these high banks of sand appears to be proved by the 
fineness of their material, which contains only particles such as could be 
carried by the wind; by their frequent occurrence on the east side of the 
valleys, where they would be formed by the prevailing strong north-west 
winds, while they are not found on the opposite side; and by the train 
of sand-drifts usually grassed over, which may be traced down in a north- 
west direction from the banks of sand now blown by the wind to the 
normal modified drift. Since the clearing away of the forest, the upper 
portion of these trains of sand has sometimes been carried several hun- 
dred feet onward, and from thirty to fifty feet higher. The excavation 
of the old drifts has been six or seven feet in depth, as shown by great 
stumps, beneath which the sand has been swept away. These dunes are 
ridged, channelled, and heaped up by the wind in the same manner as 
the more extensive dunes of a sea-coast. 
Modified Drift overlaid by Till. About Winnipiseogee lake beds of 
stratified clay are often found underlaid and overlaid by till. The clay is 
free from pebbles, and well suited for brick-making. It varies from five 
or ten to thirty feet in thickness, and occurs at various heights from the 
level of the lake to three hundred feet above it. The overlying till is from 
two or three to ten or fifteen feet in thickness, wholly unstratified, and 
very coarse, containing numerous boulders, which may be five or six feet 
in diameter. These remarkable clay-beds probably accumulated during 
the departure of the ice-sheet, in spaces melted under the ice, between it 
and the lower till. 
Modified Drift near the Coast. About Dover, and southward near the 
sea-coast, thick deposits of modified drift, sometimes forming extensive 
plains, are found occupying areas of water-shed from one hundred to two 
hundred feet above the streams, which often flow in wide valleys that are 
nearly destitute of modified drift. Some of these, as the high plains of 
coarse gravel and sand about Willand and Barbadoes ponds, near Dover, 
seem to have been produced by the rapid deposition of materials brought 
VOL, Ill. 3 
