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. III. 3 



