MODIFIED DRIFT IN NEW HAMPSHIRE. 9 
tains reached nearly or perhaps quite to the line of perpetual snow, while 
farther northward they rose far above this line. The very low tempera- 
ture which this must cause would seem to make it improbable that the 
changed proportions of heat received from the sun, such as to produce, 
if no ice existed, a mild winter and a cool summer, could melt this vast 
mass of ice and bring a temperate climate in its place. It is certain that 
this or some other cause partially melted this ice at times, and that it 
afterwards advanced, covering the territory from which it had retreated; 
but the work which the ice-sheet accomplished, the length of time requi- 
site for its formation, and the low temperature of the altitude to which 
it reached, render it improbable that it was several times wholly melted 
away, alternating with warm inter-glacial periods. The view here taken 
is, that the glacial period was principally produced by the last great 
eccentricity of the earth’s orbit, the changed proportions of heat re- 
ceived from the sun in the different seasons of the year favoring the 
accumulation and preservation of vast sheets of ice, which existed in the 
northern and southern hemispheres at the same time. 
Formation and Distribution of Till, The till or coarse glacial drift 
was produced by the long-continued wearing and grinding of the ice 
sheet. As this slowly advanced, fragments were torn from the ledges, 
and a large part of these were sooner or later held in the bottom of the 
ice, and worn to small size by friction upon the surface over which it 
moved. The resulting mixture formed beneath the ice is variously called 
the ground moraine, boulder-clay, or Lower Ti//, It consists of smoothed 
and striated stones, with fine detritus, which is usually a gravelly clay 
of dark bluish color, being always clayey, dark, and very hard and com- 
pact. The characteristics of the lower till are due to the mode of its 
formation. Most of its pebbles and boulders are glaciated, having 
rounded edges and smoothly-worn sides, which often retain striz. These 
show that the finer material in which they occur has been produced by 
the slow grinding up of these stones under the ice. The dark and usu- 
ally bluish color is due to seclusion from air and water during its forma- 
tion, as pointed out by Torell, leaving its iron principally in the form of 
ferrous silicates or carbonates. Its compactness and hardness are due to 
compression under the great weight of ice. Because of this quality, the 
lower till is commonly known as “hardpan.” 
VOL, III, 2 
