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 Till. 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 strias. 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 



