536 Surface Geology of the Merrimack Valley. [September, 
of the glaciers in the Alps. These fields and rivers of ice, several 
hundred feet in depth, are found descending from the regions of 
perpetual snow, their rate of motion being one to five hundred feet, 
or even more in their steepest portions, ina year. Many angular 
blocks and fragments which fall from the bordering cliffs are car- 
ried along on the surface of the ice, or are contained in its mass, 
with others torn from the rocks over which it moves, and under 
its vast weight these act as graving tools to round and striate the 
ledges beneath. The similar striation of all northern countries 
and the formation of the till have been effected by a uniform 
cause, namely, a moving ice-sheet which overspread the continents 
from the north. 
This continental glacier had accumulated sufficiently deep to 
cover every mountain summit in New Hampshire. That it over- 
topped Mt. Washington is fully proved by recent discoveries of 
Professor C. H. Hitchcock, the state geologist. Its thickness far- 
ther to the north was so much greater than in this latitude that 
its immense weight caused the ice to flow slowly outward, and 
the direction of its current in New England was to the south and 
southeast. By this motion 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 a small size by friction upon the surface 
over which it moved. The resulting mixture formed beneath the 
ice is the ground-moraine or lower till. Its dark and frequently 
bluish color is due to seclusion from air and water during its 
formation, as pointed out by Torell, leaving its iron principally 
the form of ferrous silicates or carbonates ; and its compactness and 
hardness have resulted from compression under the great weight 
of ice. While this deposit was thus accumulating beneath the 
ice, great amounts of material, coarse and fine, were swept 
away from hill slopes and mountain sides, and afterward carried 
forward in the ice. When this melted a large portion of the 
material which it contained fell loosely upon the surface, forming 
an unstratified deposit of gravelly earth and bowlders, called upper 
till. It will be seen that the upper member is the one usually 
exposed at the surface, and it is often the only one present where 
` only a thin covering of till is found. Its characteristics are the 
larger size of its bowlders, which are mostly angular and unworn ; 
the yellowish color of its fine detritus, produced by the hydrated 
ferric oxide to which its iron has been changed by exposure wa 
and water; and the comparative looseness of its whole mass. 
Frequently about Winnipiseogee Lake, and rarely elsewhere, de 
