4 SURFACE GEOLOGY. 
melting of an immense sheet of ice. These deposits thus had the same 
origin with the glacial drift; but they have been modified, being sepa- 
rated from the coarser portions, and further pulverized or rounded, and 
assorted in layers, by water. 
Tue GLAcIAL PERIOD. 
The indications of a glacial period abound in all northern countries 
whose geology has been explored; and in New Hampshire they are prob- 
ably as well shown as in any part of the world. Underlying the modi- 
fied drift we often find masses of earth and rocks mingled confusedly 
together, without stratification or any appearance of having been depos- 
ited in water. These are the glacial drift or z#//, Unlike the modified 
drift, till is distributed with no reference to lines of drainage, and fre- 
quently covers the slopes or lies at the summits of our highest hills and 
mountains. The boulders which it contains, or which lie upon its sur- 
face, are of all sizes up to ten feet, or rarely even twenty or thirty feet, in 
diameter; and in this state they have nearly all been transported south- 
ward from their native ledges. Where an outcrop of rock is so peculiar 
that its boulders cannot be confounded with those from other ledges, we 
may trace them southward or south-eastward, but not in other directions. 
They are abundant near their source, and diminish in numbers and size 
as we advance. The till of New Hampshire contains boulders which are 
thus known to have travelled a hundred miles. Wherever till occurs, it 
is also found that the ledges have been commonly worn to a rounded 
form; and, if the rock is sufficiently durable, it is covered with long par- 
allel scratches or s¢vig, which have the same direction with the dispersal 
of rocks in the till.) The same areas are also characterized by extensive 
deposits of modified drift. 
To explain these related facts was a most difficult task, which remained 
after nearly all other great questions in geology had been settled. The 
theory which has now been received by most who have studied this sub- 
ject was first brought out prominently by Agassiz in 1840, and was based 
upon his studies of the glaciers in the Alps. There fields and rivers of 
ice several hundred feet in depth are found descending from the regions 
of perpetual snow, their rate of motion being from one to five hundred 
feet, or even more in their steepest portions, in a year. Many angular 
