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. 



The 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 till. 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 strice, 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 



