GLACIAL DRIFT. 285 



The Distribution of the Till. 



By Warren Upham. 



Before the glacial period, the rocks of New Hampshire had been 

 through long ages subjected to the ordinary disintegrating agencies of 

 rain and frost. The loose material derived from this source was doubt- 

 less spread with considerable evenness over the surface, collecting to the 

 greatest depth in valleys, while on ridges or hill-tops it would be thin or 

 entirely washed away. Except where it had been transported by streams 

 and consequently formed stratified deposits, the only fragments of rock 

 held in this mass would be from underlying or adjoining rocks. 



Through this time temperate or tropical climates generally prevailed; 

 but it also seems probable, if the causes of the glacial period have been 

 rightly made to depend upon great eccentricity in the earth's orbit, that 

 these genial conditions were at times interrupted by prolonged cold and 

 the accumulation of slowly-moving ice-fields similar to the immense gla- 

 ciers of Greenland and antarctic lands. Scarcely any hint, however, has 

 been obtained in a full survey of our territory respecting these events, 

 all records of which appear to have been erased by the last great ice- 

 sheet, which pushed from the north and north-west straight forward over 

 all the hills and mountains of New England, terminating beyond our 

 coast-line. The beds which had been derived from long-continued de- 

 composition of the ledges or gathered by previous glacial action, together 

 with the thick fluviatile deposits that probably occupied the valleys, were 

 ploughed up by this ice-sheet, and thoroughly kneaded with each other. 

 Very large amounts of detritus were also added from erosion of the rock- 

 surface. Fragments of all sizes and in great profusion were loosened 

 and wrenched away, while the ledges were everywhere worn and striated 

 by boulders and pebbles which were rolled and dragged along under the 

 vast weight of ice, breaking up and grinding themselves and the under- 

 lying rock into gravel, sand, and even the finest clay. 



At the end of the glacial period, the material which had been thus 



gathered, mingled and swept along by the moving ice, was left in three 



different classes of deposits, namely, modified drift, upper till, and lower 



till. The first and second of these appear to have been held in the body 



of the ice-sheet, principally in its lower portion. At its final melting, it 

 VOL. in. 37 



