CHAPTER I. 
MODIFIED DRIFT IN NEW HAMPSHIRE. 
BY WARREN UPHAM. 
} a portion of geological history of which we have our principal 
record in the Modified Drift, begins with the departure of the 
great northern ice-sheet, and extends from that time to the present. 
The deposits included under this title are the water-worn and stratified 
gravel, sand, and clay or silt, which occur abundantly in almost every 
valley in the state. These river-lands comprise the intervals, which are 
annually overflowed at the high water of spring, and successive terraces 
which rise in steps upon the side of the valley, the highest often forming 
extensive plains. 
The origin and distribution of these materials present many interesting 
questions. When the term was first employed, it was the prevailing 
opinion that modified drift was gradually formed from the unmodified 
glacial drift by the ordinary action of rain and streams; and similar 
materials in small amount have been added by these causes, which are 
still at work. The boulder that is separated from the ledge by frost, and 
carried forward by the heaviest floods of a mountain torrent, is on its way 
to form a part successively of the coarse rounded gravel, sand, and silt, 
over which the river flows on its journey to the sea. It is evident, 
however, that the high terraces and wide plains bordering our rivers 
were formed by much greater floods than those of the present time, 
laden with vast quantities of alluvium. Both the materials and the water 
for sweeping them into the valleys appear to have been supplied by the 
