CHAPTER I, 



MODIFIED DRIFT IN NEW HAMPSHIRE. 



BY WARREN UPHAM. 



IJIlIHE portion of geological history of which we have our principal 

 J% 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 



