732 GEOLOGY OF OLD HAMPSHIRE COUNTY, MASS. 



If Ijushes or turf straud on the bar and take root, it is protected and its 

 increase is accelerated, and it grows in flood time above the low-water level 

 and rises as an island or promontory, and the tendency of the stream to 

 scour out evervthino- at high water generally keeps open a channel between 

 it and the mainland. This is the condition of Ellwells Islaixl, just north of 

 the west end of Northampton bridge. In the time of canal navigation, 

 sixtv years ago, the channel, 33 feet deep, ran right under the present 

 island, and in digging for the pier of the new bridge old boat hooks were 

 found under its south end. 



The island generally joins tlie mainland by the silting up of the 

 iipstream end of the side channel, and a deep, stagnant inlet runs up from 

 its south end. Tliis is the condition of two broad peninsulas opposite the 

 "oxbow" below Hockanum which have formed since 1840. 



The continued growth of the new addition to the flood plain takes 

 place by material brought in over ■ it during floods, and this decreases in 

 rapidity as the ground rises, and soon the checking of the current as it rises 

 over the flat makes itself manifest in the increased deposition along the 

 outer border of the flat, and a "glacis terrace"^ results, sloping sharpl}' to 

 the water and gi-aduall}' backward. The "glacis terrace" is thus a case of 

 arrested development of a terrace. The groove which separates the new 

 from the old remains preserved for a long time and often permanently. 



Again, as the waters rise over the growing teiTace, they are arrested 

 first over its upstream portion aiad thus build up this end most rajsidly. 

 This is most beautifully illustrated in the terrace which begins at the North- 

 amjiton bridge and extends south to the south end of Hadle}" street, and is 

 bounded by the road which leaves the main road at the bridge and joins 

 Hadley street at its south end. This road runs along the edge of the former 

 bank ( )f the river, and at its south end one looks down upon the lower plain, 

 still separated by a shallow inlet which runs up from the south. North- 

 ward, the lower plain gradualh' rises, the inlet shallows and disappears, and 

 the lower terrace is a complete "glacis terrace." Still farther north the 

 lower plain continues to rise, and the scarp which separates the two becomes 

 less in height until at the bridge the two have come so uearl)^ to the same 

 level that one might easily overlook the fact that the newer terrace extends 



' Hitchcock, Surface Geology, 1860, p. 5. 



