634 GEOLOGY OF OLD HAMPSHIRE COUNTY, MASS. 



Beniardston Pass and across the north of Greenfield brought the coarser 

 sands down over the Grreen River glacier and spread them to build up the 

 broad plain of Franklin Park 



THE LAKE BENCH FROM DEERFIELD EIVER SOUTH. 

 THE DEERFIELU DELTA. 



South of the erosion basin of Deerfield River the bench (1 s h) consists 

 of the southern half of the great delta of the Deerfield — that ^^oi'tion which 

 has escaped the later erosion of the river itself. It spreads out, fanlike, as 

 a broad, flat alluvial cone from the mouth of the rocky canyon of the 

 Deerfield, where it has an elevation of 320 feet, and slopes very gradu- 

 ally to its front edge, which is about 30 feet above the lake bottom, and 

 then drops by a steeper grade to the level of tlie latter. Its outer 

 boundary is in places not sharply marked, as broad bars molded by the 

 current of the main valley from the abundant detritus furnished Ijy the 

 Deerfield are spread in front of it and render the lake bottom unu.sually 

 irregular. A cutting of the Canal Railroad, 18 feet deej), passing from the 

 outer border directly to the apex of the cone above Stillwater bridge, 

 showed in beautiful detail the whole structure of the broad delta. It is 

 made up entirely of Avell-washed sands, everywhere coarser above and finer 

 below. The upper layer varies from 3 to 7 feet, and is made up of coarse 

 sand and fine gravel, well washed and rounded, laid down .in broad, lentic- 

 ular layers, as a whole horizontal or conforming to the slight slope of the 

 surface. Below are fine, whitish, perfectly sorted sands in two grades, fine 

 and very fine. The former are thrown down in layers 1 to 2 feet thick, 

 with delicate flow-and-plunge structure, and dipping at all angles up to 

 30° SE. — that is, radially from the old mouth of the river. These layers 

 are separated by other layers, from 2 to 8 iuclies thick, of the very fine, 

 moist, compact, almost clayey sand, which are thrown down upon rippled 

 surfaces of the coarser, and show a flow-and-plunge structure of extreme 

 delicacy. 



In an exceptional case a layer of tlie very fine sand occurs a mile 

 out in the valley, dipping 1.5° SE., which, although bounded for a long 

 distance above and below by horizontal surfaces and contained in undis- 

 turbed layers of the coarser sand, is contorted in a very complex way, and 



