DEUMLINS. 545 



If one could remove all the uewer deposits — sands and clays — which 

 still await our discussion, and then raise this great stratum of stony clay 

 which overspreads the valley, as one lifts a plaster mask from the face, it 

 would be found that its under surface had been exactly molded to every 

 line and curve of the rocky substratum; but its upper surface would have 

 the effect of a comic mask, swelling with unequal thickness over every 

 prominent feature, distorting and concealing its true fonn, and sending up 

 great protuberances due wholly to a thickening of its own mass and not 

 molded on any projecting ledge below. The protuberances formed thus 

 by the local thickening of the drift sheet appear now as drumhns — massive 

 domed hills, in shape like an inverted canoe, with the long axis pointing in 

 the direction of the glacial motion, from north to south. Where they are 

 most symmetrical they slope away rapidly and equally toward the east 

 and west, more gradually but equally north and south, and very natxu-ally 

 suggest the name "hogbacks," by which they are often known inland, or 

 "whalebacks," as they are called near the sea. They rise like islands out 

 of the sands, which wrap around their bases to a maximum height of 150 to 

 200 feet above the present low ground of the valley, and often the thick- 

 ness of the till composing them seems to be greater than that. 



The tw^o hills just north and south of the village of South Amherst — 

 named Castor and Pollux by President Hitchcock, from their close simi- 

 larity — another to the east of the fonner, and two others farther south and 

 west, are all cast in the same mold. Farther noi*th the hill south of Col- 

 lege Grove — named the Occident by President Hitchcock — the College Hill, 

 and, linally, all the group of hills occupying the space betw^een Amherst, 

 East Sti-eet, and North Amherst villages are of the same origin and pre- 

 serve in varying degrees the common form. 



In the case of all the hills ai'ound South Amherst, except Castor, there 

 are no neighboring outcrops by which one can judge of the elevation of the 

 subjacent ledges and so fix the thickness of the drift stratum forming the 

 hill. The surface of tlie rock may be concave beneath them and the thick- 

 ness of the till much greater than their height above the valley Ijottom. In 

 Castor the gneiss and granite appear high up on the shoulder of the hill on 

 the east and the west, and if it runs under the drift at the same level the 

 thickness of the latter would be about 30 feet, which is probably more than 



the real thickness. 



MON XXIX 3.5 



