THE PRESENT KOOK SUKFAGE. 519 



ard the cause of the extreme inequality of its effects over different portions 

 of the basin is to be found primarily in the unequal resistance offered by 

 the different rocks of which it is composed, and secondarily in the influence 

 of the projecting- masses of harder rock in deflecting- the ice and shielding 

 the softer rocks in their lee. Here the trap, so easily dissected by the 

 frost, proved most able to resist the onset of the ice. The ridge of trap 

 which makes the backbone of Deerfield jMountain survived after the sand- 

 stone had ))een worn down on either side and protected the Sugar Loaf ^ in 

 its lee, and, with Mount Warner, farther south, projected into the ice as it 

 wore deeply on their flanks. So Mount Toby, built of a conglomerate 

 more durable than the sandstone beneath, and protected by Deei-fleld 

 Mountain, stemmed the ice and sheltered the long ridge which runs south 

 from it, so far that a fragment of the soft incoherent sandstone still lies 

 along its eastern slope in Amherst village. 



But the Holyoke range, coming up from the south, swings around 

 eastward in a great curve, commencing at Mount Tom, and from ]\Iount 

 Holyoke on runs eastward to its end, and the great trap sheet which 

 makes its strength is so placed as to present the maximum resistance to 

 the ice moving from the north and northwest — that is, it dips every- 

 where as a continuous sheet from the crest of the ridge southward where 

 the chain runs east and west, and as the ridge swings round to run south- 

 ward the ilip of the sheet swings round to the east. It received the 

 pressure of the ice, then, as a log set to brace a falling building receives 

 its weight. The ice, lifting over this sharp obstruction set right athwart 

 its course, wore into it with great severity, and by its recoil as it raised 

 its mass over the opposing- range wore to a very exceptional depth in the 

 area just in front of the latter, which had been filled with the soft sand- 

 stone, forming the broad, deep furrow which runs along the northern and 

 western base of the range, beneath the Easthampton, Northampton, and 

 Hadley meadows, and in the southern part of Amherst, in which furrow 

 the three deep north-south depressions I have described above ended.^ 



' The table-moiiiitain form of Sugar Loaf is probably due to a capping of trap from the southward 

 projection of the I Jecrtield trap sheet, which endured to near the close of the Glacial period. It is called 

 an "Eddy Peak " by Prof. J. D. Whitney (1888) ; see bibliography in Chapter XXIII. 



-It is an interesting fact that a line at the north foot of the east end of the Holyoke range 

 forms a boundary north of which granite bowlders are abundant in the till, while south of this line 

 they are rare. This is because the ice mass was greatly shattered as it lifted over the ridge, drop- 

 ping its bowlders, while it eroded strongly on the crest ; or it may represent the closing period when 

 the ICO wore over the Leverett granite and halted at the north foot of the Holyoke range. 



