46^ Goldthwait — Old Graded Upland on the 



theory that the old graded slopes are the slopes of residual 

 mountains which grouped themselves around broad intermont 

 lowlands along the frontier of the New England peneplain. 



At the same time the evidence is so fragmentary that other 

 possible explanations should be considered. It might be sug- 

 gested, for instance, that the flatness of the crown of the range 

 is due chiefly to the more rapid reduction of the slopes above 

 the tree line, where frost action and the movement of rock 

 waste are more vigorous than in the forest. This hypothesis, 

 however, has to meet the fact that since the last passage of the 

 ice sheet over the range there has been almost no splitting 

 apart of the roches moutonnees, and surprisingly little move- 

 ment of the loose rock ; and moreover it seems very improb- 

 able that the tree line has been at all constant in position 

 through the Pleistocene period. Again, one might look for 

 some structural control of the upland flats, — some dominant 

 system of joints, or some very extensive development of 

 schistosity in planes near the horizontal. The answer to this 

 is simply that while the theory cannot be rejected, the facts 

 thus far observed lend no support to it. Finally, it may be 

 urged that the flattening out of these graded slopes, in their 

 descent, near the 4000-foot contour is not as real as it appears 

 to be ; that the original well-graded slopes of the range have 

 been so deeply and so widely scooped away by local glaciers 

 and the resultant spurs have been so much worn, especially at 

 low altitudes, by glaciation during the subsequent regional 

 glaciation that the effect is a false appearance of an uplifted 

 and extensively dissected cluster of low monadnocks. 



While suspending judgment in choosing between these inter- 

 pretations, it is interesting to note that the same gently graded 

 upland topography occurs on the top of Mount Ktaadn in 

 northern Maine. Tarr, in his paper on " the Glaciation of 

 Mount Ktaadn "* thus describes it : 



"The main part of the elevated portion of the mountain is 

 occupied by the 'tableland' . . . from which several spurs ex- 

 tend as divides between stream valleys heading on the moun- 

 tain top. In places this tableland is remarkably level. "f 



If there is any truth in the theory that the plateau-like aspect 

 of the lawns is due to an over-emphasis of the pre-glacial 

 graded slopes by the cirque cutting and general steepening of 

 the lower portions of the flanks of the Presidential Range, it is 

 to be noticed that this theory applies equally well to Mount 

 Ktaadn, whose profound " basins," as Tarr showed, are glacial 

 cirques like the " ravines " and "gulfs" of Mount Washing- 



* B. S. Tarr : The Glaciation of Mount Ktaadn, Maine, Bull. Geol. Soc, 

 America, vol. xi, pp. 433-448, 1900. 

 fOp. cit., p. 440. 



