Presidential Range of the White Mountains. 461 



been rudimentary in comparison to the cirques winch sheltered 

 them, for the only heaps of locally derived rock debris lie 

 close to the foot of the headwalls. It is, indeed, doubtful 

 whether such rock heaps are not in every case avalanche 

 deposits and rock falls similar to those described by Howe in 

 the San Juan Mountains.* 



Slight alterations of the cirque form by regional glaciation 

 may be seen in certain details of slope. Near the top of the 

 headwall of the Ravine of the Castles, for instance, the jointed 

 ledges were heavily scraped by the ascending ice, developing 

 distinct "roches moutonnees" which face up hill, and are 

 closely followed, on the top of the Jefferson-Adams col, by 

 glaciated knobs which face out over the head of Jefferson 

 Ravine. Although the Ravine of the Castles and King's 

 Ravine, which lie nearly parallel to the southeastward line of 

 movement of the ice sheet, are symmetrical in cross section, the 

 Great Gulf, which lay athwart the southeastward current, ex- 

 hibits a steep wall on its left or northwest side and an excep- 

 tionally gradual slope on its right or southeast side, on which 

 thousands of blocks lie scattered as if caught there in a pocket. 

 Obviously there has been some rounding off of the rims of 

 these cirques, where they were most fully exposed to the 

 southeastward thrust of the ice sheet, and some concealment 

 of their bowl-shaped floors by deposits of ground moraine. 

 The greatest damage to their form as cirques, however, appears 

 to have come from rock falls, avalanches, and more regular 

 slides, during the post-glacial epoch, especially at the beginning, 

 when the melting away of the ice from oversteepened and 

 loosened cirque walls caused huge masses to fall or slip to the 

 ground. The effect of these avalanches and slides has been to 

 tear away the rims and to fill up the floors of the ravines, some- 

 what obscuring their cirque form. Kame ridges and irregular 

 hummocks, and near the upper ends of the cirques moraine- 

 like heaps of great blocks complete the jumble of waste which 

 has accumulated below the cirque walls since the ice age. 



Alternative hypotheses to account for the lawns. — The con- 

 trast between the smooth lawns with their gentle grades and 

 the bold slopes and cliffs of the flank of the range is strong 

 enough to demand explanation. It has been pointed out that 

 where the graded upland surface passes farthest down along 

 the crests of spurs, it seems to flatten out near the 4000-foot 

 contour. It does not require much imagination to fancy this 

 graded slope blending with wide lowland surfaces at about this 

 altitude, even though no traces of such lowland surfaces now 

 remain. In short, one is inclined to look favorably upon the 



* Ernest Howe: Landslides in the San Juan Mountains, Colorado, U. S. 

 Geological Survey, Professional Paper No. 67, 1909. 



