546 D. W. JOHNSON LOCAL GLACIATION IN WHITE MOUNTAINS 



Where mountain ranges are being uncovered by a waning continental 

 ice-sheet, conditions may be peculiarly favorable for the erosion of cirques 

 without the formation of important moraines. After the surface of the 

 continental ice has been lowered some distance down the flanks of a range, 

 local glacial systems develop in the upper levels of the moimtain valleys 

 and carry on the work of cirque-cutting. Such cirques are of later date 

 than the continental glaciation of the levels at which they develop ; but 

 so long as the continental ice is moving past the lower flanks of the range 

 the material eroded from the cirques may be carried down to it by the 

 local glacier system, and hence be transported entirely out of the region 

 in the body of the main ice-sheet. 



It seems highly probable that tlie conditions just described obtained in 

 the White Mountain region during the waning of tlie Wisconsin ice-sheet. 

 If so, then only the insignificant amount of debris removed during the 

 very last stages of cirque-cutting could in any case have been left opposite 

 the ravine mouths. 



The contrast in slope of headM^all which exists between northwest- and 

 southeast-facing cirques is occasionally very striking; but there is some 

 evidence that this variation in form is related in part to structural planes 

 in the rocks composing the range, and in part to conditions affecting the 

 alimentation of the local glaciers, rather than to the modification of early 

 cirques by the continental ice-sheet. Parts of the Presidential Eange 

 are intersected by strongly marked joint planes dipping in a general 

 northerly or northwesterly direction. Headwalls of cirques facing in the 

 direction of dip will tend to coincide with the major structural planes; 

 for even if cirque-cutting originally leaves a headwall vertical, rock slides 

 along the planes of weakness must soon reduce the wall to a more gentle 

 slope. Where the joint planes dip into the headwall, this tendency, of 

 course, does not exist. King Eavine, facing slightly west of north, shows 

 very clearly the coincidence of much of its sloping headwall with major 

 joint planes (plate 38). It is into the bowl of this cirque that the most 

 important rock slides have been precipitated (plate 39, figure 1), and 

 Goldthwait has himself emphasized the importance of the sloping joint 

 planes in contributing to the development of the slides. 



The gently sloping southeast wall of the Great Gulf may similarly be 

 related to structural features. Although the crystallines of this region 

 are much disturbed, prominent foliation planes inclined toward the north 

 were noted at a number of places along the carriage road to the summit 

 of Mount Washington and on the slopes of Chandler Ridge. IN'ear the 

 Half-way House they seem to determine the strong contrast in slopes of 

 a small ridge, about which the road makes one of its sharp bends. The 



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