near Mount Washington. 



11 



the theory the task of showing, by an intensive study of the 

 district, what is abnormal about them. Further than this, 

 close observation in the field is likely to lead one to doubt 

 whether the original process of ravine wall recession is still 

 going oil The cascades which fall over the headwalls bring 

 down some loose material from the upland, chiefly stones 

 from the ground moraine, and gather some waste from the 

 exposed cliffs. Extensive cone-shaped " slides " of rock have 

 collected at the foot of the crags, and hundreds of great blocks 



Fig. 5. 



fcfa^ ^* 







" _^--V^'->>, 



> 





- T 



Fig. 5. Head wall of Tuckerman Ravine in early summer. The snow 

 arch lies just to the right of the center, where the brook which cascades 

 down over the headwall makes its way beneath the snow drift. Piles of 

 loose blocks are banked high against the foot of the walls on either side. 



have fallen to the ravine floors ; but these lie undisturbed and 

 generally unaltered — they are not being "acted upon by 

 streams" and "washed down the mountain" at anything like 

 the rate at which the walls are being demolished. In other 

 words, the rifting, sliding process which is going on to-day is 

 tending not to maintain the declivity of these walls, but to 

 reduce it ; not to sweep clean the ravine floors, but to build 

 them up. 



In view of the fact that the entire Presidential Range was 

 at one time buried beneath the ice sheet, as Professor Hitch- 



