GLACIAL FEATURES 295 



tion — W. D. Johnson's famous bergschrund hypothesis. The 

 critical condition of this hypothesis is the diurnal migration 

 across the freezing point of the air temperature at the bottom of 

 the schrund. Alternate freezing and thawing of the water in the 

 joints of the rock to which the schrund leads, exercise a quarry- 

 ing effect upon the rock and, since this effect is assumed to take 

 place at the foot of the cirque, the result is a steady retreat of 

 the steep cirque wall through basal sapping. 



While Johnson's hypothesis has gained wide acceptance and 

 is by many regarded as the final solution of the cirque problem 

 it has several weaknesses in its present form. In fact, I believe 

 it is but one of two factors of equal importance. In the first 

 place, as A. C. Andrews 8 has pointed out, it is extremely improb- 

 able that the bergschrund of glacial times under the conditions of 

 a greater volume of snow could have penetrated to bedrock at the 

 base of the cirque where the present change of slope takes place. 

 In the second place, the assumption is untenable that the berg- 

 schrund in all cases reaches to or anywhere near the foot of the 

 cirque wall. A third condition outside the hypothesis and con- 

 tradictory to it is the absence of a bergschrund in snowfields at 

 many valleys heads where cirques are well developed! 



Johnson himself called attention to the slender basis of ob- 

 servation upon which his conclusions rest. In spite of his own 

 caution with respect to the use of his meager data, his hypothesis 

 has been applied in an entirely too confident manner to all kinds 

 of cirques under all kinds of conditions. Though Johnson de- 

 scended an open bergschrund to a rock floor upon which ice rested, 

 his observations raise a number of proper questions as to the 

 application of these valuable data: How long are bergschrunds 

 open? How often are they open? Do they everywhere open to 

 the foot of the cirque wall? Are they present for even a part of 

 the year in all well-developed cirques? Let us suppose that it 

 is possible to find many cirques filled with snow, not ice, sur- 

 rounded by truly precipitous walls and with an absence of berg- 



8 Corrosion of Gravity Streams with Application of the Ice Flood Hypothesis, 

 Journ. and Proc. of the Royal Society of K S. Wales, Vol. 43, 1909, p. 286. 



