GLACIER ACTION IN ICELAND 759 



several years, as a rule, until sufficient pressure is reached to inaugurate a 

 strong and rapidly accelerated for\Yard movement. A conspicuous advance of 

 the glacial front results, which does not stop until the cirque is drained to a 

 low level. 



Professor Martin replied that it seemed to him that the fact that normal 

 glacial advance is spasmodic, as shown by Mr. Matthes, did not make any par- 

 ticular complication in the question discussed. The localization of the spas- 

 modic advances in and near Yakutat Bay make it evident that these advances 

 are due to earthquake-avalanching during the earthquakes of September, 1899, 

 which were central in Yakutat Bay. 



SOME EFFECTS OF GLACIER ACTION IN ICELAND 

 BY FRED E. WRIGHT 



Published as pages 717-730 of this voliTine. 



Discussion 



Prof. W. M. Davis remarked that "U-shaped" was an unsatisfactory term 

 with which to characterize glacial troughs because many of them are round- 

 bottom V's, the difference between the two forms probably corresponding to 

 young and mature phases of glacial erosion. 



CLIFF SCULPTURE OF THE Y08EMITE VALLEY 

 BY P. E. MATTHES * 



(Abstract) 



The Yosemite Valley may be epitomized as a glacial canyon laid in struc- 

 turally aberrant materials. It is to the latter circumstance chiefly that the 

 valley owes its remarkable wealth of sculptured forms. These are not inher- 

 ently a product of either stream or ice erosion ; they are a function of the 

 structure of the country rock. The granites of the Yosemite region may be 

 pictured as consisting of many huge monolithic masses imbedded in a matrix 

 of more or less strongly fissured rock. This unusual structural habit naturally 

 carries with it extreme inequality of resistance to disintegration. As a conse- 

 quence rock structure has played a prominent role in the evolution of the 

 tojiogrnphy of the region. The Yosemite landscape indeed reflects in its fea- 

 tures the structural character of the materials from which it has been carved; 

 its dominating heights consist invariably of intractable monoliths ; its canyons 

 and gulches are due to zones of easily eroded fissile rock. The glacial cross 

 cliffs and lake basins in the valley floors, the headlands and embayments of 

 the rock walls have in each case evolved in obedience to local structural con- 

 trols. The very trend and profile of each cliff has been determined by struc- 

 tural planes. Indeed every rock form and monument of the valley is to be 

 interpreted as an expression of its associated structures. This applies also to 

 those notches and niches about the waterfalls, which have heretofore been 

 explained as the result of the shifting of the falls in Glacial times. 



Introduced by M. R. Campbell. 



