152 EARTH FEATURES AND THEIR MEANING 



in desert regions where the rock surfaces are daily elevated to 

 excessively high temperatures (see Chapter XV). 



Dome structure in granite masses. In large granite masses, 

 such as are to be found in the ranges of the Sierra Nevada of Cali- 

 fornia, a peculiar dome structure is sometimes found developed 

 upon a large scale, and has had an important influence upon the 



breaking down of the rock and 

 upon the shaping of the mountain 

 (Fig. 157) . Such a structure, made 

 up as it is of prodigious layers, 

 can have little in common with 

 the veneers of weathered miner- 

 als which are the result of exfoli- 

 ation, and it is quite likely that 

 the dome structure is in some 



FIG. 157. Dome structure in granite wa y connected with the relief of 

 mass, Yosemite valley California h mass i ve rocks from their 



(after a photograph by Sinclair). 



load the rock which once rested 



upon them, but has been carried away by erosion since the uplift 

 of the range. 



The prying work of frost. In all countries where winter tem- 

 peratures range below the freezing point of water, a most potent 

 agent of rock disintegration is the frost which pries at every crevice 

 and cranny of the surface rock. Important in the temperate zones, 

 in the polar regions it becomes almost the sole effective agent of 

 rock weathering. There, as elsewhere, its efficiency as a disinte- 

 grating agent is directly dependent upon the nature of the crevices 

 within the rock, so that the omnipresent joints are able to exer- 

 cise a degree of control over the sculpturing of the surface features 

 which is hardly to be looked for elsewhere (see plate 10 A). 



Talus. Wherever the earth's surface rises in steep cliffs, the 

 rock fragments derived from frost action, or by other processes of 

 disintegration, as they become detached either fall or slide rapidly 

 downward until arrested upon a flatter slope. Upon the earlier 

 accumulations of this kind, the later ones are deposited, until their 

 surface slopes up to the cliff face as steeply as the material will lie 

 - the angle of repose. Such debris accumulations at the base of 

 a cliff (Fig. 158) are known as talus, and the slope is described as 

 a talus slope, or in Scotland as a " scree." 



