374 



EARTH FEATURES AND THEIR MEANING 



up the snow is steadily diminished, and hence cirque recession is 

 correspondingly retarded. Cirques which have approached each 

 other from opposite sides of the ridge until they have become tan- 

 gent at one point may, however, still receive nourishment at the 

 sides and so continue to cut down the intervening rock wall to 

 form a pass or col. The theoretical curve which results from 



this intersection is that 

 known as the hyperbola, 

 of which an illustration 

 is afforded by Fig. 396. 

 An approximation to this 

 form is clearly furnished 

 by most of the mountain 

 passes in glaciated moun- 

 tain districts, and a par- 

 ticularly good illustration 

 is furnished from the 

 vicinity of Glacier on the 

 line of the Canadian Pa- 



FiG.398.-Diagrams7o- illustrate the progres- dfic Railway (Fig. 397). 

 sive investment of an upland by cirques with Upon either side of the 



the formation of comb ridges, cols, and horns. co l the land mass is left 



I, early stage, youth; II, intermediate stage: i . i v . . /. 



Ill, late stage, maturity. m hl S h rellef > " frOm 



. a more or less triangular 



base (Fig. 398, III) into a sharp horn or tooth. An illustration 

 of such a horn is furnished by the Matterhorn in the Swiss Alps, 

 or by Mount Sir Donald in the Selkirks, though less noteworthy 

 examples may be found in every maturely glaciated mountain 

 district. 



The features shaped beneath the glacier. Those features 

 which are carved above the glacier the comb ridge, the col, 

 and the horn are all shaped as a result of intensive weathering 

 upon the cirque wall. The shaping at lower levels is accomplished 

 by processes in operation below the glacier surface, where weather- 

 ing is excluded and where plucking and abrasion work together 

 to tear away and grind off the rock surface. By their joint action 

 the valley is both deepened and widened, directly to the height of 

 the glacier surface, and indirectly through undermining as far up 

 as rock extends. Thus the valley is transformed into one of broad 



