ROCK GLACIERS IN ALASKA 363 



There is a sharp distinction between these rock glaciers and true 

 glaciers, although in some cases it may be difficult to draw the line 

 between the two. For the formation and existence of a true glacier 

 it is necessary to have an annual surplus of snowfall over melt, or, 

 in other words, to have neve fields to supply ice to the glaciers. The 

 greater number of rock glaciers, on the other hand, are found in cirques 

 where all, or practically all, of the winter's snowfall disappears during 

 the summer. In true glaciers, no matter how heavily moraine 

 covered they may be, there is always a tendency to crevass where the 

 ice rounds a bend or passes over an inequality of the bed, and pits and 

 irregularities of the surface are common at the lower ends where the 

 underlying ice melts out and allows the moraines to cave in. In the 

 rock glaciers no certain crevasses or cave-in pits were seen, and these 

 are not to be expected if the rock glaciers are composed, as they seem 

 to be, of talus with ice only in the interstices, for the talus itself is 

 self-supporting without the ice, and the melting of the ice would have 

 little effect on the surface appearance of the flow. This of course is 

 true only of those rock glaciers which show no glacial ice at their 

 upper ends. Of those which head in true glaciers (Fig. i. No. 3) 

 the upper ends would be profoundly altered if the ice should disappear, 

 but the lower ends would probably present about the same appearance 

 as they do now. The rock glaciers also differ from true glaciers in 

 that, although they may advance spasmodically, or at varying rates, 

 they never retreat, for their form remains intact even if the ice melts 

 out and movement ceases. 



The conditions necessary for the formation of one of these rock 

 glaciers are considered to be as follows : 



With the wane of the last great epoch of glaciation, the ice in many 

 small valleys which contained glaciers was retreating, and as its area 

 contracted in the cirques, the head walls and sides, steepened by gla- 

 cial undercutting and by Bergschrund sapping, were exposed to the 

 rapid weathering characteristic of bare rock surfaces in the high alti- 

 tudes of this region. In the more favorably situated of these cirques 

 the rock waste streamed down the valley sides and heads upon the 

 glacier below and was gradually carried down by the ice and ulti- 

 mately concentrated at its lower edge. Here, in the usual order of 

 events, it would have been piled up as terminal moraine, but differ- 



