364 STEPHEN R. CAPPS, JR. 



ing in character from the common forms of terminal moraine material 

 in its angular, talus-like appearance and the absence of finer muds and 

 rock flour which form such a large part of the moraines of active gla- 

 ciers. Here the small, fast-dying glaciers were eroding but little, 

 and were almost overwhelmed by the debris supplied them from 

 above. Into the detritus at the lower edge of the glacier the waters 

 from the melting ice and snow and from rains sank and froze, and 

 gradually filled the interstices with ice up to a point below the surface 

 where melting equaled the freezing. In these ice-cemented masses 

 an incipient glacial movement was started by the well-known process 

 of melting and refreezing of the waters, with their consequent expan- 

 sion. As the climate became still milder, in many of the cirques the 

 winter's snows all melted away during the summer, so that conditions 

 for ordinary glacial activity no longer existed, but the bodies of talus 

 which reached the cirque floor became filled with interstitial ice, 

 and the movement of the mass in a glacier-like way has continued, 

 although no doubt all true glacial ice has disappeared from many of 

 these rock glaciers. It is certain that much snow is still carried down 

 onto the surface of the rock glaciers in slides of ice and rock, and con- 

 siderable quantities of it may be covered by debris and incorporated 

 into the rock glaciers, but this snow probably forms only a small part 

 of the total mass of the flow. 



The above succession of events seems to be well established in this 

 region, for there are now all stages varying from apparently active 

 glaciers with short rock glaciers below, to rock glaciers in which no 

 glacier ice is seen, in valleys where all snows disappear during the 

 summer, yet in these the slow movement seems still to be in operation, 

 the rate of movement in each rock glacier controlled by the supply 

 of talus from above and by the shape and grade of the floor over which 

 it moves. The rock glaciers are, therefore, the true successors of 

 real glaciers. 



The particularly perfect development of these features in the area 

 of the Nizina Special Map is due to the rugged character of the moun- 

 tains, with cirques having steep heads- and sides; to the exceptionally 

 favorable conditions for rapid rock weathering and talus accumula- 

 tion; and to climatic conditions peculiar to areas on the border line of 

 glacial activity. 



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