346 L. D. CAIRNES EROSION AND EQUIPLANATION IX ALASKA 



grated and to some extent dissolved and conveyed out into ihe debris 

 flats, where practically all the material held in suspension, as well as 

 some at least of that in solution, is deposited. On account of the almost 

 perpetually frozen condition of the superficial materials of the upland, 

 the areas in which these materials occur are poorly drained, and conse- 

 quently practically all sediments and precipitates deposited therein nec- 

 essarily remain, having no means of escape. It is thus largely due to the 

 arctic climatic conditions that the equiplanation process is here mani- 

 festly in evidence. A prolonged rise in temperature would cause the ice 

 of the ujDiand to thaw, with the result that the accumulations now filling 

 many of the existing bedrock depressions would be washed out by the 

 again integrated drainage system. 



The rock terraces bordering the debris areas are largely of two types. 

 In some cases these terrace fronts are of firm unbroken bedrock. These 

 appear to represent antecedent forms that existed in the old peneplain 

 and are still, but very slowly, being destroyed, owing largely to the com- 

 pact nature of the rock constituting them. The majority of the terraces, 

 however, are composed dominantly of talus, and are the resultant mainly 

 of two forces, the relatively rapid forcing of the talus downhill by the 

 frost and slow disintegration along the terrace front. The exposed 

 limestones and dolomites of the residuals become in the course of time 

 somewhat fractured, due largely to expansion and contraction of the 

 rocks themselves and to the freezing and thawing of water filling the 

 various open spaces they include.' With further fracturing the rocks 

 become broken, and the water filling the spaces between adjoining frag- 

 ments, on freezing, forces the lowermost of these downhill. Expansion of 

 ' adjoining talus members, due in warm weather to a considerable rise in 

 temperature, also produces the same effect, only to a less degree. As the 

 ice melts, or, on the other hand, as a high temperature falls slightly 

 and the rocks contract, various blocks are left somewhat unsupported, 

 hence move slightly downward in adjusting themselves to gravitation. 

 This process continues until the various fragments are individually too 

 far apart for water to readily fill the intervening spaces, at which stage 

 the downward movement practically ceases. The talus blocks, having 

 been somewhat rapidly pushed downhill, present a somewhat abrupt face, 

 which remains practically stationary except for the slight amount of 

 material that is annually removed therefrom by the various eroding and 

 disintegTating processes previously mentioned; of which, solution, slow 

 as it is, is one of the most active. Thus these rock faces, in many places, 

 border the debris areas which are gradually encroaching on them, and if 



