o 



42 L. D. CAIR^'E^ EROSIOX AND EQUIPLANATION IX ALASKA 



eroding part in some localities, particularly in those areas where the 

 dominant bedrock consists of Mesozoic slates, phyllites, etcetera. Dur- 

 ing the seasons when snow only partly covers the surface, drifts tend to 

 gather in numerous corners, and to remain in the various, irregular, some- 

 what protected angles, on the mountain slopes. During the day the 

 water from the melting snow trickles from the lower edges of the drifts, 

 and tends to move outward the fine material there accumulated. With 

 lowering temperature during the night or on colder days, the water in 

 the ground is frozen, and in expanding breaks up the remaining coarser 

 material at the edge of the snow, and in tfiis way provides a further 

 supply of fines for the water to wash out whjfn the snow and ice again 

 thaw; thus the process proceeds. The drvfh tend to gradually form 

 steps up the mountain slopes, which, unless otherwise destroyed, yearly 

 increase in size and afford protection for larger drifts each year, until 

 eventually each replaces the one next higher. On numerous hillsides the 

 successive steps are well formed and present quite a striking terraced 

 appearance, but in most places these are removed by other erosive agen- 

 cies as fast as they are formed. 



An estimate of the rapidity with which erosion progresses in the slate- 

 phyllite localities may be obtained from the appearance of the hillsides 

 on which these rocks outcrop. In such places the surface has frequently 

 a streaked appearance, as if some huge rake had been drawn down the 

 slopes (plate 17, figure 1). The furrows represent the lines along which 

 the streams of water trickled, removing, in so doing, much of the more 

 finely comminuted rock material: the intervening ridges of unsorted 

 waste also become reduced as the process continues. This raking is also 

 often caused by melting snow, and at times, when rapid tliaws or heavy 

 showers cause w^ater to be abundant on the hills, the talus is rapidly 

 washed downward en inafise, causing frequent accuinulations of some- 

 what coarse waste to be distributed over the side hills (plate 17, figure 2). 

 This rapid downhill movement of the waste, combined with the slow 

 creep it everywhere possess.es, might also be considered as a minor phase 

 of solifluction" or land flowing, a process that has been so lucidly de- 

 scribed by Professor Andersson. 



By these various disintegrating and eroding processes the rocks of the 

 hills are being gradually broken and comminuted and moved to within 

 reach of the streams traversing the area, which annually convey vast 

 quantities of such debris downstream toward Beriuij Sea. 



■ .T. E. Andersson : "Solifluction a component of subaerial denudation." .Tour, of Geol. 

 veil. xiv. moo. pp. 91-112. 



