202 GENERAL GEOLOGY 



C. By Weathering. — The vast importance of the denudation effected by weather- 

 ino-, brought about chiefly by evaporation, by diurnal ranges of temperature, and 

 by wind, have ah-eady been described at the beginning of this chapter. We Avould 

 here specially emphasize the enormous importance of wind in removing snoAV from 

 inland seawards. The heavy cold air over the Antarctic Continent, as from time 

 to time it rushes seawards into the great trough of the atmosphere near 62° S. 

 [Der Rhine, or Die Luftfurche of Hann), not only bears seawards most of the new- 

 fallen snow, but tears out deep furrows in the old snow, thus forming the sastrugi. 

 Near the Magnetic Pole area these sastrugi are fully 3 feet in height.* 



This wind erosion may very likely remove at least as much snow annually as is 

 annually added as the result of precipitation. 



D. By Thaxv and by Running Water. — We may now pass on to consider the 

 phenomena of thaw and the erosion accomplished by running water. 



It was on November 13, at our winter quarters at Cape Royds, that the 

 thaw was first noticed to be taking effect, when many boulders of black kenyte, 

 which had snow in their crevices, showed large wet patches owing to the heat 

 from the sun raising the temperature of the dark rock and melting the snow. 

 Several large erratics of the same rock, resting on ice, or surrounded by snow-drifts, 

 developed shallow pools around themselves the next few days, and the thaw might 

 be said to have commenced. 



By the end of November the thaw had fully set in, and those drifts which had 

 with.stood the ablation of the winter and spring were fast disappearing. The thaw 

 seemed to strike along parallel lines in the drift, resulting in sharp knife-edges of 

 coarse-grained neve with deep depressions between, the ridges being parallel and 

 striking upwards at a slight angle of 10° to 20° measured from the horizontal, in the 

 one or two cases particularly noticed, towards that direction where the sun would 

 have most power on the particular portion of the drift. Thus, a drift sheltered on 

 the southern side, as they usually were, would have these ridges with their edges 

 pointing towards east on the eastern side of the drift, north on the northern side, 

 and west on the western side ; probably the depressions and ridges were caused by 

 the sun working back quicker along the bedding planes of snow charged with 

 dust particles produced annually, or perhaps, after each blizzard season, by the 

 concentration of the sediment in the drift. 



One curious feature in the thaw, partly caused by the intense blackness of the 

 rock at Cape Royds, was that in many cases the thaw proceeded much more quickly 

 from below the snow-drift than from above. We have trenched through a small drift, 

 3 or 4 feet thick, in midwinter, and found it to be snow right through to the rock, 

 yet when it was trenched again a month after the return of the sun in the spring 



* Bage, Webb, and Hurle}', of Dr. Mawson's Expedition, report that in 1912-13, in their journey 

 towards the South Magnetic Pole area, they encountered sastrugi, to the south-east of Adelie Land, 

 quite .5 feet in height. 



