The Andes of southern Peru . ue became clear. Eeferring to Fig. 198 we see atonce that the break occurs at yand means that under favorabletopographic and geologic condi-tions sajDping at y takes placefaster than at x and that the re-treat of y-z is faster than mil be clear that when theseconditions are reversed or sapping at x and at y are equal asingle wall will result. On reference to the literature I find thatGilbert recently noted this feature and called it the believes that it marks the base of the bergschrund at a latestage in the excavation of the cirque hasin. He
The Andes of southern Peru . ue became clear. Eeferring to Fig. 198 we see atonce that the break occurs at yand means that under favorabletopographic and geologic condi-tions sajDping at y takes placefaster than at x and that the re-treat of y-z is faster than mil be clear that when theseconditions are reversed or sapping at x and at y are equal asingle wall will result. On reference to the literature I find thatGilbert recently noted this feature and called it the believes that it marks the base of the bergschrund at a latestage in the excavation of the cirque hasin. He notes further thatthe lower less-steep slope is glacially scoured and that it formsa sort of shoulder or terrace. If all the structural and topographic conditions were known ina great variety of gathering basins we should undoubtedly findin them, and not in special forms of ice erosion, an explanationof the various forms assumed by cirques. The limitations in-herent in a high-altitude field and a limited snow cover prevented. Fig. 19S—The development of text, p. 299, and Fig. 199. G. K. Gilbert, Systematic Asymmetry of Crest Lines in the High Sierra ofCalifornia. Jour. Geol., Vol. 12, 1904, p. 582. GLACIAL FEATURES 301 me from solving the problem, but it offered suificient evidence atleast to indicate the probable lines of approach to a solution. Forexample it is noteworthy that in all the cases examined theschrundline was better developed the further glacial erosion hadadvanced. So constantly did this generalization check up, that ifat a distance a short valley was observed to end in a cirque, Iknew at once and long before I came to the valley head that ashoulder below the schrundline did not exist. At the time thisobservation was made its significance was a mystery, but it repre-sents a condition so constant that it forms one of the strikingfeatures of the glacial forms in the headwater region. The meaning of this feature is represented in Fig. 199, inwhich thre
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