. The Andes of southern Peru; geographical reconnaissance along the seventy-third meridian. Yale Peruvian Expedition (1911); Physical geography; Geology. 300 THE ANDES OF SOUTHERN PERU familiar with. Johnson's apparently complete proof of their genetic relation to the cirques. But it was less surprising to dis- cover the position of the few observed—high up on the cirque walls and always near the upper limit of the snowfields. A third fact from regions once glaciated but now snow-free also combined with the two preceding facts in weakening the whole- sale application of Johnson's hypothesis. I
. The Andes of southern Peru; geographical reconnaissance along the seventy-third meridian. Yale Peruvian Expedition (1911); Physical geography; Geology. 300 THE ANDES OF SOUTHERN PERU familiar with. Johnson's apparently complete proof of their genetic relation to the cirques. But it was less surprising to dis- cover the position of the few observed—high up on the cirque walls and always near the upper limit of the snowfields. A third fact from regions once glaciated but now snow-free also combined with the two preceding facts in weakening the whole- sale application of Johnson's hypothesis. In many headwater basins the cirque whose wall at a distance seemed a unit was really broken into two unequal portions; a lower, much grooved and rounded portion and an upper unglaciated, steep-walled portion. This condition was most puzzling in view of the accepted explana- tion of cirque formation, and it was not until the two first-named facts and the applications of the curves of snow motion were noted that the meaning of the break on the cirque became clear. Referring to Fig. 198 we see at once that the break occurs at y and means that under favorable topographic and geologic condi- tions sapping at y takes place faster than at x and that the re- treat of y-z is faster than x-y. It will be clear that when these conditions are reversed or sapping at x and at y are equal a single wall mil result. On reference to the literature I find that Gilbert recently noted this feature and called it the schrundline* He believes that it marks the base of the bergschrund at a late stage in the excavation of the cirque basin. He notes further that the lower less-steep slope is glacially scoured and that it forms "a sort of shoulder or ; If all the structural and topographic conditions were known in a great variety of gathering basins we should undoubtedly find in them, and not in special forms of ice erosion, an explanation of the various forms assumed by cirques. The li
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Keywords: ., bookauthorbo, bookcentury1900, bookdecade1910, booksubjectgeology
