The Andes of southern Peru . thickness required for motion at the base of thecirque wall and its change of function with transformation intoneve, we need invoke no other agent. If a bergschrund forms, itsaction may take place at the foot of the cirque wall or high up onthe wall, and yet sapping at the foot of the wall continue. From which we conclude (1) that where frost action occurs atthe bottom of a bergschrund opening to the foot of the cirque wallit aids in the retreat of the wall; (2) that a sapping action takesplace at this point whether or not a bergschrund exists and thatbergschrund a
The Andes of southern Peru . thickness required for motion at the base of thecirque wall and its change of function with transformation intoneve, we need invoke no other agent. If a bergschrund forms, itsaction may take place at the foot of the cirque wall or high up onthe wall, and yet sapping at the foot of the wall continue. From which we conclude (1) that where frost action occurs atthe bottom of a bergschrund opening to the foot of the cirque wallit aids in the retreat of the wall; (2) that a sapping action takesplace at this point whether or not a bergschrund exists and thatbergschrund action is not a necessary part of cirque formation;(3) that when a more or less persistent bergschrund opens on thecirque wall above its foot it tends to develop a schrundline witha marked terrace below it; (4) that schrundlines are best devel- 7240 ,1320 7240 TOPOGRAPHIC ENGRAVING CO. WASH.,n. C. I3S0 Edition of 19IS. THE YALE EERirVXAST EXPEDITION OF 1911 HIRAM BINGHAM. DIRECTOR OUADEAK& CoQtouT m(oiVTil200 fout. GLACIAL FEATURES 305 oped in the mature stages of topographic development in the gla-cial cycle; (5) that the varying rates of snow, neve, and ice motionat a valley head are the persistent features to which we must lookfor topographic variations; (6) that the hypothesis here pro-posed is applicable to all cases whether they involve the presenceof snow or neve or ice or any combination of these, and whetherbergschrunds are present or not; and (7) at the same time affordsa reasonable explanation for such variations in forms as the com-pound cirque with its schrundline and terrace, the unbroken cirquewall, the notched cirque, and the recessed, snow-covered mountainslopes unaffected by ice. ASYMMETBICAL CREST LINES AND ABNORMAL VALLEY PROFILES IN THE CENTRAL ANDES To prove that under similar conditions glacial erosion may begreater than subaerial denudation quantitative terms must besought. Only these will carry conviction to the minds of
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