GLACIER-FORMED HANGING VALLEYS 79 



hanging valleys would result in case a main valley glacier eroded its 

 bed more deeply than a tributary glacier deepens its bed. The differ- 

 ence in the depth of such valleys, after the glaciers which shaped them 

 had melted, would represent the excess of erosion of the receiving 

 glacier over that of its tributary. This simple and one may say self- 

 evident explanation of the origin of the numerous hanging valleys of 

 glaciated mountains is seemingly complete and satisfactory if the large 

 amount of glacial erosion it implies in numerous instances can from 

 other evidence be proven to have occurred. This matter will be reverted 

 to below. In the study of glaciated hanging valleys, however, the 

 question arises : Are there not other processes by which glaciers can pro- 

 duce similar topographic results ? To aid search in this direction, at 

 least six suggestions seem pertinent, and are embodied in the following 

 paragraphs : 



Small glaciers sometimes originate on the sides of mountains or on 

 the borders of deep valleys where, so far as can be ascertained, no pre- 

 vious depressions existed. Examples of such " mountain-side glaciers," 

 as they maybe termed,* are known on mount Rainier, Washington, and 

 on the Three Sister peaks and mount Jefferson, in Oregon, and elsewhere. 

 It is evident from inspection that typical mountain-side glaciers are 

 engaged in excavating depressions for themselves, which, at an early 

 stage in the process, have the essential features of cirques, and at a later 

 stage develop a valley of the typical U-shaped cross-profile, flat bottom, 

 etcetera, with a cirque at its head. These glaciers may be said to burrow 

 into the mountain sides by head ward extension, chiefly, as is judged, by 

 " quarrying." The lower limit to which they are enabled to excavate 

 their beds, or the local baselevel, is determined by melting. A glacier 

 of the type referred to deepens its bed to this level, and, given time 

 enough, extends it headward until a flat-bottom trench is produced. 

 Should the glacier then melt, a hanging valley would be left, the mouth 

 of which would open out on the slope on which the glacier originated. 

 Such valleys are " hung up " in reference to the country which they 

 face, but differential glacial erosion has no part in their production. 



A mountain-side or valley glacier sometimes ends at the summit of a 

 precipice,where it breaks off and descends in avalanches, to be recemented 

 at the base of the escarpment or there melt, according to climatic con- 

 ditions. In such instances it is evident that the glacier before being 

 broken might excavate a cirque or trough according to the length of its 

 duration, and on melting leave a hanging valley. In this case, as in the 



*The class of glaciers referred to have, as it seems, at least in part, been previously termed 

 " hanging glaciers." 



