78 I. C. RUSSELL — HANGING VALLEYS 



origin or have resulted from the combined action of these two and pos- 

 sibly also of other agencies. 



Examples of valleys which have been abbreviated in the manner just 

 referred to and left suspended above the ocean are present on Unalaska 

 island and open to the sea several hundred feet above its level.* Other 

 similar examples might be cited, but it is evident that whenever a sea 

 cliff recedes at a more rapid rate than streams, or glaciers, on the adjacent 

 land can deepen their channels and at their mouths maintain a position 

 at sealevel, hanging valleys must result. 



DIASTROPHIC HANGING VALLEYS 



A fault may cut across a valley so as to produce a steep descent in its 

 bed, and thus cause one of its segments to become a longitudinal hang- 

 ing valley in reference to the next lower segment. An example in line 

 with this suggestion has been described by W. 0. Crosby f near George- 

 town Colorado. 



If a region with stream eroded or other valleys becomes broken by 

 faults so as to produce block mountains, or when during periods of rest 

 in the growth of such faults valleys are eroded, it is evident that exam- 

 ples of discordant gradients of the nature of those under consideration 

 may result. Examples of such diastrophic hanging valleys are abundant 

 in the Great basin, which open into broad desert valleys or basins, high 

 above their bottoms, although in many, and perhaps most, instances the 

 steep descents at their mouths are concealed beneath alluvial deposits. 



It needs but a suggestion to make it clear that the upheavel of a bold 

 coast in a state of mature topographic development or occupied by tidal 

 glaciers might transform the valleys previously deepened approximately 

 to baselevel into hanging valleys opening in the rim of an ocean basin. 



The three "species" of hanging valleys to which attention has just 

 been directed would probably be considered as " imitative forms " by 

 persons who hold that the recognition of a glacial origin in the definition 

 of a hanging valley is essential, or consider that hanging valleys are in 

 themselves evidence of former glaciation. If, however, we assume that 

 peculiarity of togographic form is the leading fact expressed by the term 

 hanging valley, it is consistent and legitimate to recognize under that 

 term the discordances in slope produced in the several ways just cited. 



GLACIER-FORMED HANGING VALLEYS 



As has been clearly shown by Gannett, Gilbert, Davis, and others, 



*G. K. Gilbert: "Glaciers and glaciation." Harriman Alaska Expedition, vol. iii, New York, 

 1904, p. 185. 

 f'The hanging valleys of Georgetown, Colorado." Technology Quar., vol. xvi, 1903, pp. 41-50. 



