May 20, 1904.] 



SCIENCE. 



785 



are so pronounced that comparisons of the 

 records of the position of the ice-wall of a 

 tidal glacier made in different years need to 

 be corrected for the time of year. 



2. The influence on shore topography of the 

 waves generated by bodies of ice which fall 

 from tidal glaciers, or ' ice-fall waves,' is noted 

 for the first time, and the possible influence 

 of such waves on the shore features of partially 

 ice-walled Pleistocene lakes suggested. 



3. Anomalies in the fluctuations of neigh- 

 boring alpine glaciers, as, for example, when 

 one advances and another in an adjacent val- 

 ley recedes, find many illustrations in Alaska. 

 A discussion of the possible causes of such dis- 

 cordant changes when the conditions on which 

 they are believed to depend are general and 

 wide reaching is presented and is most sug- 

 gestive. 



4. The peculiar steep-sided depressions, 

 many of them containing lakes, well known in 

 certain formerly glaciated regions, are illus- 

 trated in the gravel deposits in front of sev- 

 eral of the Alaskan glaciers which have re- 

 cently retreated. The explanation of the 

 origin of such depressions, first advanced, I 

 believe by Professor W. 0. Crosby, is that they 

 are due to the melting of detached bodies of 

 ice that were surrounded or covered by gravel 

 deposits. The discovery of additional recent 

 and typical examples of this nature by Gilbert 

 gives greater confidence in the commonly ac- 

 cepted explanation. 



5. A characteristic feature of the topog- 

 raphy of formerly glaciated mountains is the 

 occurrence, on the sides of the valleys once 

 occupied by main or trunk glaciers, of tribu- 

 tary glaciated valleys which open into the 

 main valleys high above their bottoms. Such 

 tributary valleys have been termed ' hanging 

 valleys.' The coast of British Columbia and 

 southeastern and southern Alaska is excep- 

 tionally favorable for the study of the topo- 

 graphic features referred to ; and in the vol- 

 ume under review several characteristic ex- 

 amples are described and admirable pictures 

 of them presented. 



The author favors the view that hanging 

 valleys are due to the greater erosive power of 

 a large trunk glacier over that of its shallower 



tributaries. That is, the discrepancy between 

 the level of the bottom of a hanging valley 

 and the bottom of the larger valley into which 

 it opens is due to differential ice-erosion; the 

 idea being that the surfaces of two glaciers 

 which unite, are on the same level, while the 

 bottoms of the valleys they occupy are deep- 

 ened so as to be adjusted respectively to the 

 thickness of the ice streams occupying them. 



The explanation of the origin of hanging 

 valleys by differential ice-erosion, while 

 clearly and forcibly presented by the author, 

 will, I think, fail to satisfy many persons who 

 are familiar with the topography of formerly 

 glaciated mountains. In view of the fact that 

 Professor J. C. Branner reports hanging val- 

 leys on the Hawaiian Islands, where no sug- 

 gestion of former glaciation can be claimed 

 (Am. Jour. Sci., Vol. XVI., 1903, p. 301), and 

 that Gilbert in the volume under review with 

 characteristic candor presents a sketch of a 

 representative example in the mountain-wall 

 of Plover Bay, Siberia, where concurrent evi- 

 dence of glaciation is absent, suggest that at 

 least two sets of conditions may have pro- 

 duced similar topographic forms. Again, in 

 well glaciated mountains like the Cascades 

 and Sierra Nevada, the great differences in 

 level between a main valley and its tributary 

 hanging-valleys, amounting in some observed 

 instances to 1,500 or 2,000 feet, and this where 

 the main valley is short and has but a com- 

 paratively small gathering ground for snow, 

 must needs make the conservative glacialist 

 pause before accepting the conclusion that 

 such great discrepancies are due solely to dif- 

 ferential ice-erosion. Other considerations in 

 this connection might be mentioned, such as 

 the fact that a deep glaciated valley with hang- 

 ing valleys along its sides not infrequently 

 heads against a cliff, and its direct continua- 

 tion above the cliff also has the characteristic 

 of a hanging valley. Then, too, hanging val- 

 leys may be claimed to occur on the sides of 

 steep mountains, and on slopes overlooking 

 the sea, where no evidence of a controlling ice 

 body at a lower level is obtainable. 



The above suggestions are presented and 

 others might be added, for the purpose of indi- 

 cating that the explanation of the origin of 



