academy of scences] GLACIERS AND GLACIATION OF ALASKA 231 



be called — and the deep fiord trouglis was considered "the most important witness yet dis- 

 covered to the magnitude of the work accomplished by the Alpine glaciers of the Pleistocene" 

 (118). When this conclusion was reached on the Harriman expedition, it must have given great 

 satisfaction to Gannett, who was one of the party; for it was he who had only a year before 

 published his remarkable paper on Lake Chelan in the State of Washington, in which after first 

 pointing out the striking homology between branch and trunk river channels and branch and 

 trunk glacial troughs, he had so ably used this homology to resolve the vexed question of the 

 amount of erosion that the ancient glaciers of mountainous regions accomphshed. The elabora- 

 tion of his argument by so competent and so impartial an investigator as Gilbert in the presence 

 of the innumerable examples of hanging lateral valleys and deep main troughs in Alaska settled 

 the question beyond the need of further proof. 



METHOD OF DISCUSSION OF GLACIAL EROSION 



Several comments are suggested by Gilbert's discussion. It should be noted in the first 

 place that it is preceded by a critical physiographic analysis, not of the fiord troughs and their 

 hanging lateral valleys alone but of the region as a whole, and that this analysis resulted in giving 

 a readily intelligible explanatory description of the varied parts of which the whole is composed ; 

 in other words, the treatment has the breadth and comprehensiveness that is usually found in 

 Gilbert's essays. More significant still is the fact that the physiographic analysis was sub- 

 stantially the product of Gilbert's own observation in a region of complicated structure and of 

 complex forms during a comparatively short excursion. The analysis may of course be erroneous 

 in some respects and incomplete in others. No one would have recognized the desirability of 

 further work more heartily than Gilbert; nevertheless the results of the analysis are distinctly 

 valuable. It may be argued from this that a preliminary explanatory description of a region 

 can be properly made during a short excursion, provided the describer is properly trained. 

 The apprehensions of those who fear that explanatory physiographic description is useful only 

 after long-continued and elaborate investigation are not warranted; it is as available in recon- 

 naissance work as an explanatory geological description is; both may be helpfully carried as 

 far as the occasion warrants, always provided that the worker is competent. 



The phrasing of Gilbert's results is also of interest; it shows that this clear-minded student 

 of land forms found value in such descriptive terms as infantile, mature, and senile, borrowed 

 from the organic sciences where they are so naturally employed to designate certain stages in a 

 systematic developmental sequence, and applied in an inorganic science where they may again 

 represent stages in a systematic developmental sequence; and it shows further that he was not 

 in the least embarrassed by the contemporaneous occurrence of these unlike stages of form- 

 development on rocks of different resistance in different parts of a single region, although such 

 association appears to have caused trouble among certain less clear-minded students who have 

 confused stage, as representing phase of change, with age, as representing a measure of time. 

 Gilbert's explicit sanction of these biological terms as representing stage and not age is quoted 

 in another section. This must surely give encouragement to those physiographers who find 

 help in the analogy between the orderly development of land forms and of organic forms. 



The reasons that led Gilbert to a belief in the strong erosive power of the ancient Alaskan 

 glaciers also deserve emphasis. These were not based on a study of the physics of ice, or on an 

 investigation of the visible behavior of existing glaciers, or on an estimate of the volume of 

 glacial detritus removed from the region in question; they were based, first, on a competent 

 physiographic analysis of the Alaskan coastal region, as noted above; next on the striking 

 contrast discovered by means of this analysis between the peculiar features of this formerly 

 glaciated region and the forms of various other never-glaciated regions of ordinary or "normal" 

 erosion with which Gilbert had had abundant experience for 30 years previously; and finally 

 on the recognition of the striking agreement between the peculiar features of formerly glaciated 

 Alaska and the features that Alaska ought to have if its former glaciers had strong erosive 

 power where their currents were deep and relatively rapid, coupled with the recognition of much 

 less erosive power where the ice was less deep and more sluggish. In a word, Gilbert's conclu- 



