academy op sciences] GLACIERS AND GLACIATION OF ALASKA 233 



that they were called "monadnocks;" also that the local slopes on the residual hills of the pene- 

 plain were very commonly as steep as or steeper than the general seaward slope given to the 

 peneplain as a whole by its slanting uplift; and finally that the incision of young valleys by the 

 master streams beneath their old valley floors during the uplift of the region, taken with the 

 local residual slopes on the seaward side of their old valleys, would have generally safeguarded 

 these streams from being tipped into new courses "consequent to the slope" given by uplift. 



Thus interpreted the present fiord courses should as a rule follow the courses of the peneplain 

 drainage, except that streams which happened to flow with the slope given to the peneplain in 

 its uplift presumably extended their courses by accelerated headward erosion and diverted other 

 streams to their service. In so far as the fiords now follow the strike of the strata, it would also 

 seem that such courses had been developed by the larger rivers during the peneplanation of the 

 region rather than that the rivers were diverted from some other courses " to the lines of strike" 

 during the uplift of the region. It is here once more to be noted that, as already pointed out 

 at the end of the chapter on "Field work in Colorado," no class name was used for rivers that 

 had spontaneously developed their courses along belts of weak rocks; they can not be called 

 "adjusted," for in an adjusted drainage system it is quite as essential that the master rivers 

 should here and there cross belts of resistant rocks as that the remainder of their courses, as well 

 as the whole course of many smaller streams, should he along belts of weak rocks. Hence it 

 may be repeated that Gflbert does not seem to have been especially interested in this phase of 

 physiographic terminology, great as his interest was in physiographic science. 



SUBMARINE GLACIAL EROSION 



It is stated above that Gilbert did not make a study of the physics of ice or of the visible 

 behavior of existing glaciers in his attempt to determine whether the great glaciers of the past 

 were efficient eroding agents or not. It may now be added that he did make a simple but ex- 

 tremely important suggestion as to the iDvisible behavior of both existing and ancient glaciers. 

 He showed that a glacier occupying a fiord trough which extends below sea level must press so 

 heavily on the trough bottom that the sea water can not enter beneath the ice and press it up. A 

 capillary film of water may separate the ice and the rock, but this will ' ' not prevent the bed rock 

 from supporting (through the mediation of the film) the whole weight of the glacier. ... It thus 

 appears that there is no important difference, as respects its pressure on the rock bed, between a 

 glacier resting on the land and one which is partly bathed by the water of a fiord; and, so far as 

 glacial erosion is conditioned by pressure, the presence of the sea does not diminish the efficiency 

 of the glacier" (215, 216). This is probably the most novel view that Gilbert reached in the 

 course of his Alaskan journey and studies; it is an advanced term in a series of views through 

 which the explanation of fiords has progressed. It was at first thought that, although the ancient 

 glaciers occupied fiords, they did not create them; it was then believed, even by observers who re- 

 garded glaciers as effective eroding agents, that they had not eroded fiord troughs below sea level, 

 and that fiords therefore testify to postglacial submergence; it was next seen that glaciers might 

 erode their troughs to a certain depth below sea level, but that the erosion woidd proceed with 

 increasing slowness until the fiord depth was about six-sevenths of the ice thickness, when it 

 would cease because the ice would then float. Finally Gilbert adduced good reasons for thinking 

 that a glacier would, other factors being equal, erode just as effectively when nearly submerged 

 in a fiord sea arm as when on dry land. It would seem that only one further step remains to be 

 taken: That a fiord glacier woidd continue to erode, even after its trough had been so much 

 deepened that the ice surface lay below sea level, and that the limit to the glacial erosion of fiord 

 troughs would not be reached until the increasing cross-sectional area of the trough caused such 

 a diminution of the glacial current as to make it an inefficient scourer. 



Gilbert's volume closes with an enumeration of the resemblances and differences between 

 "rivers of ice and water." Of resemblances, 26 are listed. Among the differences it is noted on 

 the last page of the volume that — 



many features of rivers and river work which arise from inertia in association with swift motion are paralleled by 

 features of glaciers and glacier work which arise from high viscosity in association with slow motion. In each 



