230 GROVE KARL GILBERT— DAVIS UIem " IRS [vol T x1^ 



A summary of results declares the detected fluctuations of glacier ends to be unsystematic; 

 not even when the retarded fluctuations of long as compared with short glaciers is allowed for 

 did the advances and retreats exhibit an orderly relation. Hence Gilbert was led to speculate 

 as to the possibility of other controls of fluctuation than those dependent on climatic variations 

 as ordinarily conceived, and he therefore suggested that " the combination of a climatic change 

 of a general character with local conditions of varied character may result in local glacier varia- 

 tions which are not only unequal but opposite" (109). The results of such a combination are 

 worked out by a very ingenious discussion: It was shown that if the ocean water in the 

 Gulf of Alaska has such a temperature as is "most favorable for the development of glaciers 

 in the district as a whole, it will be too warm for the highest development of certain glaciers 

 and glacier systems and too cool for others." Hence "whenever such a condition obtains, a 

 change in ocean temperature will cause some glaciers to enlarge and others to contract" (111). 

 The analysis is regarded as serving its purpose "if it has given plausibility to the suggestion 

 that a change in some meterologic factor or factors may result in simultaneous modifications 

 of glaciers which differ not only in amount but in algebraic sign." It is then pointed out that 

 because of the greater variety of local conditions found in Alaska than in Europe or Green- 

 land, " the complexity of interacting conditions may for a long time baffle attempt at its analysis, 

 but when the complexity has been resolved, the resulting theory should have wider application 

 than one founded on simpler phenomena" (112). The two pages devoted to this topic may be 

 instanced as exhibiting Gibert's logical ingenuity in a highly characteristic form. 



GLACIAL EROSION OF FIORDS 



The second half of the volume on "Pleistocene glaciation" contains the most cogent pre- 

 sentation of the evidence for strong glacial erosion of a mountainous coast that has ever appeared. 

 It is a model of orderly, powerful, and convincing procedure. The preglacial topography of the 

 region is given an explanatory description as follows: 



After the folding and squeezing of the metamorphic rocks, there was a long period of erosion in which broad 

 tracts of the land were worn down nearly to sea level. Then came uplift, producing a [slanting] plateau from 

 3,000 to 8,000 feet high. . . . The period of subsequent erosion has been long enough for the development of 

 local peneplains [on weak rocks] at a lower level, and in that time the plateau [of harder rocks] has been greatly 

 modified. When the glacial period opened, the larger part of the region was mountainous in the ordinary sense, 

 with crests at various heights and a complicated system of steep-sided ridges, spurs and gorges. There were 

 extensive remnants of the high-lifted peneplain, its plateaus marking the areas of most resistant rocks, and above 

 these plateaus rose summits of the nature of monadnocks. There were remnants of a low peneplain .... and 

 these occupied areas of relatively weak rock. There was a system of river valleys or master lines of drainage, 

 narrow where the rocks were most resistant and more open among weak rocks. The bottoms of these valleys 

 were in part below tide level (129, 139). 



It is further stated that " at the date of the lower peneplain," the region presumably included 

 all phases of the topographic cycle, "being infantde to adolescent where the rocks are most 

 resistant, adolescent to mature .... [where the rocks are of medium resistance] and senile 

 where the rocks are weakest" (130). Before taking up the question of glacial erosion, it was 

 pointed out that the preglacial valleys of river erosion can not have been so deep as the present 

 fiords, for that would have involved a relative elevation of 3,000 feet, and "such a change would 

 carry a very large area above snow fine, and would so promote the alimentation of glaciers as to 

 flood the whole district with ice and abolish stream erosion" (136). 



The problem of glacial erosion is then considered ; and not only the cirques in the mountains 

 that surmounted the plateau of the uplifted peneplain and the high-level lateral valleys above 

 the fiords, but the deep fiord troughs also are ascribed to it. Gilbert writes: 



For this complicated system of troughs I have not been able to suggest an origin that does not involve 

 an immense amount of excavation by ice (144). 



Various combinations of river and glacier erosion are examined, but no combination makes 

 it possible to account for the discordant relations of the high lateral valleys and the deep fiord 

 troughs without powerful excavation by glaciers; and it is interesting to note that the discordance 

 of depth between the high-level valleys — or "hanging valleys," as Gilbert proposed they should 



