730 



SCIENCE 



[N. S. Vol. XXVII. No. 697 



Working Hypothesis on the Physiography 

 of Alaska: Wallace W. Atwood, 

 Chicago, 111. 



Associated with the study of the Cre- 

 taceous and Tertiary formations in the dis- 

 trict of Alaska, it has been necessary to 

 consider with some care the physiographic 

 history of the district. In the coastal 

 provinces there has long been recognized 

 a great peneplain which is now represented 

 by the summits of the Coast ranges. This 

 is shown in southeastern Alaska, along the 

 gulf of Alaska and on the Kenai Penin- 

 sula. 



In the Controller Bay region and in the 

 Cook Inlet region, distinct peneplains were 

 recognized below the summits of the Coast 

 ranges. In the Cook Inlet region the lower 

 peneplain is post-Kenai (Oligocene) and 

 the upper or summit peneplain, there 

 represented by the crest of the Kenai 

 Peninsula, is certainly pre-Kenai. Through 

 the Kenai Peninsula there are certain 

 passes which correspond in elevation with 

 the general level of the lower peneplain, 

 and which are the valleys developed dur- 

 ing the period when the second peneplain 

 was being developed. 



In crossing the Coast range at White 

 Pass, the uppermost or summit peneplain 

 is readily recognized, but below this level 

 there is a system of broad open valleys in 

 which White Pass, Chilcoot and Chilcat 

 Pass belong. Passing to the northward 

 and do>vn the head waters of the Yukon 

 system, ' the uppermost, or summit pene- 

 plain, was traceable, but the remnants 

 which reached that level became less and 

 less in extent and in number. At Cariboo, 

 in the Yukon territory, an intermediate 

 bench appears and to the northward many 

 more extensive benches and ridges reach 

 that intermediate level. Before reaching 

 Dawson the intermediate horizon is seen to 

 represent the summit of the Yukon 

 plateau. This plateau has been recognized 



by many as a great peneplain, and its cor- 

 relation with the summit peneplain has 

 been suggested. The studies during the 

 past season would indicate that these two 

 great Peneplains are distinct, and suggest, 

 at least, that the broad passes through the 

 Coast ranges are of the same age as the 

 Yukon plateau peneplain. 



In the central portion of the great 

 Yukon plateau area between Eagle and 

 Fairbanks, the Yukon plateau is well 

 shown, and rising above it there are many 

 monadnock forms which belong to the 

 earlier or summit peneplain of the Coast 

 ranges. There are also distinct benches 

 below the Yukon plateau, and from 1,200 

 to 1,400 feet above sea-level, which attract 

 attention. They are represented at Eagle 

 by a low ridge just south of town, and in 

 the valley of Seventy Mile and Mission 

 Creek, by broad open valleys above the 

 present gorges. 



Continuing the studies farther down 

 stream, the upper peneplains became less 

 and less conspicuous, and the lower one 

 grew in importance and then took a lesser 

 place, as the modern peneplain, repre- 

 sented by the broad alluvial flats of the 

 lower Yukon, Koyukuk and Kuskokwim 

 rivers became more conspicuous. 



The hypothesis stated briefly is that the 

 great summit peneplain of the coastal 

 province is distinct from certain lower 

 peneplains bordering the Pacific and from 

 the Yukon plateau peneplain of the in- 

 terior; that in the Yukon basin there have 

 been several peneplains developed and that 

 each process of peneplaination has moved 

 as a wave up that basin from the west- 

 ward, just as the modern peneplain is to- 

 day moving up that valley ; that the extent 

 and number of remnants of each peneplain 

 decrease from the headwaters of the Yukon 

 toward the mouth, just as terrace remnants 

 in a single valley may decrease from their 

 up-stream termini to the lower portion of 



