koyukuk-kobuk region 565 



of glaciers. Cirques, U-shaped troughs, and overdeepened valley floors 

 and oversteepened valley slopes bear witness to past glaciation. In the 

 lowland of the Kobuk Valley are moraines in places over a hundred feet 

 high which mark former fronts of valley glaciers tributary to the Kobuk 

 (figure 2, plate 34). The relation of these moraines to the gravel fill- 

 ing of the Kobuk is not definitely proved, but they seem to rise above 

 the general level with steep, north-facing slopes, and merge more or less 

 uninterruptedly toward the south with the general surface of the low- 

 land. These moraines are particularly prominent opposite the mouths 

 of the larger valleys, as, for instance, those of the i\.mbler, Selby, Maune- 

 luk, and Kogoluktuk. 



It is believed that practically all the glacial ice that reached the main 

 Kobuk Valley originated in the mountains to the north of the Kobuk, 

 and, after flowing down the largfer tributary valleys, on entering the 

 Kobuk lowland expanded into lobes both east and west. Perhaps various 

 lobes coalesced and a continuous Kobuk glacier was formed ; but it seems 

 as though the ice in the Kobuk Valley must have been dominantly stag- 

 nant rather than a vigorously eroding agent, and its effect was rather to 

 obstruct earlier drainage than greatly to erode the previous topography. 



As an instance of the obstruction of the glacial drainage may l)e in- 

 stanced the drainage across the hills south of the Kobuk, east of Lake 

 Selby. The details of the history of the changes have not been worked 

 out, but it is believed that before the period of glaciation the Lockwood 

 Hills formed the Koyukuk-Kobuk divide. With the advent of glacia- 

 tion, however, the lobes of ice from the Endicott Mountains obstructed 

 the westward drainage of the Kobuk and lakes back of the ice barriers 

 were produced. For instance, the Lake Selby glacier is known to have 

 formed a barrier in front of the mapped lake in the Kobuk lowland, and 

 it is believed that earlier it extended across the valley so that a lake was 

 formed upstream — that is, east of the barrier. As the lake rose, its 

 surface at last reached the elevation of the lowest sag across the Lock- 

 wood range and spilled over. The channel thus formed was eroded 

 rapidly until a narrow gorge was cut down to an elevation of 800 to 

 1,000 feet above sealevel. Evidence of the high level of the lake is shown 

 by gravel terraces and irregular gravel deposits up to an elevation of nearly 

 1,000 feet above the Kobuk, or 1,400 feet above the sea. The narrow- 

 ness of the gorge and the character and relation of the deposits point to 

 the conclusion that the erosive agency was water rather than ice. 



With the unblocking of westward drainage by the retreat of the Lake 

 Selby glacier it appears that a lower gap was uncovered in the place now 

 marked by Pah River. Downstream the large Ambler glacier still ob- 



