566 p. S. SMITH GLACIATION IN NORTHWESTERN ALASKA 



stmcted westward drainage, and it is b}^ no means improbable that the 

 Kogolnktuk and Mauneluk glaciers also crossed the Kobiik lowland and 

 abutted on the hills to the south.. It is possible that the Pah Eiver pass 

 was previously occupied b}^ a south-flowing stream during the approach 

 to maximum glaciation, but the records are so indistinct that they were 

 not discovered. Suffice to say that the large terraces from 200 to 300 

 feet above the Kobuk seem directly referable to this stage. 



With the further retreat of glaciation the Ambler obstruction was 

 removed and a westward discharge permitted. It is difficult, however, 

 to explain why the transverse south-flowing drainage b}^way of the Pah, 

 having been established, was abandoned and the present northward 

 course acquired. Possibly the cause is to be found in the unblocking of 

 the western part of the Kobuk Yalley by the retreat of the Ambler 

 glacier; but the whole explanation is undoubtedly concerned with a 

 number of correlated incidents. For instance, the glaciation of the 

 north and east sides of the Zane Hills undoubtedly had an effect on the 

 southward discharge. Thus, although it is believed that the Zane Hills 

 glaciers never extended far from the front of the range, they probably 

 fed a large amount of waste into the Hogatza lowland. This condition, 

 with the diminution in the amount of water in the Kobuk and the un- 

 blocking of the former Kobuk Yalley, may have so interacted that a 

 reversal of drainage through capture was made possible. 



Alatna Eiver 



In the Alatna Eiver basin there is evidence not only of past glaciation, 

 but even active glaciers were foimd in 1911. The existing glaciers are 

 only 1 to 2 miles in length and are located in the high granite peaks to the 

 west of the central part of the Alatna Yalley. Figure 1, plate 35, shows 

 the serrate ridges in the background and one of the glaciers. The ele- 

 vation of the foot of the glacier is about 3,000 feet above the sea and the 

 higher points are from 4,000 to 5,000 feet above the valley floor. 



That the present glaciers are but the shrunken remnants of once more 

 sizeable ones is evident from the topography and deposits at many places 

 in the upper Alatna Yalley. Topographically recognizable moraines are 

 practically absent throughout the Alatna Yalley, but resorted deposits 

 containing lapge angular boulders, apparently ice-transported, are found 

 at many places. In the central part of the valley a deposit of blue clay 

 is formed of glacial rock flour. Ice-transported erratics and outwash 

 gravel deposits have been found up to an elevation of 2,300 feet above 

 the existing main stream. On the northern divide of the valley head- 

 ing in the glaciers, at an elevation of over 2,500 feet above the stream, 



