42 STEPHEN R. CAPPS 



and on cold days, deposition is general throughout all the stream- 

 courses. 



The character of the valley gravels changes notably as one pro- 

 ceeds down stream. Near the glacier the gravels are generally 

 coarse. Farther down they become progressively finer and finer, 

 and bars of small gravel and sand take the place of the cobbles so 

 abundant above. 



All of the larger tributaries of the Nabesna River, including 

 Nikonda, Bond, Cooper, and Stone creeks from the east, and Monte 

 Cristo, Jacksina, Jack, and Platinum creeks from the west, have 

 gravel bars similar in origin to those in the main Nabesna Valley, 

 but smaller in size. 



Terraces. — ^The conditions for the deposition of the gravel beds 

 in the Nabesna Valley have not been uniform since the withdrawal 

 of the greater glacier, and it is not to be expected that they would 

 have been. The retreat of the ice probably consisted of many 

 withdrawals interrupted by halts, or even by slight readvances, and 

 from time to time the quantity of water, as well as the abundance 

 and variety of the materials to be transported would have varied. 

 At times of ice-advance the streams were supplied with an excessive 

 amount of detritus, and would therefore have built up their beds 

 with great rapidity. Later, with a comparative scarceness of gravels 

 to be carried, the streams would have been able to entrench them- 

 selves in the gravel bars already formed. That some such condi- 

 tions actually did exist is shown by the terraces of stream-laid gravels 

 which are now to be seen at various places along the Nabesna Valley. 

 These terraces have their best development between Bond Creek 

 and California Creek. At their upstream end they reach an eleva- 

 tion of about 200 feet above the river, but slope gradually downward 

 to the north, and near Bond Creek merge with the gravel bars which 

 the river is now building. 



Just south of California Creek, the plain-like surface of the ter- 

 races is broken by a number of low, irregular hillocks, composed of 

 glacial till. These indicate that the terrace gravels were here laid 

 down around and on top of a terminal moraine during the retreat 

 of the glacier. The stream has now cut its channel 200 feet into 

 these gravel beds. 



