GLACIAL EROSION IN ALASKA 



IOI 



depth. For a stream valley to pass from youth to maturity, even 

 under the most favorable conditions, requires a great lapse of time. 



The form of river valley to be expected in such a mountainous coun- 

 try as the coast of British Columbia and Alaska, would therefore 

 depend largely upon the length of time that the streams had been work- 

 ing to cut the valleys. Had the stream action been brief, we should 

 expect to find profound gorges; had it been long, broader valleys and 

 the more gentle slopes of maturity. If, as is the case in Alaska, the 

 same valleys have some of the characteristics of youth and some of 

 maturity, a special explanation must be sought. 



A second characteristic which results from the normal develop- 

 ment of stream valleys is the accordance in grade between main and 

 tributary streams. No matter how fast the main stream may be 

 lowering its valley, even though it be a Colorado Eiver, the side 

 streams, including even weak tributaries, lower their mouths at ap- 

 proximately the same rate that the main stream deepens its valley. 

 This feature is so well established as a normal condition of valley 

 development, that it may be stated as a law that, under normal condi- 

 tions of stream development, tributary valleys enter main valleys ap- 

 proximately at grade. That this is not the case in many instances in 

 Alaska will be shown below. 



A third feature normally developed during the formation of stream 

 valleys is that of a somewhat winding course with overlapping spurs, 

 alternating first on one side then on the other. Because of this 



Fig. 2. Large Tributary Valley entering Grenville Channel from the East, 

 below the Sea Level. Note steepened lower slope on left side of tributary valley. Photo- 

 graph by Lawrence Martin. 



