38 TARR AND MARTIN CHANGES OF LEVEL IN YAKUTAT REGION 



fiord, where at the former stand of the land travel along the shore at high 

 tide must have been impossible. 



ELEVATED ALLUVIAL FANS, OR DELTAS 



Practically everywhere that a stream enters the fiord, in places of 

 change of level, there is found a characteristic elevated alluvial fan. Its 

 front is usually nipped away. Its top is dissected by the stream, forced 

 to change from aggradation to degradation by the lowering of its base- 

 level (see plate 16, figure 1). Many streams have intrenched themselves 

 in their alluvial fans from sealevel to the head of the fan, and are now 

 once more aggrading at their mouths and in the lowered channels (see 

 plate 15, figure 1). 



iiccompanying this change has come a growth of both annual and 

 woody plants upon the upper part of the fan, now no longer reached by 

 the floods of the aggrading stream. As in the case of the elevated 

 beaches, the nipping of the front of the fans varies with the amount of 

 uplift and the intensity of the waves. Another factor is the amount of 

 sediment, for some of the larger streams have built deposits in front of 

 the fans and thus checked the nipping. Some small fans, in areas of 

 marked uplift, have been so nipped as to form pronounced gravel cliffs 

 from 10 to 25 feet in height, and in these cliffs, as well as in the stream 

 cuts, the internal structure of the fans is clearly seen (see plate 16, 

 figure 1). 



TILL SHORELINES 



At certain places along the present shore, but particularly in the region 

 of marked uplift on the east and west shores of Disenchantment bay, 

 parts of the beach now consist of compact, unoxidized blue till. The 

 occurrence of such a clayey deposit on a beach is evidence of the recency 

 of the movement which exposed it to the waves. How fast the clay is 

 going off in suspension is evidenced by the muddy water along the shore 

 wherever these till shores are wave*-washed. They can not last long, for 

 the waves will carry off the clay and round the angular pebbles and erase 

 their glacial scratches; then normal modern pebble beaches will replace 

 these novel ones. At present, however, uplift has brought within the reach 

 of the sea margin the boulder clay which glaciers laid down at a former 

 stage of extension. This boulder clay may be ordinary till or it may be 

 an accumulation of marine silt sprinkled with scratched, angular stones 

 floated in bergs; but its proximity to shore suggests the former explana- 

 tion. These till shorelines are in all cases found either along rock shores 



