230 BULLETIN : MUSEUM OF COMPARATIVE ZOOLOGY. 



northeast and thus runs nearly parallel to the Bay. The valley 

 terminates, at a distance of eight or ten miles from the cascade, in a 

 number of deep corries. Unlike the fiord, the lateral valley flares 

 broadly and its floor is occupied by a half dozen lakes large and small. 

 It is itself provided with two branch-valleys which join it just above 

 the cascade. One of them ends in a glaciated col, the other in a mag- 

 nificent cirque a mile in diameter. The drainage of these three valleys 

 serves to sustain the waterfall all the year round. 



It may also be noted that a line series of cirques and hanging valleys 

 look over the Kogarsuk. The most notable of these is the one whose 

 drainage, forming the Sennerkitte brook, cascades more than two hun- 

 dred feet into the Kogarsuk. It lies north of Mt. Ford, runs five 

 miles or more to the eastward, and is floored over with a number of 

 small lakes. (Plate 12.) 



An early statement of an explanation for these apparently abnormal 

 valleys was given by Helland : " If a glacier fills a tributary valley, and is 

 thinner than that in the main valley, the depth to which it erodes its bed 

 must be less than the depth of the main valley. Hence many tributary 

 valleys must debouch high above the bottom of the main valley. In- 

 stances abound of tributary valleys debouching thousands of feet above 

 the beds of the main valleys along the steep sides of the fjords of western 

 Norway." 1 Helland does not seem to have recognized the full signifi- 

 cance of his idea in the general theory of glacial erosion ; perhaps be- 

 cause he was so fully persuaded at the time of the competency of glaciers 

 in this regard, that further emphasis was not necessary. The advantage 

 of living on the ground may here have aided perception just as the an- 

 cient belief of the Swiss peasants long antedated the theory of Char- 

 pentier and Agassiz that the Swiss glaciers were once much greater 

 than now. Davis, Gilbert, and Penck have recently and independently 

 developed the idea to something like its full importance. Reference may 

 be made to the detailed memoir of Davis for fuller information on this 

 subject. 2 



Soundings showed that Nachvak Bay is an excellent type of fiord. 

 Twenty-one casts of the lead sufficed to determine the submarine relief 

 in its main features. (Plate 12.) The greatest depth, of one hundred 

 and ten fathoms, was found at a station six miles from the Narrows, 

 where the water is nearly as deep. Two miles to seaward of the Nar- 

 rows, the bottom shoals to an interrupted rock-sill which forms a long 



1 A. Holland, Quar. Jour. Geol. Soc, 1877, Vol. 33, p. 174. 



2 W. M. Davis, Proc. Bos. Nat. Hist., PJ00, Vol. 29, p. 273. 



