STREAM TERRACES OF THE YUKON. 145 



characteristic example of a rock-cut terrace occurs a few miles below the 

 international boundary. The rock is there a contorted slate and rises steeply 

 to an elevation of about 150 feet, where a broad terrace occurs, the surface 

 of which is covered by twenty to thirty feet of gravel. Back of the terrace 

 the mountain rises precipitously to a height of several hundred feet. - This 

 and other terraces of a similar character in the same general region show 

 that the Yukon at one time flowed in a comparatively broad valley, and 

 spread out characteristic flood-plain deposits. Subsequently it deepened its 

 channel and left portions of the bottom of its former rock-cut trough in the 

 form of a terrace on the mountain side. 



Most of the terraces of the Yukon are of gravel, and show that a pre- 

 viously eroded valley was deeply filled with stream-borne material, and that 

 subsequently, owing to increased grade, or perhaps to a lessening of load, 

 the stream eroded a new channel, leaving portions of its flood-plain from 

 time to time as terraces along its borders. In places from five to six ter- 

 races may be easily recognized, and not infrequently followed continuously 

 for many miles. Where they have been cut away on one side of the valley, 

 they almost invariably appear on the opposite side. No opportunity was 

 afforded, however, for examining them in detail. The most interesting con- 

 tribution that I am enabled to offer concerning them is their increase in 

 elevation as one ascends the Yukon. From about fifty feet in height above 

 the river near Anvik, they increase to over 700 feet at the international 

 boundary. Above the boundary the highest terrace is, by estimate, about 

 400 feet above the river. 



Volcanic Dust in Stream Terraces. — The scarps formed by the cutting away 

 of gravel terraces along the Yukon near the mouth of Pelly river, and at 

 many localities on the Lewes, exhibit a conspicuous white band, formed by 

 a stratum of volcanic dust from eight to twelve inches thick, which was 

 blown out of some volcano with great violence at a recent date, and deposited 

 over a very wide belt of country. This deposit has been described by 

 Dawson * and was also noticed by Schwatka. f 



I was informed by Arthur Harper, one of the most observing and obliging 

 traders on the Yukon, that a stratum of material similar to the one in the 

 banks of the Yukon below the mouth of Pelly river was seen by him at 

 Belle Isle, and also at Fort Yukon. Frank Densmore, one of the most 

 experienced frontiersmen of Alaska, reports a similar deposit in the valley 

 of the Tenanah, some 200 miles above its mouth. These observations indi* 

 cate that the bed of volcanic dust, so conspicuously exposed along the upper 

 Yukon and the Lewes, occupies a belt of country fully 500 miles broad from 

 east to west. 



* Rep. Yukon District, loc. cit., pp. 43b^6b. 



t Along Alaska's Great River. New York, 18S5, p. 196. 



