122 I. C. I : I — 1 : 1 I 1 RPACE GEOLOGY OF ALASKA. 



A section of the flood-plaiu deposits of the Porcupine where no complica- 

 tions occur presents the following divisions in their natural order and 

 approximate thickness* - : 



Peaty layer 2- 3 feet. 



Fine sill 3-5 " 



Sand 3- 6 " 



Coarse current-bedded gravels and sand. _. 1">~;0 " 



The continuity of the strata just described is broken when the river cuts 

 across a bend, as frequently happens, and a new series of deposits is begun. 



A decrease in the grade of the stream from any cause, as orographic move- 

 ment for example, would admit of the superposition of one flood-plain series 

 upon another. An occurrence of this nature seems to have taken place in 

 the low lands of the Yukon abovethe Lower Ramparts, where a layer of peat 

 is interst ratified with current-bedded sands and gravel. An increase in the 

 grade of the stream would enable it to deepen its channel and leave portions 

 of its Quod-plain as a terrace along its borders. A similar record would be 

 made by a stream descending a stable declivity, by the erosion and deepen- 

 ing of its channel, thus leaving portions of its flood-plain to record horizons 

 at which it remained for a considerable time. 



Terraces along the Upper Yukon record the fact that the stream at one 

 time flowed several hundred feet higher than at present, and in deepening its 

 channel, probably on account of orographic movement, left portions of its 

 tl 1-plaiu on the sides of its valley. 



Mum ninth Remains in the Banks of the Yukon. — Teeth and tusks of the 

 mammoth, associated with large bones, are reported to occur in abundance 

 at two principal localities along the Lower Yukon. I was not fortunate 

 enough to find any of these fossils myself, but saw several that had been 

 found by others. < »ne of these localities is near the head of the delta, but I 

 was not able to learn it- exact position. The other is On the lefl bank of 

 the river between Nowikakat and Nuklukahyet, about forty miles below the 

 mouth of the Tananah. Its position is indicated by the word " Palisades " 

 on the [J. S. Goast and Geodetic Survey map of "Alaska and Adjoining 

 Territory," and on the small map ( pi. 2) accompanying this paper. 



The bluffs at the Palisades are approximately three hundred feet high, 

 level topped, and composed of fine, light-colored, evenly stratified sediment-. 

 Back from the bluffs is a level, densely wooded table-land, with swamps and 

 ponds, bordered on all Bides, except thai adjacent to the river, by bold hills. 

 The Palisades proper are washed by the river, and form precipitous bluffs 

 entirely bare of vegetation. The same escarpment extends some ten miles 

 up the river, clothed with vegetation, and with a densely wooded flood-plain 

 along it- base. The portion ol the escarpment now washed by the river. 



