464 E. Blackwelder — Yahutat Coastal Plain of Alaska. 



mixed fluviatile deposits tying one upon the other and perhaps 

 alternating in successive wedge-shaped beds. 



The stratification planes of the various classes of deposits 

 have peculiarities of importance, and these surface features are 

 in turn reflected in the minor structures seen in cross-section. 

 The aggrading streams are all filled with fan-shaped bars which 

 have gentle back slopes and steep outer slopes. The depo- 

 sition of sediment in this form produces prominent cross- 

 bedding. More intricate cross-bedding is caused by the alter- 

 nate cut and fill of the shifting rivers and by the migrating 

 sand dunes. Cross-bedding is also doubtless a common feature 

 of the deposits now forming near the zone of breakers in the 

 ocean. On the whole, therefore, much of the deposits of the 

 foreland must be cross-bedded. 



Ripple-marks, although absent in the deposits of the swamps 

 and quiet lagoons, are characteristic of the beach and river 

 sediments. On the ocean shore, regular ripple-marks of the 

 long parallel type are made by the water currents below tide- 

 mark and by the wind currents above that line. In the chan- 

 nels of the streams the ripple-marks are usually shorter and 

 of the spatulate type, with gentle back slopes and steep 

 fronts. They are even more numerous there than along 

 the littoral zone. Some which were observed by our 

 party near the head of the Alsek delta were exceedingly 

 large. These were composed of bowlders and coarse gravel 

 and had a relief of 4 to 6 feet with wave-lengths of scores of 

 feet. In fact they were so large that I did not recognize their 

 true character while clambering over them, but only when I 

 saw them in panorama from a considerable elevation. Ripple- 

 marks of such magnitude would of course express themselves 

 in cross-section as strong cross-bedding. They give some idea 

 of the power of the current in the Alsek River when it is in 

 full flood. 



Sun-cracks, another feature of terrestrial and tide-water 

 sediments, appear to be uncommon here. I do not remember 

 having seen any. The explanation is doubtless to be sought 

 in the climate, which is so moist that muds rarely become dry 

 enough to crack. If the cracks occur anywhere, the tidal 

 sounds would furnish the most favorable conditions. 



Organic remains are being preserved between some of the 

 layers to-day and they offer a further means of identification 

 of deposits of this general class. In the submarine zone of 

 the plain, shells of mollusks, echinoderms and other aquatic 

 animals lie upon the bottom ; but as life is not very abundant 

 in this boreal sea, the shells preserved should be somewhat 

 scarce. In the terrestrial zone, driftwood is buried by the 

 dune sand, while forest trees are undermined by the rivers and 

 enveloped in deposits of gravel or sand. In the finer sedi- 



