PLEISTOCENE DEPOSITS. 95 



plauated farther and farther on the west the ground which by its destructional 

 work it claimed as its bed; while simultaneously, as it abandoned or receded from this 

 plauated ground on the east, it built thereon, notably at high- water stages, flood-plain 

 and riparian deposits, which now constitute the surficial terrane of the flats. Thus, 

 while degradation or destructional work was in progress on the west, aggradation or 

 constructional work of flood-plain building was progressing on the east. At the 

 same time in the coastal region the large amount of sediments borne down by the 

 river was built into an expansive delta plain, which landward merges into and is con- 

 tinuous with the inland subareal portion of the flats, and seaward merges into the 

 already mentioned expansive tidal mud flats, bars, and reefs at the coast. 



The Colville Flats therefore appear to consist essentially of a Pleistocene veneer 

 of flood-plain, ground-ice, and deltoid deposits resting on Tertiary beds of the Colville 

 series that have been planated by the river in its lateral migration toward the west. 



Another type of recent deposits is seen in the sand dunes that occur for a distance 

 of a few miles along the upper part of the Anaktuvuk, just before it leaves the 

 Endicott Mountains, and also over the inland portion of the Colville delta and adjacent 

 mainland. The material of the dunes in both places is obviously river sand and silts 

 derived from the wind-swept bars and flats exposed during stages of low water. 

 This dune work is probably accomplished mostly during the dry, frozen periods of 

 winter, when the bare ice surface is favorable for wind transportation of the material. 



COASTAL EEGION. 



From Colville River to Point Barrovj. — Though the Tertiary beds of the Colville 

 series may come to the Arctic coast in some localities, they have not been positively 

 identified there. From Colville River to Point Barrow the shore line materials 

 consist mainly of recent beach deposits of mud, muck, some sand and gravel, and 

 deltoid mud flats, backed in places by low banks of older dark muck and ground ice 

 (see PI. IX, A and PI. X, D). West of the Colville, in Harrison Bay, deposits of 

 sand or loam are occasionally seen, which, from similarity in character of material, 

 seem to belong to the Gubik sand. 



The eastward swing, or retreat, of Colville River ; in relatively recent time 

 from bluffs of its own cutting on the west, leaving a portion of the Colville Flats on 

 its west or left side, along the section extending from Ocean Point to the coast, a 

 distance of nearly 40 miles, suggests for this particular part of the region a local tilt 

 to the eastward in Pleistocene time, or at least subsequent to the production of the 

 bluffs and the Colville Flats, which must have been formed by the river. If this tilt 

 or differential uplift to the west of the Colville has been far-reaching in its westward 

 extent, it favors the existence of a coastal belt of late Pleistocene as far as it extends. 



To local differentials in elevation along this costal line of uplift are to be ascribed 

 the occasional ground-ice cliffs as at Cape Hal kett and Cape Simpson, alternating with 



