V_> RECONNAISSANCE IN NORTHERN ALASKA IN L901. 



sea floor seems i>> be its continuation. Thus the subaerial tundra country and the 

 submarine coastal shelf almost appear u< be parts ot' the same plain. Beneath the 

 siM. deposition is still active, while on most of the surface of the subaerial portion of 

 tin' plain the work of erosion has hardly yet begun. 



The series consists principally of heavy-bedded silts, soft sandstone, limestone, 

 shale, and lignite. It is best exposed along the (Jolville in the region of the mouth 

 of the Anaktuvuk. where it forms steep-faced bluffs about 200 feet in height, extend- 

 ing for a number of miles both up and down the river (PL VIII). 



Structure. — The Colville series has been but little disturbed. The beds lie nearly 

 horizontal. On the left bank of the Anaktuvuk, 10 or 12 miles above its mouth, 

 they dip southward at an angle of -4° or 5°, but from near the mouth of the Anaktu- 

 vuk, along down the Colville nearly to the coast, they dip northward or northeast 

 ward (see PI. VIII), so gently that great care is required to detect the introduction of 

 successively newer and younger beds, as one ascends the geologic section in pro- 

 ceeding northward downstream. In the vicinity of the mouth of the Anaktuvuk 

 some very broad, shallow depressions or folds are revealed. Above the mouth the 

 dip is northward, at an angle of 8° to 10°, but a few miles below it is southward at 

 about the same angle, so that the lower part of Anaktuvuk River seems to lie in a 

 very shallow syncline. 



Besides this gentle folding, the beds have also suffered slight faulting (see PI. 

 VIII, J3). The maximum vertical throw in the faults observed is about 2 feet. 

 Lateral crowding has also taken place, producing local crumpling or folds and small 

 overthrusts, as shown in PL VIII, B, C, and PL XIII, A. In nearly all cases 

 the crowding or thrust seems to have come from the south, indicating that crustal 

 movements have apparently taken place in the mountain region in geologically 

 recent time. The inland or upstream side of any particular stratum which has been 

 broken or disturbed has been crowded or overthrust onto the coastal or downstream 

 side of the same, often producing a short overlap. Disturbance, however, is hardly 

 noticeable in the younger Pliocene portion of the beds toward the coast. 



Observations made along Colville River indicate that the series has a thickness 

 of 500 or 600 feet. 



On account of difference in degree of consolidation of the beds, together with 

 their fossil and lignite contents, the series has been separated by the writer into two 

 parts, upper Colville and lower Colville. It is possible that an unconformity may 

 exist between the lower and upper parts .of the series, but if present it must be very 

 slight. 



Lower Colville (Oligocene). — This constitutes the main portion or lower three- 

 fourths of the section exposed at the mouth of the Anaktuvuk. It is about 150 feet 

 thick, while the upper Colville at this same locality is about 40 to 50 feet thick. 



