PLEISTOCENE DEPOSITS 251 



has been generally supposed by geologists who have drawn their deduc- 

 tions concerning this remote region from observations made on a trip 

 down the Yukon or along the western coast. The Endicott mountains, 

 as illustrated in the topography shown in plate 42, do not seem, so far 

 as observed, to have been overridden by an ice-sheet, but in the valleys 

 nearly everywhere there is such evidence of ice drainage as striae, ter- 

 minal moraines, and deposits of till. The breeding ground for these 

 glaciers was in the Endicott range, with the zone of maximum accumu- 

 lation probably somewhat north of its median line. Here the mountains 

 were doubtless largely overlain by an ice cap or neve, but the ice move- 

 ment was confined essentially to the drainageways leading off to the 

 north and to the south. But on the north slope of the range the ice 

 seems to have moved off, at least locally, in a continuous sheet or small 

 regional glacier, with its front reaching north beyond Willow creek, some 

 35 or 40 miles beyond the base of the mountains. This is evidenced by 

 the more or less continuous till sheet overspreading the entire region 

 and by deposits of drift and erratics on the highest portion of the Cre- 

 taceous plateau. In the valleys this sheet or ground moraine attains a 

 thickness of about 150 feet. From the edge of the ice-sheet ice drainage 

 in the form of valley glaciers continued about 40 miles farther north- 

 ward, to near the mouth of the Anaktoovuk, but none crossed the Col- 

 ville, whose drainageway does not seem to have been interrupted since 

 the Tertiary.' 



On the south of the Koyukuk basin similar, but not so pronounced, 

 evidence extends to beyond the Arctic circle, a distance of 50 or more 

 miles southward from the base of the range. Here, however, the gla- 

 cial phenomena, so far as observed, are more of the valley glacier type, 

 but the deposits are undoubtedly till and contain striated pebbles of 

 distinctly glacial type. Along the route of traverse, omitting the mound- 

 like remnant, about 300 feet in diameter and 60 feet in height, near the 

 middle of the range in John River valley, the glacial ice has disappeared 

 from the country. 



GROUND ICE, MARSH, MUCK, MUD FLATS, ETCETERA 



The northern 30 miles of the section, between the point where the 

 Tertiary bluffs of the Colville series leave the river, lie in marsh flats 

 whose inland half is continuous with the ground abandoned by the Col- 

 ville river in its lateral migration or drifting of 30 or more miles west- 

 ward and its simultaneous down-cutting into the Tertiary terranes, 

 while the coastal half lies in the Colville delta, both of which features, 

 however, slope down to low marshes, and finally expansive tidal mud 



