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Glaciers originating in the central Brooks Range extended south into the lower Alatna Valley during three major episodes of Illinoian and Wisconsin glaciation. During the oldest glaciation, coalescing glaciers formed a piedmont ice sheet that extended 60 miles south of the range and covered most of the Koyukuk Lowlands. The later Kobuk Glaciation, subdivided into two stades, was marked by a smaller lobe that terminated in the Koyukuk Lowlands, receded, then readvanced to a frontal position within the Alatna Valley 25 miles closer to the Brooks Range. Drift of the Itkillik Glaciation, which marks the final period of ice advance into the lower Alatna, is subdivided into four moraine belts. Siruk Creek moraines, the oldest and most extensive Itkillik deposits, form an arcuate belt that extends 30 miles south of the range front. The later Chebanika moraine marks a brief readvance or stillstand of glacier ice farther upvalley. Ice then stagnated in the Helpmejack Lakes area, and massive bodies of ice-contact stratified drift were deposited in a broad belt 8 to 15 miles south of the Brooks Range. Moraines at the range front represent the last major event of the Itkillik Glaciation.

Subdivisions and correlations of the glacial sequence are based on extent and position of the drift sheets, inferred glacier regimens, postglacial modification of glacial deposits, radiometric dates, and ice limits within the upper Alatna Valley. The four episodes of Itkillik glaciation within the Alatna drainage system appear to be equivalent to four Itkillik stades in the Anaktuvuk Valley, and radiocarbon dates from both areas indicate a late Wisconsin age for Itkillik deposits. The preceding ice advance, correlated with the Kobuk Glaciation farther west, may represent the early Wisconsin. The oldest glaciation is correlated with an ice advance of Illinoian age that filled the Kobuk Valley and extended west into Kotzebue Sound.

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