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The east-central Alaska region extends west from the Canadian border to the southeast edge of the Koyukuk basin, and north from the Yukon-Tanana upland to the southeast flank of the Brooks Range (Fig. 1). The region encompasses all or parts of 13 1:250,000 quadrangles and covers an area of about 140,000 km2.

The physiography of the region (Wahrhaftig, this volume) is extremely diverse (Fig. 1). Steep, mountainous areas having high to moderate relief include parts of the Ogilvie Mountains, White Mountains, Yukon-Tanana upland, Ray Mountains, and southeastern Brooks Range. Areas of dissected plateaus and rolling hills with moderate to low relief include the Porcupine plateau and the Kokrines-Hodzana upland. Lowlands characterize the Yukon Flats basin and extend upstream into other parts of the Yukon River and Porcupine River drainage basins.

Bedrock exposures are severely limited in most of the east-central Alaska region. Mountainous areas are locally glaciated, but except in the most rugged parts of the Ogilvie Mountains and parts of the Brooks Range, bedrock is extensively mantled by surficial cover and the colluvial products of alpine weathering processes. Stream cuts commonly offer the best exposures in generally forested or tundra-covered areas of moderate relief. Lowland areas have few pre-Cenozoic bedrock exposures because of extensive surficial cover and tundra.

The pioneering reconnaissance studies by J. B. Mertie and his predecessors were carried out in the east-central Alaska region prior to 1940. A relatively small group of geologists conducted the basic geologic mapping during the 1960s and 1970s that defined the

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