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West-central Alaska includes a broad area that stretches from the Bering and Chukchi seacoasts on the west to the upper Yukon-Tanana Rivers region on the east, and from the Brooks Range on the north to the Yukon-Kuskokwim delta on the south. It covers 275,000 km2, nearly one-fifth of the entire state—and all or parts of 29 1:250,000 scale quadrangles (Fig. 1).

Rolling hills with summit altitudes between 300 and 1,000 m and isolated mountain ranges that rise to a maximum altitude of 1,500 m characterize the area (Wahrhaftig, this volume). The uplands are separated by broad alluviated coastal and interior lowlands that stand less than 200 m above sea level. Bedrock exposures are generally limited to elevations above 500 m and to cutbanks along the streams.

The bedrock underlying this huge area consists of six pre-mid-Cretaceous lithotectonic terranes, which were assembled by Early Cretaceous time and were subsequently overlapped by mid- and Upper Cretaceous terrigenous sediments (Figs. 2 and 3; Jones and others, 1987; Silberling and others, this volume). The bedrock in the east-central part is composed of lower Paleozoic sedimentary rocks and Precambrian metamorphic rocks that belong to the Nixon Fork and Minchumina terranes. A broad mid-Cretaceous uplift, the Ruby geanticline (Fig. 4), borders the Nixon Fork terrane on the northwest and extends diagonally across the area from the eastern Brooks Range to the lower Yukon River valley. The core of the geanticline consists of the Ruby terrane, an assemblage of Precambrian(?) and Paleozoic continental rocks that was metamorphosed

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