The lower Kuskokwim-Bristol Bay region is an area of about 25,000 square miles in southwest Alaska which includes most of the mountainous province between the Bethel lowlands on the west and the Nushagak lowlands on the east. The region has been repeatedly subjected to strong compressive forces operating northwest and southeast, and strata of Late Cretaceous age and older are severely deformed.

The rocks range in age from Precambrian to Quaternary. Except for continental volcanic rocks of Tertiary and Quaternary age, the formations were deposited under geosynclinal conditions. From oldest to youngest, the stratigraphic column consists of: (1) metamorphic rocks of Precambrian age, (2) limestone of Devonian age, (3) the Gemuk group consisting of sedimentary and volcanic rocks ranging in age from Carboniferous to Early Cretaceous, (4) andesitic volcanic rocks of Middle and Late Jurassic age, (5) the Kuskokwim group consisting of graywacke and shale of Cretaceous age, (6) continental basalt of Tertiary and Quaternary age, and (7) surficial deposits of Quaternary age.

Volcanic rocks of Jurassic age which crop out on the west flank of the Kilbuck Mountains were, until recently, thought to be of Late Cretaceous age. The Jurassic age of these rocks indicates that they are part of the Ruby geanticline and that the Kuskokwim geosyncline probably does not extend southwest beneath the Bethel basin.

The Kuskokwim group is characterized by coarse-grained clastic rocks which were derived from local geanticlinal uplifts and deposited in the Kuskokwim geosyncline. Several lines of evidence show that these rocks were deposited rapidly and constitute an unsorted "poured-in" sediment of low porosity.

Major structural elements of the region are the Ruby geanticline, Kuskokwim geosyncline, and Goodnews arch which were formed in Middle to Late Jurassic time. During Late Cretaceous and early Tertiary time the rocks were folded and faulted and intruded by igneous rocks of several kinds.