The Miocene Bear Lake Formation--chiefly sandstone, shale, and conglomerate--crops out on the Alaskan Peninsula between Port Heiden and Pavlof Bay. As thick as 1,600 m in outcrop and 2,368 m in subsurface, the Bear Lake Formation appears to have been deposited mostly by tidal processes in a semi-enclosed back-arc basin that was bordered to the southeast by volcanic uplands of the Aleutian arc. To the northeast, the basin originally extended beneath Bristol Bay as part of the North Aleutian basin. The Bear Lake Formation, which rests unconformably on Oligocene volcanogenic sedimentary rocks and is unconformably overlain by Pliocene volcanic rocks, contains few, if any, interbedded volcanic rocks. Sandstone of the Bear Lake Formation contains more quartz, locally as much as 65%, than most Tertiary strata of the Alaska Peninsula. Rounded clasts of granitic rocks as large as 25 cm were probably derived from large batholithic complexes to the southeast.

Sandstone beds are characterized by large-scale trough and tangential-tabular cross-strata, herringbone cross-strata, shale drapes on cross-strata, reactivation surfaces, channeling, superposition of small-scale cross-strata or current ripple markings on large-scale cross-strata with reversal of flow directions, scattered megafossils, local coquinas, and local burrows that include Ophiomorpha. Shaly sequences are characterized by flaser bedding, current and oscillation ripple markings, starved ripple markings, abundant small-scale bioturbation, load casts, abundant mica and plant fragments, and synsedimentary slumps. Coarse-grained fluvial deposits at the base and fine-grained marine shelf deposits at the top of many sections suggest deposition during a major transgression, possibly a a result of subsidence of the Aleutian arc during an interval of relative volcanic quiescence.

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