Unalaska Formation

The Unalaska Formation is a Miocene Era geologic formation on Amaknak Island and Unalaska Island, in the Aleutian Islands archipelago of southwestern Alaska.

It consists mainly of volcanic breccia, flows, tuff, and intercalated sedimentary rocks. It is exposed over about 70 percent of Unalaska Island. The Dutch Harbor Member is a 126 m thick Early Miocene sequence of the formation.

It preserves fossils dating back to the Miocene era of the Neogene period.