Located near the Fidalgo Complex in Washington's
San Juan Islands, Mount Erie has a unique and somewhat puzzling composition. The mountain is composed mostly of
feldspar- and
hornblende-rich
diorite. It is thought to have been formed during the
Jurassic period (155–170 Ma) by invasive volcanic flows creating batholithic masses of diorite, which also helped to create the island arcs associated with the San Juan Islands. By the end of the late Jurassic period the area experienced uplift and erosion and was subjected to intense folding and faulting. At this point most of the deposited Paleozoic and Mesozoic sediments had been eroded away, exposing the plutons of diorite, which is the primary igneous rock found on the mountain. At some point the diorite plutons experienced intrusion by
gabbro dikes associated with the formation of oceanic seafloor, indicating that the area may have experienced periods of submersion as the Juan de Fuca plate subducted underneath the North American plate accreted the area. This whole process of constant uplift and erosion followed by accretion is thought to have created the unique
ophiolites associated with the San Juan Islands and especially the Fidalgo Complex, of which Mount Erie is a part. During the Quaternary Period the area experienced many periods of glacial advance and retreat. Evidence for the glacial erosion of Mount Erie can be observed at the summit of the mountain as large striations in the diorite and gabbro. These periods of glacial erosion were separated by warming trends and continue to the present day. Big Rock in
Coupeville, Washington, and the
Wedgwood Rock in
Seattle, Washington, are both
glacial erratics that originated on Mount Erie before being transported to their current locations. ==In popular culture==