Located near the Fidalgo Complex in Washington's San Juan Islands, Mount Erie has a unique and somewhat puzzling composition.

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.

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.

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.