September Geology Image of the Month: Angular Unconformity at Beach 4, Olympic National Park

The September Geology Image of the Month was taken by State Geologist, Dave Norman. It shows a well-exposed angular unconformity in the rocks at Beach 4, located along coastal Highway 101 between Ruby Beach (to the N) and Kalaloch (to the S), and within the bounds of Olympic National Park.

An angular unconformity is created when strata that were originally deposited horizontally are then uplifted, deformed, and eroded before being covered by a later horizontal deposit (see diagram below). An unconformity represents a gap of geologic time missing in the rock record.

Generation of an angular unconformity.
From EarthScience Australia (Paul Michna where no other author is indicated) / CC BY-NC-SA 3.0

At Beach 4, the tilted strata are comprised of Miocene-aged turbidite deposits overlain by weakly stratified Pleistocene glacial outwash deposits. An excellent field trip guide to this area is available on (WWU researcher) Dave Tucker’s geoblog.