Geomorphology

Sombrio beach is located on the active Cascadia subduction zone where the Juan de Fuca and North American plates converge. (Mosher et al. 2004).  These first order processes (Klee, 1999) such as widespread faulting are present at this site with deep, rugged mountain terrains carved out by the Cordilleran glacier that dominate the coastal area near this boundary (Alley and Chatwin, 1979). With ice sheets ranging in thickness from 1000m to 450m from Victoria to the edge of the Juan de Fuca Strait, the ice sheet was integral to the formation of features including the splitting of the Puget and Juan de Fuca lobes (Alley and Chatwin, 1979). This rugged terrain consists of pre-tertiary igneous rock and metamorphosed limestone (Alley and Chatwin, 1979). With these defining features of a narrow continental shelf, a rugged shoreline and deep basins that result in small, fast running streams, this tectonically active site is indicative of a leading edge coastline (Klee, 1999). Wind exposure, high energy waves and powerful storm surges slowly erode the rugged, rocky shore. Evidence of this erosional process is seen in beach cobble consisting mainly of island lithology (Alley and Chatwin, 1979) with finer grains deposited along the southern portions of the beach through longshore drift (Klee, 1999). Steep berms at swash zones and the movement of large logs provide further evidence of high energy waves along the shoreline; while boulders deposited via glaciation processes along the intertidal and breaker zones depict a rocky, reef dominant benthic environment (Alley and Chatwin, 1979) that provides a steady surf with both ground and wind swells influenced by North Eastern winds (surf forecast).