The Franciscan Complex sandstones (predominantly arkoses), although indurated, are susceptible to erosion when exposed to wave activity. This erosion has led to numerous sea stacks and the occasional sea tunnel, as seen in this photo. Undercutting by wave activity has led to substantial mass wasting, resulting in the steep slopes seen in the left hand side of the photo. The top of the tunnel, and the adjoining cliff to the left both show evidence of beveling by waves to a gently- sloping surface while at or slightly below sea level. Subsequent (and recent) uplift of the coast has exposed this surface about 10 m above sea level, subjecting it to erosion. Photo by Alan Grannell.