Geology
While the island is not a volcano it is composed primarily of Miocene volcanic rocks (basalt) interbedded with marine sediments.
The steep wave-cut cliffs of its shoreline indicate that this is one of the younger Channel Islands. It exhibits at least six marine terraces; evidence of repeated tectonic uplift and subsidence (so called porpoising). Arch Point, on the north-east shore of the island is a 130 ft (40 m) arch causd by wave erosion of fault weakened rock.
Offshore, there are two named rocks: Shag Rock off the northerly shore (1-acre (4,000 m2)) and Sutil Island off the southwest end (12 acres (49,000 m2)).
Read more about this topic: Santa Barbara Island