Anacapa Island is actually composed of three separate small islands
that are together part of a volcanic ridge. At low tide they are nearly joined by sand spits. All together, the island covers 1.1 square miles between the East, Middle, and West Islands. Its highest point is Summit Peak at 930 feet, the summit of West Anacapa. It is thought that the volcanic activity that pushed an extrusion of igneous rock up to the ocean bed surface was long ago joined by seismic activity to finally lift the island up above sea level. Over thousands of years, the wind and weather has etched the volcanic rock, but it has not lost is craggy, rugged surfaces that seem impenetrable to visitors. Reportedly it is possible to observe submarine lava flows long ago hardened to stone on now-above-sea-level exposed surfaces on the Island.
One of the most picturesque features of Anacapa Island is the natural
Arch Rock, a graceful arch of stone extending from the cliff of East
Anacapa out into the Channel as a bridge between the land and the sea.
Another interesting geologic feature on Anacapa is Cathedral Cave. This
cave is accessible by kayak or skiff, and its multi-chambered rooms can be
explored with caution during calm weather.