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Geology of Angel Island
The rocks of Angel Island are part of the Franciscan Complex, an extensive belt of marine sedimentary and igneous rocks which were deformed and metamorphosed during the Mesozoic. Metamorphism of the Franciscan Complex occurred at high pressures and low temperatures, producing indicator minerals jadeite and glaucophane, characteristic of subduction zone metamorphism. The rocks of Angel Island have been grouped with similar rocks displaying similar metamorphic minerals in the East Bay Hills and on the Tiburon Peninsula as the "Angel Island Nappe". The rocks are diverse, including well-exposed serpentinite in the old quarry, sandstones and conglomerates containing clasts of glaucophane schist on Kayak Beach, meta-volcanics and cherts with dark blue amphibole and brown needles of stilpnomelane on Perles Beach. However, their relationships to one another are not well understood. The Franciscan Complex rocks are unconformably overlain by flat-lying sediments of the Colma Formation near Blunt Point on the south coast of the island. These sandstones are only weakly consolidated and are eroding to provide a supply of sand to the south coast of the island, in contrast to the northern and western beaches which are dominated by pebbles and cobbles. The shape of the hillslopes on Angel Island include the scars of pre-historic landslides and mass wasting, and deposits of eroded material may have been transported away from the island by currents in the San Francisco Bay.