the Franciscan Assemblage, the same range as Albany Hill, 3 miles south, and the Coyote Hills, 25 miles south in Hayward. This Franciscan formation is derived from sediments laid down in the sea west of San Francisco during the Late Jurassic and Cretaceous (150-66 million years ago) and then scraped off onto the edge of the North American plate during the subduction of the Farallon plate. The main portion of the island covers an area of approximately , with the tidal lands along the breakwater covering approximately more. A short distance from the southwest coast of the main island is Bird Rock or Bird Island, occupying approximately . The sandspit that extends for about two miles west of the main island formed along a breakwater that was installed in the 1920s to preserve a deepwater channel to the Richmond Inner Harbor. During the last ice age San Francisco Bay was a valley, and Brooks Island was just one hill of many along its western slope. As sea level rose, the valley was flooded and Brooks Island was cut off from the rest of the East Bay. Although the island is quite small, there is a permanent spring, which is used by the caretakers as a water source. Ecology The island hosts salt marshes, tidal flats and has a rise of . It is home to many bird species, including herons and egrets, and provides their nesting sites. Some parts of the islands are off limits to visitors to protect nesting sites. Mammals Brooks Island had no native land mammals prior to European contact. Domestic mammals including cattle, sheep, goats, horses and dogs were introduced intentionally between the late 18th and mid-20th centuries. House mice (Mus musculus) and Norway rats (Rattus norvegicus) were apparently introduced accidentally during this period. Although mice were later displaced by voles, Norway rats persist along the island's shores, where they have access to carrion and marine food sources, such as bay mussels (Mytilus trossulus). During dry periods, rats may occasionally prey on voles. At one point, the island caretaker controlled the rat population through poisoning. One mammal species was apparently introduced deliberately by biologists. In the summer of 1957, seven California voles (Microtus californicus), four males and three females, were introduced to Bird Island with the apparent aim of measuring the population growth rate on an unoccupied land mass. Very quickly, voles were discovered to have