An image of Alcatraz from Crissy Field, Presidio. Alcatraz is an island composed of Franciscan sandstone and shale, of Creatceous age. Alcatraz is home of the famous inoperative maximum-security prison, and has a great deal of cultural history. For more information visit the National Park Service's web page, at http://www.nps.gov/alca/index.htm.