The metropolitan areas of Miami, Fort Lauderdale, and West Palm Beach are located on a rise in elevation along the eastern coast of Florida, called the Eastern Coastal Ridge, that was formed as waves compressed ooids into a single formation. Along the western border of the Big Cypress Swamp is the Immokolee Ridge (or Immokolee Rise), a slight rise of compressed sand that divides the runoff between the Caloosahatchee River and The Big Cypress.[19] This slight rise in elevation on both sides of the Everglades creates a basin, and forces water that overflows Lake Okeechobee to creep toward the southwest.[20] Under both the Miami Limestone formation and the Fort Thompson limestone lies the Biscayne Aquifer, a surface aquifer that serves as the Miami metropolitan area's fresh water source. Rainfall and stored water in the Everglades replenish the Biscayne Aquifer directly.[16]