The Port of Redwood City is the only deepwater port in southern San Francisco Bay. It developed as early as the 1850's, as a loading area further up Redwood Creek for the redwood trees that were harvested from the hills, and taken up to San Francisco as construction material. Slowly land was filled in and the port area extended further out into the marshlands of the Bay, and the Army Corps kept the waterway dredged for ships. Bulk industries continue to dominate the port, mostly material recycling, including construction debris, recycled industrial fuel, concrete and asphalt, and scrap metal. Cargill Salt also uses the area for salt crystallization and harvesting, though most of this activity takes place across the Bay in Newark. Piles of recycled bulk material can be found throughout the port. Gypsum is imported from Mexico and is used by wallboard companies; shredded scrap metal from 200,000 vehicles per year are loaded on ships bound for Asia; and used concrete is ground up and reused. Portland Cement was once made from oyster shell deposits dredged from the bottom of the Bay. Now the RMC Company imports cement and stores it in silos at the port for use in construction projects throughout Northern California.