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The extensive, gray urban area that surrounds the southern end of San Francisco Bay contrasts markedly with the surrounding rugged ridges, hills, and low mountains. The darker hills and ridges reflect dense vegetation, and the browner landscape shows less vegetation. Apparent at the extreme southern end of the bay are manmade dikes for harvesting salt. The variety of colors in the adjacent salt ponds occurs because of the different saline content of each pond. As the process progresses from seawater to solid salt, different species of algae produce different colors.