The Gulf of California is an elongate trough bordered by highlands on the west and mostly by lowlands on the east. Fault scarps divide its floor into a succession of closed basins separated by ridges, some of which are capped by islands. Water flows into the open sea along the east side and leaves along the west side after partial evaporation. About half of the runoff from land reaching the gulf enters at its head via the Colorado River, but dilution of the gulf water is not evident. Winter upwelling caused by northwesterly winds brings high-nutrient (including high-silica) water to the surface where diatoms and other phytoplankton flourish so abundantly that they discolor the water. On death, the siliceous frustules fall to the bottom forming diatomaceous muds in the middle third of the gulf. High-organic matter in the muds is also a result of high production in surface waters and of anaerobic conditions within the sediments, although not in the water above them. Sediments of the northern third of the gulf are deltaic, and those of the southern third are normal marine green muds. Sands and gravels composed of shell debris and detritus from land are concentrated along the shores and in channels between islands where tidal currents are swift. The physiography and geological history indicate that the gulf is a taphrogeosyncline. Its sediments should form chiefly shale and gray-wacke, fades considered typical of geosynclines, although climatic and oceanographic factors may be more responsible for their deposition than are tectonic factors.