Millions of years ago, most of California lay under the ocean until the Pacific plate collided with the North American plate and was forced beneath it, scraping rock from the sea bottom into a jumbled mix of shale, sandstone, and deep oceanic crust that became serpentine. Pressure along the San Andreas fault slowly pushed this chaotic mix of rocks above the ocean surface, creating the Coast Range seen today, where different rocks weathered into different soils that support different plant communities on high, dry mountains and in river valleys. Serpentine soils are low in nutrients and high in metals, so few plants can grow there, leaving gray, treeless barrens on the mountains, while nearby sedimentary soils support dense stands of pine, oak woodland, and chaparral. Water also shapes the ecosystem, with willows and cottonwoods growing along the river bank, grassy oak woodland thriving in moist valley bottoms, and chaparral taking over on dry, well-drained hillsides.