Plains : Broad and flat, plains are well named. Some appear when glaciers and streams erode away elevated terrain; others spread where rising magma pushes, erupts, and spews. Some plains spill into the oceans, and others are bound by mountains on several sides. They all hide a tumultuous geologic history beneath their level disguise. The base of the vast Great Plains in North America formed when several small pieces of continental crust collided and welded together more than a billion years ago. As time marched forward, the base was filled with marine sediments as periodic shallow seas covered the region and glaciers, rivers, and streams eroded the Rocky and Appalachian Mountains. Today, mountain erosion continues to carry debris out onto the plains. When melting snows and heavy rains fill rivers beyond their banks, they flood. The waters spread out over the surrounding landscape and drop the load of mud, sand, and silt they normally channel downstream. Over thousands of years, the sediments build up floodplains. Alluvial plains often form where steep mountain valley rivers gush onto more level lands, forcing the rush of heavy waters to spill over their banks and drape their sediments out like a fan. Out on the wide-open valley floors, rivers twist and turn on a constant search for passage to the sea. The meandering path continues to widen the valley floor as falling sediment forever alters the course and builds up a river plain. The Snake River Plain stretches from Oregon across northern Nevada and southern Idaho into Wyoming. Its geologic history is a complicated tale of normal fractures in the Earth's crust on its western edge to a more complex plot of basalt lava flows perhaps stemming from a hot plume of magma now beneath Yellowstone National Park. Coastal plains are stretches of lowland next to oceans that are separated from the interior by highland features such as mountains and plateaus. Often the plains are portions of the ocean floor built up from the sediments rivers carry towards the sea. Geologists call the submerged part of coastal plains thecontinental shelves.