Coastal plains are formed when ocean levels fall on continental shelves that are at sea level, or when rivers carry sediment to areas of the ocean. These plains can take hundreds of years to form, and a few are found hundreds of miles inland.
Many coastal plains that formed on continental shelves were made following the ice age. When the ice age ended, warm waters entered low lying areas, forming plains. Scientists use fossils to identify coastal plains that rest hundreds of miles inland. For example, marine fossils are present in Kansas, indicating they were able to move significant distances.
Another means of coastal plain formation is sediment, rocks and other hard substances moving from rivers to low lying areas of the ocean. Over a period of years, these materials form several layers, eventually resulting in a plain. This takes place over millions of years. For example, one coastal plain in Alaska features layers of sand and shells, alongside fossilized whale bones. Much of the sediment came from the nearby Appalachian mountains, and the plain required several thousand feet worth of material to form. Sometimes these areas give rise to fertile lands, like the Oxnard Plain in California, which is particularly suitable for growing strawberries.