Early coastal classification schemes were based on sea-level variations. Thus, submerged coasts are drowned river and glacial valleys (rias and fjords), while emerged coasts are characterized by coastal plains. A second type of classification distinguishes between primary coasts, which result mainly from non-marine processes and include drowned river valleys and deltas, and secondary coasts, mainly formed from marine processes and/or marine organisms, for example, barrier coasts, coral reefs, and mangrove coasts.