has part adjacent to composed primarily of plain alluvial plain SWEETRealm:AlluvialPlain EcoLexicon:alluvial_plain https://en.wikipedia.org/wiki/Alluvial_plain A relatively flat and gently sloping landform found at the base of a range of hills or mountains, formed by the deposition of alluvial soil over a long period of time by one or more streams coming from the mountains. seamount range of mounts alluvial soil