fan pass

A fan terrace is a fan-shaped or conical alluvial deposit formed by a stream or a river at the edge of a plain where it becomes less steep. The natural sequence of land formation in a river valley is fan, terrace, floodplain. Terraces may be alluvial, or they may be strath terraces consisting of a flattish bedrock surface created through lateral erosion by a river, with a thin layer of alluvium on top. If a floodplain is isolated from the active channel—for example, if the channel is deeply cut— then the floodplain becomes a terrace. So—from a geographer or geologist’s point of view—the distinction consists in whether the land adjacent to a river is susceptible to flooding. If it is, it’s floodplain. If it isn’t, it’s terrace.

Robert Hass