Final answer:
An erosional surface cut into bedrock stream-derived sediment composing former floodplains is known as an eroded floodplain.
Step-by-step explanation:
An erosional surface cut into bedrock stream-derived sediment composing former floodplains is known as an eroded floodplain. Erosion is a process that shapes the earth's surface, and rivers are a prime example of where erosion occurs.
Over time, flowing water can carve gullies and create erosional surfaces in bedrock and sediment, forming features like floodplains.