Final answer:
A steep slope separating a floodplain from terraces above it is known as a bluff. This geological feature is typically found alongside riverbanks or coastlines where rivers have created steep-sided valleys.
Step-by-step explanation:
The steep slope separating a floodplain from terraces above it is called a bluff. A bluff is an elevated landform with a steep, often vertical, slope, found typically alongside riverbanks or coastlines.
These features can be prominent in areas where rivers have cut through layers of sediment and rock, creating steep-sided valleys and cliffs.
When these features are associated with a river system, they mark the transition between the lower floodplain and the higher terraces that were formed during periods when the river level was higher.