Pediment
"any relatively flat surface of bedrock (exposed or veneered with alluvial soil or gravel) that occurs at the base of a mountain or as a plain having no associated mountain. Pediments, sometimes mistaken for groups of merged alluvial fans, are most conspicuous in basin-and-range-type desert areas throughout the world." (Britannica, 2020)
Sources
Britannica, T. Editors of Encyclopaedia. "pediment." Encyclopedia Britannica, January 29, 2020. https://www.britannica.com/science/pediment-geology.