Wiktionary
valley fill
n. The waste produced by mountain top removal mining. So named because it is typically piled high in nearby valleys.
Usage examples of "valley fill".
We made no move, however, content to sit and watch the valley fill with thick, grey mist.
If water ran so fast here, why didn't the valley fill right up to make a lake?
Each afternoon he stops on the bluff to watch the valley fill with air, light wrapping the fine branches of trees rising from the surf ace full-grown but leafless, though no wind has blown for thirty-seven years.