What do we call the area in the valley where waste materials are dumped from mountaintop mining?

The area in the valley where waste materials are dumped from mountaintop mining is referred to as "valley fills." This terminology is specific to the practice of mountaintop removal mining, where the excess rock and soil, known as overburden, that is removed to expose coal seams, is deposited in adjacent valleys. Valley fills can significantly alter the landscape and have considerable environmental impact, as they can bury rivers and streams, disrupt ecosystems, and affect water quality. This practice is regulated due to its potential effects on the environment, and the term "valley fills" is crucial for describing both the action and the resulting geographical features in this context.

The other options, while they may seem relevant, do not accurately describe this specific practice and its implications in mining operations.