What do you predominantly find in valley fills after mountaintop mining?

In valley fills created by mountaintop mining, the predominant material found is waste materials. This practice involves the removal of the summit of a mountain to access coal seams underneath, and the surplus rock and soil, known as overburden, is often deposited into nearby valleys, creating these fills.

These waste materials typically consist of a mixture of soil, rock, and other debris that is not economically viable to extract for coal. As a result, the landscape changes significantly, often burying valleys and disrupting existing ecosystems. Due to the nature of the mountaintop mining process, the main purpose is to dispose of the excess materials, making waste materials the most prevalent constituent in valley fills.

Other choices like coal, vegetation, and mining supplies play lesser roles in this context; coal is the target resource but is not what fills the valleys, while vegetation is usually disrupted by the mining process, and mining supplies are transient and not a product of the valley fills themselves.