In different regions of the Appalachian Plateau, enough plant debris accumulated to form peat, which upon burial, compaction and heating was made into the coal of the Appalachian coalfields. Due to the abundant coal in the Appalachian Plateau, coal mining has been a staple of the area and has proved to be a very successful mining hub. Iron ore was once an extremely abundant natural resource but due to the thin layer of iron, over time it was mostly depleted.