Here's why:

* Formation: Coal forms from the remains of ancient plants that died and were buried under layers of sediment. Over millions of years, heat and pressure transform this organic matter into coal.

* Sedimentary Rock: The process of sediment accumulation and transformation creates sedimentary rock, and coal is a type of sedimentary rock.

While coal is found in the sedimentary layer, it can be located at different depths depending on the geological history of the area.