Theodore Roosevelt national park - Red Rock

Similar to that of a kiln baking ceramics, when a coal vein ignites (due to lightening strikes, forest fires, etc.), the heat will bake whatever rock sediment is above. Whether is be a sandstone, mudstone, and/or Bentonite clay, the layer above a burning coal seam will turn a reddish color and harden. Because of it's similarities to a baked brick, this red rock is known as "Clinker."