Related Clues:

• Possible source of petroleum
• Petroleum source
• Brick ingredient
• Shelf material
• Fine-grained rock
• Layered rock
• Fuel-yielding rock
• Oil-bearing rock
• Fossil medium
• Mudstone, e.g.
• Mud, after centuries
• Coarse-grained rock
• Banded metamorphic rock
• Oily rock
• Metamorphic rock