Definition of "mudstone" []

• A fine-grained, dark gray sedimentary rock, formed from silt and clay and similar to shale but without laminations. (noun)

American Heritage(R) Dictionary of the English Language, Fifth Edition. Copyright (c) 2011 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.

• "The "shale" is a sedimentary rock known as mudstone, and the "oil" is a waxy solid called kerogen."
• "Anyway, the ground was a very light grey, almost purple in places, a weathered clay or mudstone or siltstone of some sort."
• "Researchers spotted the 45cm horn while hunting for fossils in the Hell Creek Formation, a 100m-thick slab of mudstone in south eastern Montana."