Mudstone

In short

Mudstone is a fine-grained sedimentary rock formed when a mixture of silt- and clay-sized particles is squeezed and hardened. It differs from shale in that it does not split into thin sheets, which means it is massive. Its grains cannot be told apart without a microscope; it looks like hardened clay and can resemble cracked, dried mud. Mudstones and similar rocks make up about half of the sedimentary rocks in the geological record.

Photos

How to recognise it

1. 1

   It is very fine-grained: no grains show even with a hand lens, the surface is dull and smooth, and it looks like hardened clay.

2. 2

   It does not split into thin sheets like shale; it breaks in blocks and sometimes shows cracks like drying mud.

3. 3

   It is soft, can be scratched with a knife, and may crumble over time in air and water.

4. 4

   Its colours vary widely: it can be red, purple, brown, yellow, green, grey or black.

Myths and misconceptions

• MythMudstone and shale are the same rock.

  FactBoth are fine-grained rocks of the same family, but shale splits into thin sheets and mudstone does not. The lack of fissility may come from the original texture or from burrowing animals that disturbed the layers as the mud settled.

• MythFinding organic matter in Martian mudstone proves there was life on Mars.

  FactOrganic compounds can form by non-living processes as well as by living things. The kerogen and complex organic compounds NASA found in Martian mudstones are not evidence of life; they show only that carbon compounds were preserved there for billions of years.

Did you know?

Where is it found?

• Lyme Regis and Charmouth, Dorset, England · Type localityThe type section of the Early Jurassic Charmouth Mudstone Formation is in the cliffs between these two towns. The formation is famous for ammonites, ichthyosaurs and plesiosaurs; Mary Anning collected fossils here in the 19th century.
• Cheshire Basin, England · Known localityThe thickest part of the Triassic Mercia Mudstone Group lies here (over 1,350 m). The rock is reddish brown from iron oxide, and halite (rock salt) beds within it are worked around Northwich.
• Dinosaur Provincial Park, Alberta, Canada · Known localityThe upper part of the Late Cretaceous Dinosaur Park Formation consists of organic-rich mudstones full of root traces; the park, with its dense concentrations of dinosaur skeletons, is a UNESCO World Heritage Site.
• Ischigualasto Provincial Park, San Juan, Argentina · Known localityFloodplain deposits of the Late Triassic Ischigualasto Formation include mudstone layers; fossils of early dinosaurs such as Eoraptor and Herrerasaurus come from here. The park has been a UNESCO World Heritage Site since 2000.
• Petrified Forest National Park, Arizona, USA · Known localityThe colourful bands of mudstone and sandstone in the Late Triassic Chinle Formation give the Painted Desert its multicoloured look; the formation is also famous for petrified wood.

Locations are approximate to protect sites. Red dots mark places in Türkiye. Open in the big map →