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What is Mudstone?

By M. Haskins
Updated: May 21, 2024

Mudstone is a common sedimentary rock with a very fine-grained texture that is sometimes called by various other names such as mudrock, argillite, claystone and siltstone. Shale is also a type of mudstone rock but differs from most other kinds of mudstones in that it is made up of visible layers. The geological process leading to mudstone formation begins when sediment like clay, mud and silt is deposited in areas such as lakes, the bottom of the ocean, or tidal zones. This material is then buried under more sediment and becomes lithified, meaning the fluids contained in it are removed while the remaining material is compacted under pressure. This type of rock looks like dried clay, and it comes in a wide variety of colors, including black, orange, white, gray, brown, and green.

Sedimentary rock is common in the outermost layer of the Earth's crust, and an estimated 65% of all sedimentary rock is mudstone. Mudstone is made up of very fine particles no larger than 0.0025 inches (0.0625 mm) in diameter that can only be seen in a microscope. The color of mudstones is determined by its mineral content. Rocks containing iron oxide are commonly red, orange, or yellow, while rocks rich in pyrite or carbon are black.

There are various types of mudstones with somewhat different characteristics when it comes to texture and hardness, depending on various differences in their mineral composition and formation. The term argillite can refer to any type of mudstone, but is sometimes used specifically to refer to deposits that are well-formed and harder than other types of mudstones. Marl is a term used for a specific type of this sedimentary rock that is soft and contains a lot of carbonate. Shale mudstone is characterized by its layered appearance, also referred to by the geological term fissile layering.

Mudstone is generally too soft for construction or similar purposes. However, certain types of argillite have been and continue to be used by some Native American peoples, primarily for carvings and jewelry-making. The general softness of the material, which makes it easy to shape and cut, has contributed to its popularity for such purposes. The Anasazi people in New Mexico commonly used both turquoise and orange argillite for jewelry and inlays. In Canada, the Haida people of British Columbia are famous for their carvings using a hard, very finely textured black argillite that can only be found in one quarry on Graham Island.

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Discussion Comments
By seag47 — On Aug 10, 2011

When I was a child playing hopscotch, I used mudstone without even knowing it. During family reunions, we would always meet at the lake, and us children would draw out our own hopscotch game.

We would go down to the water’s edge and look for the white claystone. We used this as chalk for drawing the blocks on the sidewalk. We didn’t know what it was, so we thought that chalk came from the lake.

It felt a lot like the chalk we used on the blackboard at school. It was powdery, smooth, and easily broken. It left white lines when rubbed across a hard surface.

By kylee07drg — On Aug 09, 2011

I love going to a nearby huge manmade lake and collecting claystone and siltstone. Certain areas of this lake have large quantities of mudstone, and I like to collect it to use in my flower garden.

I have seen orange, red, yellow, white, and gray claystone here. It is so smooth, and it has a powdery feel when broken open.

The siltstone is often brown or gold. I have seen pieces of it floating in the water near the shore that look like gold flecks. I collect them and bring them to my young nieces, who think that they are real treasure. These are the prettiest of the mudstones at the lake.

By lighth0se33 — On Aug 08, 2011

I have always been fascinated by geological processes and the resulting rocks and minerals. I remember learning about mudstone and how it can turn into gneiss, a metamorphic rock with bands of mineral grains. The bands of gneiss are very pronounced, and they are often gray, white, and pink.

When mudstone receives tremendous amounts of pressure and heat deep underground, it will recrystallize and form gneiss. As the rock is lifted up, it will erode away into tiny particles of silt and clay. These eventually will settle, and they will become cemented together, forming siltstone.

By Perdido — On Aug 08, 2011

In geology class, we learned how to physically tell mudstone apart from shale. Mudstone will break off into blocky pieces, and shale will break off into thin chips having mostly parallel tops and bottoms. We had samples of both in our classroom, and we did the break test.

We also learned how to tell two types of mudstone apart. You can’t tell the difference between siltstone and claystone by looking, but you can by rubbing them against your tooth or nibbling them. Siltstone will feel gritty when you bite it, but claystone will feel smooth, because it formed in an environment with still, quiet water.

By anon203553 — On Aug 06, 2011

Thanks Wise Geek. You are so much better than Wikipedia!

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