Mudstone

First of all, what’s a sedimentary rock?

A sedimentary rock is made up of sediments that have accumulated in a ‘basin’ over geological time to form a rock. When I talk about geological time, I’m meaning over millions to 10’s of millions of years, no just 100’s to 1000’s of years… These sediments can be made up of clay, silt, sand, gravel, cobble or even boulder sized particles. So basically clay particles that are the finest end up turning into mudstones, silts - siltstones, sand - sandstones and anything larger than that turns into either a conglomerate (if the clast’s are rounded) or breccia (if the clasts are angular) surrounded. You find these rocks forming in different types of depositional environments which can be shown in Fig 1 below.

Fig 1: Main depositional environments for sedimentary rocks.

What’s mudstone?

Mudstone is a type of fine-grained sedimentary rock primarily composed of clay- and silt-sized particles. It can be dark, grey, dark grey, brown, green, purple and even red depending on the environment it’s deposited in. 

Fig 2: Mudstone from the Scottish Upper Coal Measures formation - Rock cycles that formed during the carboniferous period between 315.2 to 308 million years ago during the carboniferous period. Note the fine grained ‘muddy’ look to the mudstone.

What are the processes involved in forming a sedimentary rock?

1. Erosion and Weathering: Parent rocks, often igneous, metamorphic, or other sedimentary rocks, are broken down through physical weathering (e.g., freeze-thaw, abrasion) and chemical weathering (e.g., dissolution, hydrolysis). This process produces fine particles like clay and silt.

2. Transportation: These fine particles are transported by water, wind, or ice. Rivers and streams, for instance, carry these sediments to lower-energy environments, such as lakes, floodplains, or the ocean.

3. Deposition: Mudstone forms in quiet, low-energy environments where the fine particles can settle out of suspension. Examples include the bottoms of lakes, lagoons, deep-sea floors, or tidal flats. These areas allow for the slow accumulation of clay and silt particles because there is minimal disturbance.

4. Compaction: Over time, layers of fine sediment build up and are buried under additional sediments. The weight of the overlying layers exerts pressure, squeezing the sediments together. Water is expelled, and the particles are compacted into a dense layer.

5. Cementation: Minerals, such as silica or calcite, precipitate from groundwater flowing through the compacted sediment. These minerals bind the particles together, transforming the sediment into a solid rock.

Where can I find mudstone in Scotland?

You can find mudstone wherever there’s sedimentary rocks - a good place to start in Scotland is in the Midland Valley Terrane in Scotland where we have most of our Devonian rocks (400 Ma) Carboniferous rocks (330 Ma).

Fig 3: Midland Valley Terrane (Phillips et al, 2009)

The old red sandstone group

Scotland was situated south of the equator during the Devonian period between 419.2 Ma to 358.9 Ma on a continent known as Laurussia. The great mountain building event known as the Caledonian orogeny was coming to an end and you just had the collision of Laurentia, Baltica and Avalonia creating this continent known as Laurussia. Laurussia was known as the ‘old red sandstone’ continent and was a very dry, semi-arid to arid continent. During this time, we saw the deposition of the old red sandstone group within the Midland Valley Terrane and in North east Scotland. There was a massive lake known as lake Orcadia in North east Scotland where you had the accumulation of mud, silt and sand during the Devonian period. This period of time was also known as the ‘age of fishes’ and we can find fossilised fish in these rocks up in north east Scotland as well.

Fig 4: Caithness Flagstone, the Old Red Sandstone Group, Spittal Quarry, Scotland. Interbedded layers of sandstone, siltstone and mudstone that were originally deposited as sands, silts and muds in to the Orcadian basin.

Fig 5: Orcadian Basin, North East Scotland - The old red sandstone group

Fig 6. Thinly laminated reddish brown mudstone from the Devonian period (old red sandstone group), Scotland.

Carboniferous period

Moving on into the carboniferous period ‘Scotland’ moved further north as part of this continent and drifted into a humid environment, similar to the amazon rainforest today. We can find cyclic sedimentary formations during this period of time that were deposited between 299 Ma to 358 Ma. These cyclic formations usually repeat themselves and represented different depositional environments, with mudstones being deposited in lagoons, flood plains, or even deep marine environments when the sea level rose. You can find these cyclic sequences of rocks across the Midland valley as part of the Scottish coal measure formations and carboniferous rocks.

Fig 7: Scottish Lower Coal Measures formation - Chatelherault Country Park, Hamilton

Fig 8: Notice the bottom layer (mudstone) is a lot weaker than the top layer (sandstone)

Learn more about this rock on YouTube - https://youtu.be/Lx5bD4KsonA