Derived from the Latin Argilla – meaning ‘clay’, this group of rocks primarily comprise particles of the finest grade, including clay- and silt-sized clasts up to ¹/₁₆ mm in diameter. They may be divided into subclasses of shales, mudstones and siltstones.

Both shale and mudstone are composed of the finest particles of sediment less than ¹/₂₅₆ mm in diameter, and can be distinguished by the way in which they cleave. Shale is generally finely-laminated and fissile, able to be split easily along its bedding planes, mudstone on the other hand has no preferred axis of cleavage and tends to exhibit a ‘blocky’ fracture¹. Siltstone follows similar principles of cleavage but comprises grains between ¹/₂₅₆ mm and ¹/₁₆ mm in diameter.

Shales of the Whitby Mudstone Formation (grey) form the foreshore and lower cliff beneath Middle Jurassic sandstone (yellow) at Rosedale Wyke. The remains of Kettleness alum quarries form the headland in the background.

Clay minerals (alumino-silicates) make up the bulk of such rocks and may include kaolinite, illite, chlorite and montmorillonite-smectite. Argillites are rarely pure but include a mixture of minerals. For example the Alum Shale Member of the locally exposed Whitby Mudstone Formation contains all four of the above mentioned clay minerals plus pyrite (FeS₂), quartz (SiO₂), siderite (FeCO₃), calcite (CaCO₃), collophane (apatite), goethite (FeO(OH)), gypsum CaSO₄ • 2(H₂O), jarosite (KFe³⁺₃(OH)₆(SO₄)₂), mica, feldspar, zircon and anatase. The latter three minerals in only minor amounts.

Shales and mudstones may also frequently contain inclusions in the form of calcium carbonate, siderite or other minerals. These features form after deposition of the originating sediment during the process of lithifaction. They grow in-situ when minerals distributed through the body of the deposit are drawn toward a single point through ionic transportation. Often a shell fragment or fossil will provide a nucleating point around which the inclusion develops as the accreting mineral is drawn from the surrounding sediment. In the image below showing the Jet Rock Member at Rosedale Wyke, bedding can be seen to pass around the outside of weathered calcium carbonate nodules.

Laminations in the Jet Rock Member of the Whitby Mudstone Formation passing around weathered limestone nodules.

Argillites are all sedimentary in origin, their components being either water or wind-borne. They are the products of fairly low energy environments such as deep sea floor, tidal flats, lakes and (in the case of loessite – lithified wind-borne rock dust from a number of sources) continental environs. They may occur in a variety of colours ranging from the dark red-brown, blue-grey, or tea green Triassic deposits to the light brown or black Bituminous Shale and Jet Rock Members of the Lower Jurassic containing hydrocarbons.

Red-Brown Permo-Triassic mudstone as seen at Seaton Carew.

The fine-grained nature of argillites make them ideal for the preservation of detailed fossil specimens. One example of such excellent preservation is the enigmatic suite of remains discovered in the Burgess Shale of Canada by Charles Doolittle Walcott (1850-1927) in the early 20th century². These Middle Cambrian fossils were reappraised in the 1970s and found to represent the remains of creatures with a number of body plans previously unknown to science such as Marrella a kind of extinct crustacean.

Marrella - An extinct crustacean from the Burgess Shale of Canada with no modern day relatives.

Historically, argillites have been exploited locally for a number of reasons. Millions of tons of Lower Jurassic (Toarcian) Alum Shale were quarried and processed to serve the local alum trade at over twenty sites in and around the Tees Valley. At Ravengill, near Commondale, Middle Jurassic (Aalenian) mudstone was quarried and milled for the brick and tile trade.

When sedimentary argillites become altered (or metamorphosed) by heat and/or pressure to form rocks such as slate, hornfels, etc. the resulting fine-grained metamorphic rocks tend to be referred to as Pelites.

Notes

^[1] The ironstone miners of Cleveland had their own terminology for many kinds of rock and tended to refer to mudstone units inter-bedded with ironstone seams as shale.

^[2] If the anecdote concerning this discovery is to be believed, although C.D Walcott recovered the fossils from this lagerstatten, it was actually his horse which drew his attention to their presence.

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1. Site Description
2. Site Map
3. Site Assessment
4. Surveyors

Grid Reference NZ 459 096
BGS Sheet 33
OS Sheet 93
Forwarded as RIGS 30/09/2003

Site Description

Site Status SNCI
Description of Geodiversity River-cliff exposure revealing the contact between the Sherwood sandstone and the overlying Mercia mudstone. The Sherwood sandstone forms a washed platform and waterfall.
Literature References

Site Map

Site Assessment

Surveyors

Andrew Carter, John Waring

1. Site Description
2. Site Map
3. Site Assessment
4. Surveyors

Grid Reference NZ 465 146
BGS Sheet 33
OS Sheet 93
Forwarded as RIGS 30/09/2003

Site Description

Site Status SNCI
Description of Geodiversity Small river-cliff feature revealing an exposure of Mercia mudstone
Literature References

Site Map

Site Assessment

Surveyors

Andrew Carter, John Waring