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.

PLEASE NOTE: TVRIGS Group cannot be held responsible for the content of external sites.

©2011 TVRIGS Group.

This page provides a glossary of geological terms to be found on the TVRIGS website and elsewhere. It is an evolving document and will grow as the site expands.

T

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«Glossary(L-S)

PLEASE NOTE: TVRIGS Group cannot be held responsible for the content of external sites.

Shown below is a representation of the rocks underlying the Tees Valley and Darlington districts in Northeast England. The diagram depicts the relative thickness of the different systems. Absolute ages are given alongside. Click on the different areas to view more details about a particular time period.

Geological column showing the relative thickness and absolute ages of rocks underlying the Tees Valley and Darlington.

The place for you to find out more about the geology and industrial heritage of Redcar & Cleveland, Middlesbrough,
Stockton, Hartlepool and Darlington.

Alum, Alchemy and Ammonites Page
 
Future Events Page
 
Past Events Page

Why not check out our new site description pages and geo-trails

                        Boulby – Cowbar                           Hummersea
                        Loftus Quarries                              Boulby Quarry

Latest News

RIGS News

• Tuesday 8th November 2011: November’s Rock of the Month offering can now be viewed by clicking here…

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• Thursday 13th October 2011: The RIGS Group’s recently published Geodiversity Action Plan has been recognised and used as a case study by Geoconservation UK. More details can be seen here and by clicking on the navigation bar at the top of this page.

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• Wednesday 12th October 2011: Why not view Tees Valley Wildlife Trust’s Alum, Alchemy & Ammonites pages.

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• Wednesday 1st June 2011: Our latest Rock of the Month article is now online. We would like to thank RIGS Group member Scott Bradley for providing the article.

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• The group’s expansion into the Darlington district is approaching the end of its first phase. A number of sites have been identified and five summarily surveyed.

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Announcements

• For details concerning the next RIGS meeting please contact the RIGS Group by e-mail.

Ramblers walking with the RIGS Group descend Boulby Bank after visiting cliff-side alum quarries near Staithes.

TVRIGS are always keen to recruit new members. So if you have an interest in the region’s geology, would like to find out more about the Tees Valley’s industrial heritage, or simply wonder what all of the fuss is about, then why not join us, it’s free – and we have only the very best biscuits at our meetings…

Unless otherwise stated, all images on this site ©2010 & ©2011 TVRIGS Group.

1. Introduction
2. Sherwood Sandstone Group
3. Mercia Mudstone Group
4. Penarth Group

Introduction

Column showing divisions in the Triassic System of the Tees Valley.

During the Triassic the geography of North-western Europe differed greatly to that existing today. At this time, all of the landmasses of the Earth had agglomorated into a single continent named Pangaea. An episode of mountain building, known as the Variscan Orogeny, locally raised parts of the Earth’s crust to form a mountain chain extending between North Devon and The Wash. The so-called Mercia Highland was a fundamental influence, controlling the nature of the rocks deposited during the Triassic.

Triassic deposits have their type locality in Germany and Austria where thick marine deposits indicate the presence of an epicontinental sea in the German Basin with poor connections with the southerly Tethys Sea. Deposits in the UK are generally continental sabhka – a hot arid coastal environment – or shallow water deposits. Rain falling on the Mercia Highlands washed eroded debris down the slopes of the mountains onto a broad flat tropical plain with a shallow sea to the east.

The Lower Triassic deposits found in the Tees Valley do not correlate well with the German type locality and hence have been redefined to suite the local representatives of these rock units. Here the episode is represented by the Sherwood Sandstone Group, a suite of yellow and red sandstones deposited primarily in shallow water. The Middle Triassic, represented by the Muschelkalk of Germany, is missing in Britain as an episode of erosion ensued, represented by an unconformity know as the Hardegsen Disconformity which extends into the modern North Sea Basin. The Upper Triassic locally is represented by the Mercia Mudstone Group, a suite of red-brown and blue-grey mudstones with occasional halite and gypsum deposits. All of these deposits are unfossiliferous demonstrating the harsh conditions locally at this time.

Triassic rocks crop out at very few localities in the Tees Valley making any exposure all the more valuable. The Triassic succession is mostly obscured by the Tees Estuary on the coast, and thick glacial deposits inland. However, an example of Sherwood Sandstone can be seen at Foxton Bridge, and exposures of Mercia Mudstone can be viewed alongside the River Leven downstream of Hutton Rudby.

Sherwood Sandstone Group

During the Late Permian and Early Triassic the Tees Valley constituted an arid coastal plain. It was subject to rapid deposition of sediment carried by flash floods descending from the Mercia Highlands, an upland area extending from North Devon eastwards through Norfolk. Adjacent to the base of the uplands, thick pebble beds developed when descending water dumped the heavier material it carried as it flowed onto flatter ground. Lighter particles of sediment, sand and mud, were spread in great fans away from the uplands across the flat plain, and it is these that make up the Sherwood Sandstone Group in our region. Alternating beds of yellow or red sandstone and thin mudstones exhibiting desiccation-cracks typify the rock unit, which underwent various episodes of both deposition and erosion. It has a distinct lack of fossils, partly because of the harsh environment but also because life on earth appeared to be still recovering in numbers from the Permian Mass Extinction. An erosion surface (unconformity) marks the top of the Sherwood Sandstone Group.

Mercia Mudstone Group

The next episode in the area’s development saw a shallow sea advance across the coastal plain as an intermittent marine connection opened to the southeast. Circulation of water within this sea was restricted and it frequently cut off from the main water body. When it was isolated there was evaporation which then concentrated the dissolved salts. Saline lagoons, pools of hot mud, and glittering beds of salt developed. As a result, the Mercia Mudstone Group is comprised of beds of dark red-brown or blue-grey mudstone, frequently mottled. The Mercia Mudstone Group is a product of fine-grained material washed or blown into the sea from the flat plain. The mudstones alternate with occasional evaporite beds including halite, gypsum, and anhydrite deposited from evaporating sea-water. The uppermost beds in this sequence have been dubbed the Tea Green Marls.

Penarth Group

The close of the Triassic locally saw the Tethys Sea readvance across the saline Triassic plain. The Penarth Group of rocks were deposited over a peroid of c.4 million years. The highly poisonous saline surface, over which the sea advanced, combined with restricted circulation of its waters, produced considerable variations in salinity and oxygen content at first. The lack of oxygen formed the black sulphurous shales typical of the Westbury Beds in the lower part. In this layer a bone bed indicates mass mortality of many creatures, this is believed to have been caused by algal blooms.

As the sea deepened marine creatures of many kinds began to flourish. Consequently, higher in the Penarth Group succession, and hence later in time, lie the brown and green mudstones, known as the Cotham Beds, containing the fossils of shelly creatures, indicating life beginning to establish itself locally in the improving waters.

« Permian                               Lower Jurassic »

©2011 Tees Valley RIGS Group.

The the addition of the letter ‘o’ to spell geo was simply to make the word sound better: “gelogy” would sound awful!

It is the rocks of the Tees Valley that have brought industry and money into the area. Some of the rocks to be found here are more important than others. But all of them have in some way played a part in building the Tees Valley.

To find out more about any of the rocks in the Tees Valley select a geological period from the list below. The geological periods are given in chronological order (order of time) with the oldest period at the bottom of the list.

• Quaternary
• Tertiary
• Jurassic
• Triassic
• Permian
• Carboniferous

Click here for a detailed geological timescale for rocks in the Tees Valley.

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

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

Grid Reference NZ 526 304
BGS Sheet 27
OS Sheet 93
Forwarded as RIGS No designation

Site Description

Site Status None
Description of Geodiversity Small exposure revealing a wave washed scar consisting of Sherwood sandstone
Literature References The Geology of the Country Between Durham and West Hartlepool – Dennis Smith 1967

Site Map

Site Assessment

Surveyors

Andrew Carter