Fossil wood is not usually found associated with the rest of the tree (leaves and roots) and identification can be difficult, in these cases the specimens are given a special botanical name. These usually feature the term xylon, along with the plant type it is assumed to be part of, to show that the identification is not bases on whole specimen. (e.g. Arucarioxylon – linked to the Arucaria (Monkey Puzzle) or related genus)

A well-preserved specimen of fossil wood recovered from the Redcar Mudstone Formation near Staithes.

This example of a piece of fossilised wood was found on the beach near Staithes. It has been preserved through a process known as permineralization. The original piece of wood was buried amongst sediment deposited millions of years ago. Over time the minerals from the rock soaked into the wood and replaced the original organic material, whilst keeping the structure of the wood. (in this case it was part of the Redcar Mudstone Formation which was laid down approximately 190 million years ago).

In the specimen shown above you can still clearly see the growth rings hundreds of millions of years after the tree, which grew on land, was transported by rivers to the ancient Tethys Sea where it eventually became incorporated into the sea floor sediment.

©2011 Tees Valley RIGS Group.

In situ fossils in the Redcar Mudstone Formation on Redcar Scar.

The ancient seas that covered Britain millions of years ago are now continually revealing the extinct organisms that once roamed the oceans.

Living within the Tees Valley area provides us with many opportunities and locations to acquire a whole host of these interesting objects. The abundance and diversity of fossils throughout our region is wide and varied. From the famous Jurassic ammonites to the alluring fossil gastropods (sea snails).

Once collected most fossils will need preparing in some way to enhance their natural beauty, this can take many hours even weeks to complete and requires patience and a good eye for detail.

A selection of fossil preparation equipment.

When starting out on preparation most hobbyist collectors will find the use of a rotary tool kit with interchangeable heads immensely versatile. Along with an electric engraver, steel probes and craft knives, these can be helpful for carefully picking away and removing the surrounding sediments. Fortunately the tools just mentioned do not take up much space and can be purchased at very little expense to the user.

A selection of prepared fossils and pertinent literature.

As the collector becomes more proficient in preparation they may eventually want to turn to more robust specialist equipment. There are many tools and supplies on the market to choose from, and one recommendation has to be the pneumatic fossil preparation pen – using compressed air to vibrate a tungsten tip at high speed these can remove the hardest of matrices accurately and effectively in a fraction of the time of other methods. Although professional equipment can prove somewhat expensive the end results almost always outweigh any of the costs.

We would like to thank RIGS Group member Scott Bradley for providing this month’s article.

Please Note: Fossil collecting must be done responsibly to preserve key beds and specimens for the enjoyment of others. Please feel free to collect loose specimens from the beaches, but leave in-situ specimens for the enjoyment of those who follow in your footsteps. Click here to see an informal fossil collecting code.

PLEASE NOTE: Tees Valley RIGS Group cannot be held responsible for the content of external sites.

©2011 Tees Valley RIGS Group.

1. References

Frosterley Marble is a dark grey to black limestone which has been used as an ornamental stone locally and internationally in churches and buildings such as Durham Cathedral.

Frosterley Marble is not a true marble as it is not a metamorphic rock (true marble is limestone which has recrystallised during thermal or regional metamorphism). Stonemasons use the term marble for some limestones which can take a high polish.

Much of the Frosterley Marble has been worked from quarries around Frosterly, in Weardale, County Durham. Frosterley Marble was formed during the Lower Carboniferous Period (325 million years ago) when the northern Pennines area was closer to the equator. The area alternated between a deltaic and tropical marine depositional environment. The Great Limestone Member was deposited in a marine environment and is in places, 22m thick. The Frosterley Marble forms a fossil rich band in this rock.

Frosterly Marble containing numerous fossil corals.
Image: Carole Rushall.

This photograph shows a boulder of the marble which contains abundant fossil specimens of the coral Dibunophyllum bipartitum. Unlike many corals which live as colonies, Dibunophyllum bipartitum was a solitary coral (it is now extinct). The solitary coral organism had a curved cone shaped calcareous skeleton in which lived a soft-bodied polyp whose tentacles captured organic matter from the sea water. When the organism died, the skeleton settled into the limy ooze on the sea floor which eventually formed limestone.

You can download an illustrated leaflet about Frosterley Marble produced by the North Pennines AONB Partnership by clicking here…

References

Stone, P., Millward, D., Young, B., Merritt, J.W., Clarke, S.M., McCormac, M. & Lawrence, D.J.D. (2010). British Regional Geology: Northern England (Fifth edition). Keyworth, Nottingham: British Geological Survey.

Scrutton, C. (ed) (2004). Northumbrian Rocks and Landscape : A Field Guide. (Second edition). Yorkshire Geological Society.

Our thanks go to Carole Rushall for providing this month’s article.

PLEASE NOTE: Tees Valley RIGS Group cannot be held responsible for the content of external websites.

©2011 Tees Valley RIGS Group.

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.

Trilobite of the species Paradoxides.

They are one of the earliest creatures to have evolved hard-parts, presumably to protect against predators and, as such, are well-represented in the fossil record. The oldest rocks in the Tees Valley, the Carboniferous and Permian strata, may hold fossilised examples of these remarkable creatures. Trilobites have three parts to their bodies: the head or cephalon, body or thorax and tail or pygidium. They were marine animals which lived on the sea floor at a variety of depths.

Trilobites are so named for the three longitudinal lobes: 1 – left pleural lobe; 2 – axial lobe; 3 – right pleural lobe. The trilobite body can also be divided into three major sections (tagmata): 4 – cephalon; 5 – thorax; 6 – pygidium.

Trilobites survived until the end of the Palaeozoic Era, marked by the Permo-Triassic Mass Extinction which occurred around 250 million years ago.

This trilobite is Walliserops trifrucatus from Djebrl Oufaten in Morocco.

TVRIGS Group cannot be responsible for the content of external sites.

This mineral is able to assume almost any colour but commonly brown, yellowish-brown, or grey specimens can be found. It occurs in Britain’s Carboniferous strata as nodules and beds of impure iron carbonate known as Clay Ironstone. Once a valuable source of ore, alongside a dark carbonaceous form known as Blackband. In Cleveland the well-known ironstone through which which Teesside became a major industrial force from 1850, is of Jurassic age (c.188,000,000 years old), contains iron-rich berthierene rather than siderite, and occurs with a distinctive texture known as oolitic. An amalgamation of small rounded concentric structures, which form through the same colloidal processes as those reponsible for oolitic limestones, make up the bulk of the rock. Siderite can also be found in massive, granular, or concretionary forms, produced in a variety of environments including within hydrothermal veins along with pyrite and galena, within intrusive pegmatites, and as sedimentary Bog Iron Ore in high latitude lakes and swamps.

At its purest, siderite forms rhombohedral crystals with a vitreous (inclining to pearly) lustre, perfect cleavage, a white streak, and uneven fracture. An allied mineral Hydrated Iron Oxide or Limonite (FeO(OH)·nH2O), commonly forms pseudomorphs (perfect copies) of siderite crystals.

It is however more usually found in the local area as red-weathering nodules within grey mudstone scars, exposing part of the Cleveland Ironstone Formation that crops out on the foreshore at Jet Wyke, Staithes. In the 1700s, such nodules were collected from the scars by local villagers and loaded onto boats which eventually disgorged their cargoes at furnaces on Tyneside, long before the significance of the Cleveland Ironstone Formation was suspected.

Images above are of:
Manganoan Siderite with Albite;
Siderite Pseudomorphosis in Limonite with quartz;
Red Siderite Nodule in Grey Mudstone at Staithes.

More correctly, the Mica Group of minerals are sheet silicates. This means that instead of growing as a large crystal they form in lots of very thin layers. The minerals are often transparent and in the past were used as a substitute for glass. It is a widely distributed mineral occurring in igneous, metamorphic, and sedimentary rocks and can form the planet’s largest-known crystals reaching several tens of metres in length, particularly within granitic pegmatites.

China is the greatest world producer of mica, with the largest deposits being found on the Indian sub-continent, USA, South Korea, and Canada. The mineral has multifarious uses including within Geiger-Müller Tubes, heating elements, capacitors, refractory windows (isinglass), in toothpaste, and adds a bright shimmer to make-up.

Throughout the ages, fine powders of mica have been used for various purposes, including decorative purposes. The coloured body paints used by Hindus of North India during holi festival contain fine small crystals of mica. The majestic Padmanabhapuram palace, 65 km (40 miles) from Trivandrum in India, has colored mica windows.

Mica occurs in the Tees Valley most commonly as small specks within the Jurassic sedimentary strata and is generally indicative of the sediment being deposited in quiet water conditions.

Around 58 million years ago, as the Atlantic oceanic basin formed, adjacent areas of crust became stretched and weaknesses could be exploited by molten material (magma) being forced into the crust by pressure from below. This magma cooled very quickly surrounded by local rocks and became the Cleveland Dyke.

Stretching for c.350 miles between Mull in Western Scotland and the Tees Valley and North Yorkshire the hot magma cooled to form a dark blue-grey, finely crystalline rock referred to by geologists, more correctly, as dolerite. Dolerite is chemically similar to basalt, the major difference being that basalt is erupted at the Earth’s surface, whereas dolerite solidifies within the Earth’s crust.

Following removal of the overlying strata by erosion, primarily through glaciation, the dyke was exposed at the Earth’s surface. In the west of our region it can be traced crossing the river at Preston-on-Tees, but perhaps its most notable feature occurs near Great Ayton where the more durable rock making up the dyke, and softer Jurassic strata into which it is intruded, exhibit a phenomenon known as differential erosion. The softer sedimentary rock is preferentially removed by erosion leaving the harder whinstone to form a bold ridge called Langbaurgh Ridge.

The geater hardness of whinstone relative to sedimentary rock makes it ideal for use road-stone and cobbles, and it was for this purpose that Leeds City Council leased land around Great Ayton, where the ridge is best developed, in 1869. Large quantities of the rock were quarried at Cliff Rigg, as well as elsewhere along the length of the dyke, for example at Preston-on-Tees, Ingleby Barwick, and at a variety of locations on the North York Moors. The now-abandoned workings today form an unmistakeable scar on the landscape, though the former quarry’s remains allow geologists to study the effects of metamorphism, i.e. the baking of the surrounding sedimentary rock when the hot magma was injected.

What the Persians were actually importing was Asbestos, one of six fibrous minerals known to geologists as chrysotile, amosite, crocidolite, tremolite, anthopyllite, and actinolite.. The natural shape of the mineral as it grows is as fine fibres that are strong enough to weave into material.

Its widespead use as insulation and a fire retardant commenced with the Industrial Revolution since which time it has been used in the manufacture of concrete, bricks, pipes, gaskets, flooring, roofing, ships, aircraft, and a wealth of other products. In the 1950s it was even used by one cigarette manufacturer to produce filters!

We now know that Asbestos can be extremely harmful when the fibres become lodged in the lungs and the substance is now restricted in its use.

Alum Shale is an unremarkable, grey, thinly-bedded, pyritic mudrock that weathers readily to thin crumbly flakes, the detritus often forming steep talus slopes below the working faces in numerous alum quarries that today reside peacefully along the coast and hills of Cleveland and North Yorkshire. The quarries and boiling houses operated for over 260 years here, commencing around 1600, in the only district in Britain where rock suitable for the important industry of alum-making was, and still is, able to be extracted.

The Tethys Sea supported a diverse fauna of, now mostly-extinct, creatures amongst which can be counted a wide-range of ammonite species, belemnites, fish, and a number of large reptiles including crocodiles, ichthyosaurs and plesiosaurs. At the end of their lives, the remains of these creatures would settle on the sea-floor and occasionally become buried and preserved as fossils. Given the rock’s mode and time of creation, who amongst us could have imagined that the remains of these leviathans, tokens of antiquity from a long lost world millions of years after their lives had ended, would once again see the light of day by way of a quarryman’s hands.

Ammonites achieved their evolutionary zenith during the Jurassic as a result of which some species are only found within a very small stratigraphic range. The usefulness of this to geologists in ascertaining the relative ages of strata was noticed by alum-maker’s son Louis Hunton (1814-1838), who collected data at coastal quarries and made valuable contributions to the young science of biostratigraphy in the 19th century.

The large reptile fossils began to come to light during the 18th century at a time when the science of geology was in its infancy. They provided some of the earliest, best preserved, fossils to be examined by early palaeontologists and found their way to academic establishments across the world. Specimens of these and many more fossils can be seen today on display in Pannett Park Museum, Whitby.

The image above shows Lower Jurassic Alum Shale (grey) making up the lower part of the cliff at Rosedale Wyke, Port Mulgrave. The overlying yellow-brown sandstone belongs to the Middle Jurassic Saltwick Formation.