Drawing fossils

OGG member Andrew Orkney describes a bivalve from the Wessex Basin.

This is a depiction of a single bivalve valve, from a beach in the Wessex Basin. Bivalve fossils with both valves are rare, because Bivalves spring open after death, and the valves separate.

The shell retains some original aragonite, especially near the dorsal side, and is filled in with glauconitic green sandstone.

The original aragonite shows that the fossil is relatively young, and the glauconite is consistent with a source from the Greensand, which was deposited in the early Cretaceous. The Greensand was deposited slowly on a submerged continental shelf, accumulating many fossils and providing the right conditions and time scales for the diagenetic alteration of iron-bearing minerals into glauconite.

The Greensand is friable, which means it releases its fossils when it is weathered at outcrop.



FOCUS ON: Dry Sandford

Dry Sandford Pit  is a 4 ha site has been a SSSI since 1950 and is now a BBOWT nature reserve. The rock exposures at Dry Sandford Pit are of Middle Oxfordian Stage (Late Jurassic) deposited circa 140 million years ago, in shallow coastal waters close to coral reefs.

Two geological formations are present at Dry Sandford:

Stanford Formation (Coral Rag member)

Kingston Formation (Beckley Sand Member)

The middle Oxfordian Stage is sub-divided into two ammonite time zones, each defined by the occurrence of a diagnostic species. One zone is characterised by Cardioceras densiplicatum and the other zone by Cardioceras tenuiserratum. As both ammonite zones are represented at Dry Sandford Pit the site is regarded as scientifically invaluable, local and national middle Oxfordian reference section.

Oxford Geology Group has published a new ‘FOCUS ON: Dry Sandford’ page on the OGG website http://www.ogg.uk.com

The page features photos, a log, bed by bed description and video.

Geological Illustration

This image demonstrates the difference between schematic and tonal illustration. A tonal illustration will be time-consuming, and in some cases tone fails to pick out, or even obscures details that are evident to the eye. This is one reason why cameras, which make images based on only tone and hue, and neglect prominence differences that are evident with stereoscopic observation, can often be misleading.

A schematic illustration is a ‘short hand’ way of expressing observation of a fossil. Details which are not relevant to your interest can be neglected, lending emphasis to the important features you have preserved.

Schematic illustrations usually denote changes in a specimen’s, texture, geometry, prominence or colour, with a sharp edge, producing a cartoon that is simple and accessible, compared to a truer rendition.

In this example, I have emphasised the trilobite anatomy’s segmentation.


Deep Time.

OGG member Paul Goodrich writes…

When astronomers talk of distances, they talk in AU’s ; Astronomical units, or Parsecs. The AU is the mean distance to our star, Sol, the Sun. It is 149,597,870,691 km, or thereabouts. A Parsec on the other hand is; 3.26 light years or 31,000,000,000,000 or 1.91735116 × 1013 miles. Even a parsec with all those noughts is still only three quarters of the way to our nearest intra galactic neighbour, Proxima Centuri. And most of the stars you can see at night unaided, are a thousand times more distant than that. Particle and nuclear Physicists on the other hand, go in the other direction. They go to microns, nanometres and Angstroms. More noughts with a decimal point way over to the left this time.

Numbers with a lot of zeros behind them begin to lose meaning, so terms like Parsec and Angstrom are used – to give a clearer concept of scale and for easier handling of equations. Well, at least for Astrophysicists. Conceptualising scale is important. I mean, when asked, how big is that water melon, one wouldn’t suggest that its about forty seven squillion bajillion oxygen atoms across. Thirty centimetres would be more appropriate, or one foot in old money. Geologists have similar difficulty in calculating quantities. Amounts of recoverable reserves in hydrocarbon reservoirs for example, or the volume and mass of the Earth. We use powers or orders of magnitude.

So when asked by a student, how heavy is the Earth? the long answer is 6,000,000, 000,000,000,000,000,000 kilograms. The short answer is, it has an approximate mass 5.975e24 kg. Neither makes much sense, so analogies are often used, like piling up bags of sugar all the way to the moon. Then there is pressure. If I dig a hole 35 km deep by one metre square and placed a hand specimen of mudstone and a pressure gauge at the bottom, back filled and tamped it neatly down, what would the pressure be down there with all that backfill over it? Excavate the hole for a second time and our gauge will have recorded about 30 MPa/km or 1050 MPa, or something over a billion tonnes per square metre. Actually, our pressure gauge would not have survived the experiment and the lump of dull mudstone would now be a nice shimmering piece of slightly smaller slightly denser – schist.


But the most perplexing concept of all, to Geologists, Physicists and Philosophers, is that of time. We all experience it, seconds and hours – months and years. A decade is a long time. A century is barely beyond the experience of everyman and a millennia is utterly unknowable. In the 17th Century, Bishop Ussher calculated that the Earth and all things in existence came into being on October 23, 4004 BC. Which would make the Earth, its vast oceans, it deserts and mountains, flora and fauna, just over 6000 years old. A reasonably comfortable number. Still big, but agreeably comprehensible. So when occasional heretics came along and questioned this concept of time, or at least – that there could have been so little of it, they were met with scepticism, derision and often outright hostility.

Geology as a science was in its infancy, it was lumped in with ‘Natural Philosophy’ practiced by gentlemen of means, aesthetes and ironically, some notable clergymen. When they pointed at rocks with their great bands of strata piled on top of each other, they wondered out loud, how it could be that it took so little time for grains of detritus to gather together and create such immense edifices as observed by Hutton, Lyle, Sedgwick and Murchison? And so the concept of Deep Time was postulated. To stand on a beach and see the waves slowly wear away the rocky cliffs and beach outcrops, 6000 years could not seem enough, not nearly enough. Rivers, winds and ocean currents carrying sand and clay minerals must surely need vast tracts of time to deposit enough material to create rocks thousands of miles square and thousands of metres deep.


Only for those rocks to be worn away, tilted on their side and the whole process repeated in great successions above an unconformable division or contact between the two rock type of different color and composition. Yes, Siccar Point! Tens or hundreds of thousands of years – perhaps, blasphemously, even millions of years were required to deposit, sculpt and shape the lands these pioneers of reason, science and art walked upon in search of answers and enlightenment. Over the next two and a half centuries, the Science of Geology and its Earth Science siblings of Palaeontology, Geophysics, Geochemistry and Geomorphology were providing answers, missing pieces of an enormous puzzle.
By the time of Lord Kelvin, an age of 100 million years was – while remarkable, acceptable.

His calculations based on heat reached the not unreasonable conclusion that a molten globe the size of the Earth and the dissipation of heat therein would require that amount of time to cool to its present level. Kelvin as an old man, was present at a lecture given by Ernest Rutherford, a luminary of the University of Manchester – wherein Rutherford spoke of radioactive decay and the energy/heat it produces. The products of that decay, the parent and daughter isotopes could be established, Kelvin heard of Uranium transmuted to Lead, of unstable and stable elements, and lastly – how this process dates the Earth. For the last century, accurate dating has made sense of the great piles of strata, ocean floors, mountains and the fossilised flora and fauna therein.

Radiometric dating is now a commonplace concept. Modern geologists often find it difficult to grasp just how the early pioneers worked, severely disadvantaged as they were by the lack of accurate chronology. Relative dating produced the now familiar periods from the Cambrian to the Cainozoic. These were accurate only in their division of time, of periods, eras and epochs when sands, muds and extinct biota were deposited, eroded and redeposited. The Ordovician for example was known for its Graptolites, Trilobites and other marine fauna in the rocks of the ancient Avalonian North Wales Basin. But over how long these rocks were formed and for how long the benthic creatures endured, was unknown for over a century.

Now, a visit to the labs at the Williamson Building in Manchester with a small sliver of Graptolite bearing slate will give an absolute date with a narrow margin of error. We can say that the Ordovician was a period of about 45 million years, book ended by mass extinctions at 488.3 mya and 443.7 mya. My small fragment of it was from the Caradoc, Soudleyan age, about 462 million years old. Give or take.

Finding Zircons in the Canadian Shield or the Cratons of South Africa and Australia, give reasonably accurate dates back to the Archean at 3850By to 4.400By. A mind boggling temporal distance from – for example, the Cambrian Explosion – the great diversification of life still over three billion years in the future.

Fragments of meteorites push the Earths formation further still to 4.54By. Or 4.54 × 109 years. More incomprehensible numbers. We can’t see these great periods of time, touch them or feel them. Its not something easily visualised or conjured up in the minds eye. So we go back to analogies and thousand word pictures. There have been have been the familiar clocks where the human race stands at a few seconds before midnight, and Sir David Attenborough’s moving band of time, diminishing to a vanishing point behind us.

I am a Geologist with an appreciation of the concept of Deep Time, but I value these depictions of past eons. To times when there was a Heavy Bombardment , the softened craters of which exist only in the Cratons, the remainder wiped clean by erosion and tectonics.

To the time of great oxidation events, of Snowball Earth, of Large Igneous Provinces, the first Terrestrial Plants, the great Mass Extinctions, to the closing of the Tethys and the opening of the North Atlantic, to the country where I live, only very recently sculpted by glaciers and planed by mile high Ice Sheets, ending a very short 8,000 years ago.

A comprehensible number indeed.

All Images, Open source Creative Commons.

Echinoid test

Kristian Fox writes:



Above is a relatively intact echinoid (sea urchin) test, found in Jurassic Inferior Oolite. Echinoids are members of the Phylum Echinodermata. As with other members of the Phylum – think of Asteroidea (starfish), echinoid show in the mature form 5 fold symmetry, the larval form is bilateral. This is rare in the animal kingdom but relatively common in plants, roses for example.

The major distinction in Echinoids are between regular and irregular. Being relatively spherical this is a regular echinoid, and much like a sea urchin you may picture. The mouth is on the underside and crunching mouthpieces (often referred to as an Aristotle’s lantern) are useful for eating other benthic sea life. Irregular forms, such as the ‘Sand Dollar’ are flatter and have the anus offset from the centre, this better allows them to burrow in sediment.

The Masaya Experience

OGG member Nikola Rogic writes about a recent visit to Masaya, Nicaragua…

The Masaya Volcano in Nicaragua is persistently active—that is, it erupts constantly—but it does not spew out molten rock; instead, it releases a steady plume of gas continuously.


When most people picture an active volcano, they probably don’t imagine one like the Masaya in Nicaragua—they picture giant ash clouds and lava flows that wipe out forests and towns. But just as a violent eruption dramatically alters the landscape around it, the Masaya’s plume powerfully shapes its environment. We know, for example, that crops often fail downwind of the volcano. But what else does this constant flow of gases do to the world around it?

NR 5

A research study may systematically analyze how the Masaya Volcano affects its environment.To trace the path and ultimate fate of the gases emitted by the volcano, we must start measuring from the volcano’s magma and moving through the gas plume into the soil, water, and ultimately, plants and animals. Knowledge of how volcanic pollutants travel and where they end up can help local people live more harmoniously with the Masaya by cultivating more acid-tolerant crops, for example, and developing safer evacuation plans.


Masaya Volcano releases harmful gases—it’s critical to know where they enter the food chain.


The volcano can also give us insight into the global impact of persistently active volcanoes. To address human-induced climate change, we need a better understanding of natural processes like volcanic eruptions. Although eruptions increase the amount of carbon dioxide in the atmosphere and contribute to the greenhouse effect, the impact of volcanoes is much smaller than that of burning of fossil fuels by humans. Volcanic eruptions also have a cooling effect, because the sulfur-rich gases they propel into the atmosphere form clouds that reflect solar radiation back into space.


With continuous scientific research at Masaya we hope to learn more about how volcanoes like the Masaya fit into this very complex picture.

NR 3




Photos credit: Nikola Rogic
Gravity measurements being taken at various localities (scientific stations) Masaya volcano, Nicaragua, Feb 2013

Dicynodonts at large

Kathy Kavanagh writes about the trace fossils and natural history of the anomodont therapsid dicynodonts…


A dicynodont trackway was found in 1998 on the private game reserve Asante Sana, near Petersburg, some 45 km east of Graaff-Reinet in South Africa. Excavation of the palaeosurface revealed clear footprints and trackways of six large tetrapods that walked across what was a semi-consolidated muddy substrate. Clear heteropodous impressions of the footpads and individual toes and claws are preserved.

These tracks appear to have been made by a group of anomodont therapsids, specifically medium-large dicynodonts and possibly Aulacephalodon sp. The tracks are assigned to the ichnospecies Dicynodontipus icelsi.

A continuous trackway and numerous less distinct individual prints and markings made by smaller tetrapods are also preserved, perhaps left by Diictodon, a fairly common burrowing dicynodont, the size of a small dog, or by Emydops.

Further surveys of the site have yielded numerous therapsid fossils and a second trackway. The trackways and fossils all occur in the Cistecephalus Biozone of the Beaufort Group (Karoo Supergroup) and are dated to c. 253 Ma.

Dicynodonts became dominant in the Late Permian and continued into the Early Cretaceous. They were herbivorous animals with two tusks, hence the name which means ‘two dog teeth’. The skull is highly specialised, light but strong, and with the synapsid temporal openings at the rear greatly enlarged to accommodate larger jaw muscles. At the front of the mouth is a horny beak, as seen in turtles and ceratopsian dinosaurs. Dicynodonts were the most successful and diverse of the non-mammalian therapsids.


Dicynodonts have been known from the Karoo Basin since about 1845 when Andrew Geddes Bain made the first description, referring to them as ‘bidentals’ in a letter published in the ‘Transactions of the Geological Society of London’. The palaeontologist Richard Owen named two species of dicynodonts: Dicynodon lacerticeps and D. baini. Dicynodon later became a ‘bucket genus’ and has been much revised. D. baini was subsequently reassigned to the genus Aulacephalodon in 1898, and is its type and possibly only valid species. ‘Aulacephalodon’ comes from the Greek, meaning ‘furrow-head tooth’.

Further reading:
De Klerk W.J., 2002 A dicynodont trackway from the Cistecephalus assemblage zone in the Karoo, east of Graaff-Reinet, South Africa. Palaeontologia Africana, 38: 73-91.
Kammerer, C.F., Angielczyk, K.D. and Fröbisch, Jörg 2011 A comprehensive taxonomic revision of Dicynodon (Therapsida, Anomodontia) and its implications for dicynodont phylogeny, biogeography and biostratigraphy. Journal of Vetebrate Palaeontology, 31, Memoir 11.

1 Dicynodont trackway at Asante Sana, Graaff-Reinet, South Africa (photo: WJ de Klerk)
2 Model dicynodont and trackway at Asante Sana (photo: WJ de Klerk)