Tag Archives: geology

Object of the Month – June 2020

June’s Object of the Month celebrates Volunteers’ Week. These fossils have been cleaned and recorded by two dedicated geology volunteers, helping to audit the thousands of fossils held in the Museum’s stores. The project is suspended at the moment, but we all look forward to getting back together when times are better.

These fossils are from the Red Crag layers, which are the reason Walton-on-the-Naze is famous for marine fossils. The sandy Red Crag rocks and fossils were laid down in the late Pliocene and early Pleistocene epochs between 3.3 and 2.5 million years ago, when a warm, shallow sea and bay covered most of Essex. The fossils have stained red-brown over time due to iron-rich water washing through the sandy rock.

The first fossil is a species of whelk, Neptunea contraria, which is still alive today (extant, rather than extinct). This species has an unusual left-spiral shell, hence the word contraria in its scientific name. Almost all species with a coiled shell have a right-hand spiral.

Neptunea contraria

Cardita senilis

Cardita senilis is a species of bivalve, a group which also includes oysters, mussels and scallops. These molluscs have a flattened body protected by two shells or valves joined by a hinge. A bulge near the hinge, called the umbo, is the oldest part of a growing shell, and is at the centre of the growth rings that can sometimes be seen on the surface.

Spinucella tetragona is an extinct species of predatory sea snail, in a group known as murex snails or rock snails. This species’ shells are highly ridged, but other extant species (such as Chicoreus aculeatus) have exaggerated and complicated patterns of spines on their shells, which makes them very popular with shell collectors.

Chicoreus a

Spinucella tetragona

Chicoreus aculeatus

Oyster: Ostrea species

Later Pleistocene fossils from Essex, such as the oyster, don’t really ‘belong’ here at all. They were brought south or churned up from older rocks by glaciers during the Pleistocene Ice Age, which lasted from 2.5 Mya to 12,000 years ago. They appear in glacial drift deposits left behind as the glaciers grew and shrank. This fossil of Chicoreus aculea is actually from the Jurassic period (201-145 Million years ago).

All images © Saffron Walden Museum, except C. aculeatus: H. Zell – Own work, CC BY-SA 3.0

Identification – flint, fossil sponge

Figure showing flint nodule from chalk

In Essex and south east England, almost every pebble on the beach and in gardens is flint. It’s a hard rock found in the Chalk, a soft, white, limestone layer that is up to 200m (600 ft) thick in north Essex and Cambridgeshire. In north west Essex, this chalk is between 90 million and 66 million years old and lies just below the soil, north of a line running from Stansted to Sudbury.

Diagram showing bedrock geology of Essex

Diagram showing the main bedrocks across a section of Essex. Chalk appears as the bedrock across northern Essex. Credit: reference 1.

Chalk started out as a thick mud on the floor of a tropical sea that covered most of Britain and north west Europe. This mud contained the remains of tiny sea creatures (plankton) which grew shells of calcium carbonate. When they died, these plankton and their shells fell to the sea floor to form a thick mud, which compacted into chalk over millions of years.

As it compacted, it squeezed out the seawater containing dissolved quartz, or silica (which comes from the skeletons of tiny sponges, a very simple animal).This silica was pushed out into gaps, cracks and burrows in the chalky mud to form nodules or layers of flint. These flints have a white outer layer (cortex), and are black inside. They can come in very complicated, bulging shapes, or with spikes, holes and cavities. Because of this, they can be easily confused with fossilised bones.

Figure showing flint nodule from chalk

An irregular flint nodule with a white cortex. Credit: reference 2.

Some flints do contain fossils, often urchins, or cockles or other small shellfish. Sometimes, the whole flint looks like fossil, and this may be because the silica that created it was forced into a hollow space in the hardening chalk which contained a sponge. Sponges are very simple animals which live on the sea floor. They still exist today, and the earliest known fossil sponges are  580 million years old.

The silica fills the gaps in the sponge’s skeleton and, over millions of years, the skeleton itself can dissolve away and be replaced by other minerals. This skeleton is a fossil, and the flint fills the spaces left by the soft parts of the animal after they rotted away.
Sponges are hollow tube or cone shapes and have no muscles, stomach, brain or nerves. They are filter feeders that catch bacteria and microscopic plants & animals from seawater that flows through tiny channels (pores) in their body.  Sponges are open at the top, and water currents flowing across the opening helps pull in water through the pores and remove it from the centre chamber, like wind blowing across a chimney.

Diagram showing water flow through a sponge's body

A simple diagram of a sponge’s body showing the pores in the sponge’s body, and the direction of water flow (blue arrows). Credit: reference 3.

Figure showing a living sponge

A living sponge, showing the typical hollow tube shape. Credit: reference 4.

The first sponge below is preserved in chalk and is a typical funnel shape. Some fossils may have a textured ring around the top, showing the rough pattern of the sponge’s surface and pores, like in the second photo.

Figure showing typical funnel shaped sponge

Fossil of a sponge (Ventriculites species) that lived in the Chalk sea. This sponge attached to the sediment with its branching roots. © SWM.

Figure showing rim imprint of a sponge's body in flint.

A flint nodule showing the imprint of the upper rim of a sponge’s body. Credit: reference 5

References

  1. Essex Bedrock, Essex Rock 1999. GeoEssex.org, retrieved 11:36, 24.4.2020
  2. © G Lucy. GeoEssex.org, retrieved 11:31, 24.4.2020
  3. Adapted from: Porifera_body_structures_01 By Philcha – Own work, CC BY-SA 3.0
  4. NOAA Photo Library reef3859 By Twilight Zone Expedition Team 2007, NOAA-OE. , Public Domain,
  5. Flint rim print. flint-paramoudra.com, retrieved 11:47, 24.4.2020

Identification – Limonite

Yellow limonite on brown goethite.

Limonite (pronounced “lime-on-ite”) is an iron ore similar to the more well-known iron oxides haematite and magnetite. It often forms as existing deposits of these other minerals react with water in an oxidation reaction, turning the iron oxide into iron oxide-hydroxide. This interrupts the regular crystal structure and opens up microscopic gaps that trap other water molecules in positions where they can’t chemically react and bond with the iron atoms. Water which forms part of the molecular structure of in this way is called ‘water of crystallisation’.

Yellow limonite on brown goethite.

Limonite can be ground up to produce the pigment yellow ochre, famous from prehistoric cave paintings. This sample from the Museums’ mineral collection has yellow limonite on brown goethite, another form of iron hydroxide.
Image: © Saffron Walden Museum.

Scientifically, limonite does not meet the criteria of a ‘true’ mineral, which must have a consistent chemical formula and molecular crystal structure. Because limonite forms as a replacement for several other minerals, this means that the crystal structure is not consistent. Variations in the original mineral, the compounds dissolved in the water and the environment where it forms, also mean the relative amounts of iron oxide, iron hydroxide and water of crystallisation are not constant either.

Four small, rounded pieces of limonite

These pieces of limonite were originally pieces of the gemstone garnet. Iron-rich water filtering through these stones replaced the original garnet mineral with limonite, keeping the shape.
Image: Eurico Zimbres FGEL/UERJ CC BY-SA 2.0 br (Wikimedia Commons)

Limonite may be any colour from a rich yellow to a dark brown, and was used historically to make the yellow ochre pigment which is still produced in this way in Cyprus. Despite this variation in colour, an easy way to distinguish it from haematite is the ‘streak test’. This can be used to separate many minerals which may appear similar to the eye, by rubbing the mineral along a piece of un-glazed white porcelain. Limonite will leave a yellow-to-brown streak, whereas haematite produces a red streak.

Two forms of haematite leave a rusty red streak on ceramic, central.

Two different forms of haematite both leaving a rust-red streak.
Image: KarlaPanchuk [CC BY-SA 4.0] (Wikimedia Commons)

Deep red botryoidal (grape-like) haematite.

This is an easily-recognised form of iron oxide, haematite. The rounded, bulbous form is described as ‘botryoidal’, meaning grape-like in Greek.
Image: © Saffron Walden Museum

 – James Lumbard, Natural Sciences Officer.

Identification – Ammonite in sandstone

One of the most interesting parts of working in museums is helping people discover something new (and I usually learn something new myself). A really important way for museums to do their job as a welcoming public source of information is by identifying mystery objects that you might find on a walk, on a seaside holiday or even in your garden or attic.
Anyone can bring in an item for us to identify, for free, and you should have an answer within a few weeks. It might look a bit like this:

Ammonite in sandstone

This piece of stone is a Jurassic fine-grained sandstone or sandy limestone, which may be from the Lias Group rock unit found on the Dorset coast, although it has a sandier appearance and rougher texture than the rocks usually found in this formation. If it is from the Dorset Lias formation, the rock is roughly 195 to 200 million years old, and the fossils it contains would be a species of Promicroceras ammonite, which are common along the Dorset coast.

Fossil of a Promicroceras ammonite.
Image: Ammojoe CC BY-SA 3.0 (Wikimedia Commons)

The bristleworm, Polydora ciliata. Image: Yale Peabody Museum of Natural History [CC0] (Wikimedia Commons)

 

 

 

 

 

 

The surface pattern of pores in the rock was made much more recently. They were probably made by a species of Polydora worm, probably Polydora ciliata. P. ciliata is a small, rock- or shell-boring worm which can grow up to 30mm (1 1/8 in.) long, and is also known as a bristleworm.

P. ciliata burrows in stone. Image: Rosser1954 CC BY-SA 3.0 (Wikimedia Commons)

Bristleworms are thought to burrow into rock or shell by scraping away at the surface using specialised bristles on the fifth segment of its body, although it may also secrete chemicals such as weak acid to help. It digs a U-shaped burrow, which appears on rocks as distinctive small slots or a ‘sunglasses’ shape.

 – James Lumbard, Natural Sciences Officer.

 

A bit behind the scenes

An update from James Lumbard, Natural Sciences Officer.

The Geology Gallery received a lot of attention in the run-up to the festive period thanks in no small part to the help provided by Cali, the latest addition to the natural sciences volunteer team. After a short training session in how to carefully clean specimens using a conservation vac and a paintbrush, we were away, and have already cleaned around half of the objects on display at the time of writing. It should be a fairly quick job to finish the rest of the objects in the ‘table-top’ display cases, leaving only a dozen or so in wall-mounted cases. This is part of regular ‘deep cleans’ that help care for museum objects, and will help us double-check and update the information we hold about each object. Many museums are also ‘Accredited’ which means that they uphold certain national standards of collection care, and this work contributes to Saffron Walden Museum maintaining its Accredited status year-on-year. Meanwhile, the photos we take can be used for everything from social media to encouraging researchers to visit the collection.

Fossil ammonite found in Saffron Walden. 150-200 million y.o.

At the start of December I visited the Essex Field Club’s (EFC) annual exhibition and social at Wat Tyler Country Park, near Basildon. The EFC is a volunteer-run society of amateurs and professionals who compile and look after a county-wide database of the wildlife and geology of Essex. The club’s secretary, Fiona Hutchings, very kindly introduced me to members from each specialty so I could speak to them about the natural sciences exhibition this summer, called Take Away the Walls. My plan is to hold a museum-based exhibition showcasing the wildlife of north-west Essex, and to run activities bringing together wildlife organisations and community groups across Uttlesford to help people enjoy the outdoors in new ways that will benefit their own health, and the health of the local environment. The exhibition and activities will really start to take shape behind the scenes soon, so keep your eyes peeled for more updates in the coming months.

Fossil bryozoan in flint. Tiny bryozoa live in coral-like colonies (above), but are much more complex internally.

At the end of this month I will be attending a short training seminar entitled ‘Finding Funds for Fossils, Ferns and Flamingos’, hosted by the Natural Sciences Collections Association (NatSCA) at the World Museum, Liverpool. NatSCA are a nationwide ‘subject-specialist network’ of museum professionals working in the natural sciences who have an active programme of meetings, training courses and conferences throughout the year. This particular event is all about how to successfully attract funding and support to care for and promote natural sciences collections in museums, and I look forward to putting my new-found knowledge into use to benefit the tens of thousands of natural sciences specimens at Saffron Walden Museum.

Hello to James

Hello! As I’ve been here since the end of April, it’s well-and-truly time to introduce myself. I’m James Lumbard, and I’m delighted to have been chosen to share the post of Natural Sciences Officer with Sarah Kenyon. I’ve really enjoyed my introduction to the job, the museum and the friendly staff and volunteers who make it such a pleasant place to work and visit. I’m originally from South Wales and moved to East Anglia two years ago. I’ve moved around a few times since then, moving to Tendring district at the start of this year, so it’s great to have the chance to explore Uttlesford and the lovely town of Saffron Walden.

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