Tag Archives: geology

Object of the Month – March 2023 – Cast of the Ashdon Meteorite

One hundred years ago, on 9 March 1923, a meteorite came from space and landed at Ashdon in north-west Essex. The fall was witnessed by a thatcher called Frederick Pratt who was working in a wheat field at Ashdon Hall Farm. He heard a ‘sissing’ sound and looked up to see ‘the earth fly up like water’. Later he dug up the stone from a depth of two feet with another farm worker called Curven. Frederick took it to the police station in Saffron Walden, then to his vicar in the village of Wendens Ambo. Reverend Francis W. Berry purchased the meteorite and donated it to the Natural History Museum where it was investigated by the Keeper of Minerals George T. Prior. He classified it as a stony chondrite meteorite. It contains minerals feldspar, pyroxene and olivine, white specks of nickel-iron and other minerals from which the solar system formed 4.5 billion years ago.

SAFWM : 1923.3 Cast of the Ashdon Meteorite ©

Chondrites
Chondrites are by far the most abundant class of meteorites, but they are also the most interesting, as they are thought to have been formed at the same time, and from the same stuff, as the inner rocky planets of our Solar System. They contain a mixture of fine-grained crystalline and glassy materials, and there are several different types, but they are all characterised by the presence of ‘chondrules’ – tiny, near-spherical beadlike objects about a millimetre in size. Chondrules are named after the Greek word chondrus, meaning grain, and are only found in meteorites. There is no scientific consensus on how chondrules were formed but they are thought to have once been molten droplets in space, formed at very high temperatures, which solidified and aggregated into asteroids.

The Ashdon Meteorite
The Ashdon meteorite is 12cm x 9cm x 6cm in size and weighs 1.27kg. When it landed it weighed 1.3kg, however two pieces were chipped off to find out what it was made of and the Natural History Museum took a thin section for microscopic examination. This plaster cast was made in 1926 using a mould of the original meteorite.

It is a flight-orientated stone. The smooth face of the meteorite was melted by the heat generated as it travelled through the Earth’s atmosphere from space. The face became a shield-shape as white hot molten rock was forced backwards by hypersonic flight. The other side of the meteorite is rough and irregular, as shown in the photograph below.

Meteor Shower Landing Soon! To celebrate the centenary of the fall more meteorites will be on display from Sunday 12 March to Friday 14 April. These rocks from space include meteorites that landed in Africa, Greenland, North America and Russia. You will also be able to see pieces of rock ejected from Mars and the Moon which landed on Earth as meteorites.

In association with the upcoming meteorite display, the museum shop will be stocking Gerald Lucy & Mike Howgate’s booklet, The Ashdon Meteorite for a very reasonable £3 per copy.

Valentines in our collections

A bit of Valentine’s related history for February from our collections!

An invitation to a Valentine’s ball at Wimbish Village Hall, 12 February 1943. Miss McQueen was well-known locally she had a small farm at Rowney Corner from which people could buy fresh eggs and she also played the organ in the 1970s at the church in Wimbish.

We also have a selection of Victorian Valentines cards. These 19th century designs typically include floral decoupage, lace doilies, ribbon details and lace trimmings. Inside the cards are lovely little poetic verses.

In our Natural sciences collections we also have these amethyst gemstones associated with love and romance.

Amethyst is the birthstone for February, but as a symbol of love, St Valentine is said to have worn an amethyst ring so Christian couples in Ancient Rome could identify him. Valentine was a priest who carried out forbidden Christian marriages and married young couples, when the Roman empire persecuted Christians and preferred their soldiers to be unmarried men.

Lapis lazuli can represent truth and friendship, and in Christianity represents the Virgin Mary. With the blue of the sky and gold of the sun, it represents success in Jewish traditions, while beads found in the ancient town of Bhirrana from 7500 BCE are its oldest known use by people. The remains of Bhirrana are in the Indian state of Haryana.

Sapphires are popular for engagement rings, as used for Lady Diana’s engagement ring from Prince Charles. Sapphire is the traditional gift in the UK for a 45th wedding anniversary and can symbolise truth and faithfulness.

In Ancient Greece and Rome, the word sapphire was used for lapis lazuli, as sapphire was only widely known from the Roman Empire onwards.

Identification – Fossil sponge in flint

Some flints do contain fossils, or look like whole fossils. Fossils inside the flints are often sea 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 filled by a sponge. 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.

A grey, funnel-shaped fossil on a wooden surface

Typical funnel-shaped sponge, fossilised in flint. © L Hodgson.

The shape of this piece of flint looks a lot like a small sponge that lived on the sea floor, and was fossilised in flint as the thick mud solidified into chalk. It may have patterns inside it that show the structure of the sponge’s skeleton. The wiggly line around the widest part of the flint shows the top rim of the sponge and the rough texture of the line is the surface texture of the sponge preserved as a fossil.

More info on flint and chalk in Essex in this post from 2020. https://saffronwaldenmuseum.swmuseumsoc.org.uk/identification-flint-fossil-sponge/

Object of the Month – October

Dark stone with faint tracing of a fossil flower in two petal shapes

October’s Objects of the Month are pieces of fossilised plants.

Fossils can form in different ways depending on where they form and the type of plant or animal. Most fossils come from the hard parts of animals such as bones, teeth or shells. For plants, wood is the most common material to fossilise because it is quite hard, and takes longer to rot away than other parts.
Soft leaves and flowers need to be buried quickly in deep sediment like mud or volcanic ash where the low oxygen levels mean they won’t rot. Once underground, plant material can fossilise in different ways.

Compression

This flower is probably preserved by compression, like pressing and drying it in fine mud over millions of years. Heat and pressure deep underground turned the mud to stone and forced moisture and gases from the leaf at the same time.

The main ingredient in living plants is carbon, so a thin, black, carbon-rich film is all that’s left. In most fossils, new minerals replace the original material. But because this is a compression fossil, the carbon-rich film is the exact same carbon that was in the plant millions of years ago. Soft-bodied animals like squid can also be preserved like this.

Impression

Dark stone showing inpression of a fern leaf, with fronds alternating in an exaggerated sawtooth pattern

This fern leaf, or frond, is preserved as an impression. When something soft is preserved by compression, the shape of it is also preserved as an impression, like pressing a leaf into soft mud or clay and then removing it.

This fossil is one part of a small rock nodule which was split in two to show the leaf – this part shows the impression of the frond. Because compression and impression fossils usually form together, the word ‘adpression’ describes both at the same time.

Petrification

Wedge of dark fossil wood, narrow at left. Lines of pale grey run top-bottom showing growth rings.

Fossilised wood is often called ‘petrified’ wood, meaning wood ‘turned to stone’. It happens when the materials (cellulose and lignin) that make up the solid part of wood are replaced by minerals, turning it to stone.

Minerals dissolved in groundwater seeping through the sediment settle as solids in the microscopic cell walls of the wood as the cellulose and lignin slowly rot. This can create a perfect stone copy of the original structure of the wood.

See these objects up close in Curiosity Corner throughout October.

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

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.

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.

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

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

Identification – Limonite

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’.

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.

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

[Show slideshow]

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.