War Memorial #2627 ~ Chedgrave EarthCache

Introduction

The Chedgrave Lychgate War Memorial contains several different construction materials, but this EarthCache concentrates on one particular geological feature visible within the flint used in the wall.

Although these stones now form part of a man-made structure, the geological features inside them formed millions of years before the memorial was constructed.

Look carefully and many of the flints show two very different areas:

• a pale or white outer layer;
• and a much darker interior.

Some also contain smooth, curved broken surfaces.

These features allow us to investigate the cortex of a flint nodule and the way its extremely fine-grained interior fractures.

Chedgrave Lychgate – Inside a Flint Nodule

•  Logging Tasks

To log this EarthCache, please visit the published coordinates and examine the flints in the memorial wall.

The tasks are based on geological features visible within the flints at this location. You will need to combine your observations with the information in the EarthCache lesson below.

Please send your answers to the cache owner through the Geocaching Message Centre or via the cache owner's profile.

Please do not include your answers in your online log.

There is No need to touch, climb on or remove anything from the memorial. All observations can be made visually from the publicly accessible path.

 1. Cortex and Interior

Examine several of the flints in the panel.

Describe the colour of:

a) The exposed interior of the flints;

b) The lighter outer cortex.

Now compare at least two different flints.

Do they have approximately the same amount of cortex remaining, or does one have considerably more?

Using what you have learned from this page, explain what this difference tells you about how much of the original outside of each flint nodule remains visible.

 2. Examine the Boundary

Choose one flint where you can clearly see both the lighter cortex and darker interior.

Describe the boundary between them.

 Is the transition:

• sharp and obvious;
• slightly irregular;
• or gradual?

Without touching the wall, visually estimate how thick the cortex appears to be at one point:

• only a few millimetres;
• approximately 1 cm;
• or more than 1 cm.

How does the appearance of your chosen stone help you distinguish the original outer cortex from an exposed interior surface?

3. How Has the Flint Broken?

Find one dark exposed surface where a flint has been broken or knapped.

Describe the shape of the fractured surface.

Look particularly for features such as:

• curved surfaces;
• sweeping arcs;
• ridges;
• ripple-like markings;
• or shell-shaped fracture patterns.

Do the features you observe appear consistent with conchoidal fracture?

Explain your answer using both your observation and the geological information in this listing.

Optional Photograph

You are welcome to add a photograph of yourself, your group, GPS or another personal item at the Lychgate.

Please avoid showing anything that clearly reveals the answers to the logging tasks.

• Geological Concept 1: Flint within Chalk

Flint is a very hard form of chert composed mainly of silica.

The crystals making up flint are microscopic. For this reason it is described as **cryptocrystalline**, meaning that its individual crystals are too small to distinguish with the naked eye.

Flint commonly occurs as irregular nodules and bands within Chalk.

The Chalk found across large parts of eastern and southern England was deposited during the Cretaceous Period when the area was beneath a warm sea.

Chalk accumulated from calcium-carbonate material produced by microscopic marine organisms.

Silica was also present in these marine sediments. Some came from organisms such as siliceous sponges.

After burial, silica could dissolve and move through water contained within the sediment. Under suitable chemical conditions it became concentrated and replaced parts of the original chalk sediment.

Over time this produced hard silica-rich flint nodules within the much softer Chalk.

•  Geological Concept 2: The White Cortex

Fresh flint nodules taken directly from Chalk commonly have a distinctive appearance.

Their interior is normally:

• Black;
• Dark grey;
• or bluish-grey.

Around the outside there may be a much lighter white or cream-coloured layer called the cortex.

The cortex forms the outer rind of the flint nodule.

Historic England describes fresh quarry flint as having a white outer cortex surrounding a darker black or dark-grey interior.

The proportion of cortex remaining on a piece of flint can vary considerably.

A flint showing a large area of pale cortex retains much of its original outer surface.

Where the nodule has been broken, the darker interior is exposed.

This means that examining the amount and position of the cortex can help us interpret which surfaces formed the outside of the original nodule and which were created later when the flint was broken.

Example of a flint core showing areas of remaining outer cortex and exposed flint interior.

Image: Portable Antiquities

Scheme / The Trustees of the British Museum.

Licensed under Creative Commons Attribution-ShareAlike 2.0 (CC BY-SA

2.0).

1. https://commons.wikimedia.org/wiki/File:Flint_core_(cortex)_(FindID_394362).jpg?utm

• Geological Concept 3: Why the Interior Looks Different

The difference between the cortex and the freshly exposed interior is particularly noticeable because the inner flint is extremely fine-grained and dense.

Individual quartz crystals within flint are microscopic, so a freshly broken surface does not usually look gritty or show large individual mineral grains.

Instead it can appear:

• smooth;
• glass-like;
• waxy;
• or finely textured.

This very fine internal structure also affects the way the flint breaks.

• Geological Concept 4: Conchoidal Fracture

Flint has a particularly well-developed type of break known as **conchoidal fracture**.

Rather than separating into obvious layers or breaking around large visible crystals, a fracture can pass smoothly through the extremely fine-grained stone.

This can create curved surfaces resembling part of a shell.

The broken surface may contain:

• sweeping curves;
• curved ridges;
• arcs;
• or ripple-like marks.

The word conchoidalcomes from the shell-like shape produced by this type of fracture.

Freshly fractured flint can also form very sharp edges.

This property is the reason humans have deliberately broken or knapped flint for thousands of years.

At this location, knapping has exposed dark interior surfaces of some of the flint nodules, allowing the contrast between the original cortex and the fractured interior to be studied.

The human workmanship is not itself the geological subject of this EarthCache.

Instead, it has exposed geological features that allow us to examine the internal structure and fracture behaviour of the flint.

---

• What Are You Looking For?

While examining the flint panel, concentrate on four observable features.

Cortex

The pale white or cream outer rind of a flint nodule.

Interior

The generally dark grey, bluish-grey or black silica-rich material beneath the cortex.

Cortex Boundary

The point where the pale cortex meets the darker interior.

Its appearance and thickness may vary between individual flints.

Conchoidal Fracture

A curved or shell-like broken surface produced because of the extremely fine crystalline structure of the flint.

By comparing different stones within the panel you should be able to see that the amount of original cortex remaining varies considerably.

You should also be able to distinguish between an original outer surface and an interior surface exposed by fracture.

These observable differences form the geological focus of this EarthCache.

2.
https://commons.wikimedia.org/wiki/File:Diagram_showing_the_conchoidal_fracture_of_flint._Wellcome_M0015946.jpg?utm

Protecting the Location

This EarthCache can be completed entirely by observation.

• Please:
• Remain on the established publicly accessible path;
• Do not climb on the memorial;
• Do not strike or scratch any of the stones;
• Do not remove any material or samples;
• and leave the site exactly as you found it.
• EarthCaches are about observing and understanding geological features, not collecting them.

 Sources of Information

British Geological Survey

British Regional Geology – East Anglia

Used for information concerning the Chalk of East Anglia, the occurrence and formation of flint within Chalk, silica replacement and the contribution of siliceous organisms.

British Geological Survey

Geological descriptions of flint/chert

Used for information concerning flint being composed of microscopic quartz crystals and its characteristic conchoidal fracture.

Historic England

Building Stones of England

Used for geological descriptions of fresh quarry flint, including its white outer cortex, dark grey or black interior, conchoidal fracture and use of knapped flint.

Historic England describes fresh flint as an extremely fine-grained, hard form of silica containing microscopic quartz-crystal aggregates.

Historic England / British Geological Survey

Building Stones of England – Norfolk

Used for information concerning flint as a characteristic building material within Norfolk and its relationship to the regional Chalk geology.

Breaking New Ground

Brandon Flint Mine Pits – The Geological Story of Flint

Used as an additional regional source concerning East Anglian flint, including the dark colour of fresh flint, its white chalky cortex and subsequent surface alteration and weathering.

EarthCache / Geocaching

Official EarthCache Guidelines and EarthCache Help Centre

Used when designing the Earth-science lesson and site-specific logging tasks.

Artificial Intelligence Disclosure

Artificial intelligence was used to assist with the organisation, wording and formatting of this EarthCache listing.

The geological information used to create the Earth-science lesson has been checked against the geological and heritage sources listed above.

The geological subject, location observations and suitability of the features at the EarthCache location remain the responsibility of the cache owner.