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This gives us a record of changes in the environment.

To learn about these changes we need to study the layers of sediment in their proper sequence, without mixing up the different layers.

The method we chose was to produce cores of sediment.

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The coring machine

The coring machine is called a Terrier Rig

This machine thumped a metal cylinder vertically into the ground to cut out a column of sediment with the layers intact. We pushed down a plastic tube that scooped up the column called the core. Each tube is 1 metre long and 100mm in diameter. We took out the filled tube, thumped the metal cylinder down another metre and scooped up the next column of sediment into a new tube.

The full core can be up to 8 metres long in 1 metre sections.

The Terrier can extract cores to a depth of eight metres providing it does not hit bedrock. The core example used here has 7 sections.

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Sampling a core

Each core was taken back to the laboratory and sliced open right down the full 1 metre length.

Our expert, William Mills of the Museum of London Archaeology Service, then carefully recorded and measured the layers. He took small samples from the most distinctive layers. These were then looked at and analysed with a microscope. A typical sample size is a 10mm cube.

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Thorney Island Core (Borehole 1)

We bored holes at 7 locations around the Harbour. The core from Thorney Island has the full range of sediments from the end of the last Glacial period, through the Holocene (last 10,000 years) up to the present day.
The Thorney core was taken at the upper end of Emsworth Channel and shows us what conditions have been like for the Emsworth area through the ages.

Using the Thorney Core to uncover the past

We have taken lots of samples from the seven metre long core of sediments from Thorney Island.
Evidence found in the samples will tell us about the environment when the sediment was laid down. We look in the sample for the following types of evidence:

• Peat or organic material such as plant roots – these can be dated using radio-carbon dating
• Foraminifera – these are marine single-celled organisms found in saltmarshes and marine channels. They are very useful in habitats like our harbour because they leave behind shells, which are different if the water has become saltier.
• Pollen – higher plants produce pollen grains which are very small ( typically 25-120 microns) and so a microscope is needed to identify them. They are spread by wind and water. They give a good indication of the types of plants which were growing in the Thorney area when the pollen got stuck in the sediment.
• Diatoms – these are freshwater and marine algae that have a hard silica shell (sometimes called a glass house!) around them that survives. They are again very small (usually 5-80 microns) and certain species are particularly good at showing if the sediments have been frequently exposed to the air, for example, when the tide goes in and out.

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Results of the study

Look at the page "Exploring the core" to see what was found in the core samples and how these have allowed archaeologists to build up a picture of the harbour environment as it changed over the years.