Land, water and air

Geology

Geology is the science, or the study of, the earth and its history as recorded in rocks.

Chichester Harbour was formed at the end of the last ice age when the water from melting snow and heavy rain carved out deep channels on its journey towards the sea.  The powerful force of moving water also helped to expose the geology beneath the harbour that we can see today in many places.

The rocks beneath Chichester Harbour are sedimentary – that means that they were formed over millions of years from sediments deposited in layers on the surface of the earth by wind, water, or ice. Sedimentary rock is formed as the sediments become more deeply buried and compacted beneath younger deposits. Sometimes they are naturally cemented into layers of hard rock.

The main layers found in or near the harbour are Chalk, Reading Formation, London Clay Formation, Bracklesham Beds, raised beaches and brickearth.

Geology diagram

Simplified Geological map and cross  section                                                                                                     

Chalk.

The oldest and deepest layer of sedimentary rock found in Chichester Harbour is chalk.  This was formed during the Cretaceous period (about 90 million years ago) when this area was under the sea.  Chalk is a type of limestone mostly made up from coccoliths, which are tiny calcite plates produced by photosynthesising micro-organisms.  Flints and fossils can be found within the chalk and loose on the surface when the chalk has been eroded away.

Reading Formation

Clay from Reading Beds
Clay from Reading Beds
In the period between 65 to 55 million years ago the sea floor was raised above sea level by earth movements and the Chalk was eroded. The next deposit, the Reading Formation is made from red and orange clays and sands deposited on top of the Chalk. Red clay can be found on the beach at Dell Quay.
 


         


London Clay Formation

During the early Tertiary Period (now known as the Palaeogene) from 54 to 50 million years ago sea levels gradually rose. Layers of mud were deposited and compacted together to form grey and brown clay often rich in iron pyrites, known as the London Clay Formation. It sometimes contains fossils. On the beach just south of Ella Nore fossil Turritella shells can be found and at Copperas Point, small pieces of pyrite and fossilised wood can be found in the mud.

“Bracklesham Beds”

Along the open coastline from Hayling Island to Selsey Bill are the “Bracklesham Beds”.  These mostly sandy deposits were deposited in a shallow sea from about 49 to 42 million years ago. Some of the beds contain fossil shells and sharks’ teeth, which may be washed along the coast to East Head.

Superficial deposits

Recent sediments form superficial deposits, which are not included in maps showing the underlying solid geology. There are several types of superficial deposit, including raised beaches and brickearth. These were formed in periods of climate change during the Ice Ages. (There have been several Ice Ages in the last 2 million years). The raised beaches were generally formed during periods of higher sea level, when ice sheets were at a minimum, and the sand and shingle deposits can be seen in many low cliffs around the Harbour. The brickearth is originally a wind-blown dust deposited under extremely cold, dry conditions but much has been re-deposited by flood water and mixed with flints. The brickearth forms the base of many of the soils around the Harbour.

How can we see the older layers?

Although the layers of rock form one on top of another, the combination of earth movements and erosion mean that in some places the older rocks are exposed at the surface.  The rock layers slope down below the harbour; the chalk is nearest the surface at the north and as you go south the younger beds become exposed.

Other interesting geological features:

Ice age rocks
Ice age rocks..

Erratics
There are several unusual large boulders dotted around the Harbour. These are of two types, erratics and sarsens. Erratics are boulders of granite and other “foreign” rocks that were probably brought here in floating ice from Brittany and south-west England during the Ice Ages. Sarsens are cemented sandstones that may have been brought by drifting ice but may also be all that is left of Tertiary deposits that once covered the Chalk of the South Downs.



Fossils

IIf you look carefully you may find some fossils. These can often be found in the areas where the London Clay is exposed. One of the most common fossils is that of the Turritella, which is a gastropod.

Fossils
Turritella Fossils

Iron Pyrites
IIron pyrites can also be found in the London Clay.  This is a mineral that comes in the form of greenish-grey nodules that weather to a rusty brown.  It is a natural sulphide of iron and its chemical formula is FeS2.  Iron pyrites (known in the old days as copperas stones) was collected by local people during the 19th century and sold for the manufacture of a chemical known as copperas, which was used in the tanning industry, to make ink and the pigment Prussian Blue, and sometimes as a mordant.

More information and references

  • Chichester District Museum
  • The Quaternary Geology of Chichester Harbour  Cordiner, R.J. August 2005. (Available at the Harbour Office at Itchenor)
  • Looking back over fifty-five million years. David Bone Chichester Harbour News and Guide 2006
  • Chichester Harbour Archaeology Research Framework document
  • Search the harbour library for information on publications and research documents relating to all aspects of the harbour.
  • Look at the landscape timeline to see some of the changes in the area.


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Harbour Office, Itchenor, Chichester, PO20 7AW.   Tel: 01243 512 301