Isle of Purbeck and Swanage - Map, draw and make notes on features associated with a concordant geology.

Isle of Purbeck and Swanage

Map, draw and make notes on features associated with a concordant geology.
These include:

Headlands at Durlston, Peveril Point and Handfast Point
Intervening bays especially Studland
Erosional Features at the Foreland
Double Spit at Poole Harbour

Durlston

At Durlston Point the Middle Purbeck limestone is clearly visible in the cliff. The Cinder Bed, a useful marker oyster bed in the middle of the Purbeck strata descends to the shore. Softer mudstones of the Lower Purbeck can be seen beneath the Middle Purbeck limestone. The scar of dumped Portland Stone beneath some flats, constructed surprisingly close to the cliff edge, covers part of the Lower Purbeck exposure and affects the scenery of the bay. Thin-bedded lagoonal limestones alternate with shales. This part of the succession is high in the Middle Purbeck Formation with plant debris and pyrite being more abundant. Kaolinite also occurs here amongst the clay minerals in this part of the succession. Some of the limestones were cemented early with early loss of aragonite and these are particularly good for showing dinosaur footprints. Other thin shell beds remain at or just below the water table and when they were buried they preserved aragonite.

Peveril Point

This stone headland is formed of a hard bed of limestone (known as Purbeck Marble), which runs from Herston to the west of Swanage to Peveril Point and then eastwards under the English Channel. The small building on Peveril Point is currently used by the 'National Coast watch' as a lookout. Throughout the second half of the 19th century, Peveril Point was a base for the Dorset Artillery Corps who were a part of the voluntary force that made up Britain's home defence. The Corps built a fort on the headland that contained two, thirty-two pound cannons that fired 6.4-inch balls.

Handfast Point

Adjacent to the headland the cliff is a strongly crenulated headland, with small bays and small outcrop cut along joint planes. Erosion has separated the stacks of Old Harry Rocks and the Pinnacles. These have been gradually reduced over the centuries, while the breaching of narrow isthmuses has formed new ones. There was once a pinnacle seaward of the castle at Studland. In 1624 only a blockhouse remained there. Three or four guns were mounted at this headland and created the development of stacks on the coast around here. The present island of No Man's Land is a long and quite large stack with an arch through the west end of it. This was connected to the mainland in the not too distant past. Two Chalk stacks called the Pinnacles are present to the southwest of Harry Rocks.

Studland

The South Haven Peninsula is formed by a strip of sand is 3.6 km long and widening from zero at Redend Point to 0.6 km in width. The main blown sand area consists of three broad parallel ridges, separated from each other and from the Tertiary land at the back by strips of marsh with scattered pools, of which the innermost widens southward into a freshwater lake about 1.5 km in length, called the Little Sea. The old cliff is aligned with the present seaward shore at Sandbanks on the opposite side of Poole Harbour mouth, with ...

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Studland

Unlike the other major coasts where there is much sand Studland is relatively undeveloped. At Studland and the Poole Formation (Bagshot Beds) provide ample sources of supply of beach material. The accumulation of sand, however, is controlled more by relationship to waves and long shore drift than to sources of supply. Worbarrow Bay, for example, has the same rocks as Swanage, but it faces west and has only shingle and a little coarse sand at the extremities Studland on the other hand, faces east like Swanage and Weymouth, and the beach is banked on Oxford Clay with no adequate source of sand in the neighbourhood. Eastwards, towards Hengistbury Head, where this shelter decreases, the beaches have large quantities of pebbles. The tendency for sand to accumulate on east facing shores and shingle on windward coasts is merely an extension of a tendency that can be observed in individual bays.

The Forleand

At the Foreland there is a basal ledge, prominent around the stacks, and sectors of intertidal wave-cut ramps with clear white scoured chalk. There is no shore platform, but the vertical cliffs and fallen boulders rise behind an abrasion platform that is submerged even at low tide.

Poole Harbour

There is an interesting generalisation regarding sediment movement in Poole Harbour and was based on studies of tidal movement. The explanation is that he ebb tide is stronger than the flood tide in the lower part of the harbour (i.e. near to the South and North Haven Peninsulas). On the other hand the flood, at least at the bottom, is stronger than the ebb in the upper reaches of the harbour. Thus, at both ends of the harbour there is evidence of a tendency to sweep material out of it, either up into the river mouth or out to the sea. Sediment is therefore trapped on the mudflats and salt marshes of the margins. This could be the reason why this feature has remained so long as a rather anomalous feature of the coast.

The tidal regime at Poole harbour is unusual, although the total rise and fall of the tide there is not that great. The double high water, which is such a well-known feature at Southampton, is much more marked at Poole, and in fact at neap tides each month it is possible to detect "triple high water". At neap tides, however, the total range is little more than two feet (0.6m), whereas after new and full moon it rises to somewhat over six feet (1.8m). It will not be exactly the same at the South Haven Peninsula but this gives some idea of the rather limited tidal range. A consequence of this is that although there is much shallow water there is not an extensive area of exposed sand-flats or mud flats off the peninsula at low tide (such as you find in the Bristol Channel region with its high tidal ranges).

Christchurch

Visit, research and make notes on coastal erosion at Barton-on-sea and the management response to the challenge of rapid erosion.

Barton-on-sea

The stretch of Barton cliffs on either side of Barton Court and the cliff-top houses is protected to some extent by sea-defences. It extends from Barton Cliff House to the end of the Barton Clay outcrop. It is backed by predominantly residential suburban development, with a small cluster of shops and other facilities. There are approximately 150 beach huts along the under-cliff. The cliffs here have been seen by geologists in the past as a marvellous natural coast with excellent exposures of he Barton Clay, and a great attraction for tourism. However, the land above has gradually become a suburban area of housing and the cliffs have threatened a few older houses near Barton Court. The cliff was receding at almost a metre per year and collapsing into terraces. As a result a revetment for protection was built, drainage works took place and the slopes were covered with gravel and regraded. The sea defences are now major and expensive features here and are intended to give protection to houses on the cliff top at Barton Court itself.

Research the depositional features to be found at Hurst castle spit and Hengistbury head.

Hurst Castle Spit

Hurst Castle spit stretches for 2.5 km from the mainland at Milford-on-Sea to Hurst Castle at its tip. It is a shingle spit comprising of mainly large pebbles. The spit probably started to form when the Isle of Wight became separated from the mainland, some 7000 years ago. There is some evidence to suggest that the spit reached a maximum length of 5 km before the water levels rose (at the end of the last glacial period) when sea levels all over the globe rose to their present height.

Hurst Castle used to be replenished by beach material from Christchurch Bay to the west, together with some eroded sediment from the local cliffs at Milford-on-Sea. This was reduced as a result of extensive defence works involving rock armouring and land drainage at Barton-on-Sea, the construction of a sea wall at Milford-on-Sea and groynes which prevented long shore drift.

With its supply of sediment reduced, Hurst Castle spit began to suffer erosion and it became much more vulnerable to high seas during storms. It is estimated that some 30,000 tonnes of sand and shingle were being lost from the spit each year. In the 1960’s the spit was breached several times, causing flooding in the village of Keyhaven. Following a breach in 1965, the army was called in to re-build the spit. Further breaches occurred in the 1980s. Nine breaches occurred in 1983-4 and in 1989, 800 m of its length was flattened and the spit was rolled back towards Keyhaven across the salt marsh by some 80 m.

After research had been completed and a cost-benefit analysis conducted, a £5 million stabilisation project was undertaken in 1996. The scheme involved a number of features including rock armouring, revetments and the construction of a rock breakwater. Since this stabilisation project has been completed, he spit has not suffered any breaches. Despite the engineering works, the feature maintains many of its 'natural' characteristics.

Reasons why the spit is important:

Flood protection - the spit helps to protect Keyhaven and Lymington from the full force of the sea.
Conservation - the spit provides a protection against a much-valued salt marsh and wetland habitat. Breaching of the spit would destroy this habitat.
Recreation - the spit is used for walking and fishing, and the sheltered creeks are used for sailing and windsurfing.
Commercial fishing - a small fishing fleet is harboured at Keyhaven. If the spit were breached, the harbour would no longer be such a safe mooring.
History - the castle (Hurst Castle) was constructed in the reign of Henry VIII to defend the country from feared invasions by the Catholic powers of Europe. It has been altered many times since it was first built and it was manned during both World Wars.

Hengistbury Head

Hengistbury Head is a headland and hill of about 36m in height and is situated between Christchurch Harbour and Poole Bay. It is less than one square kilometre in area and is a particularly valuable piece of unspoilt countryside. Because Hengistbury Head is notable for metalworking in Iron Age times, the archaeology and geology have a direct relationship. Iron was manufactured from the local siderite and other minerals were imported to the area for silver extraction. Gold and bronze was worked here and Kimmeridge oil shale was turned on lathes to make armlets. This map by John Lavender summarises the discoveries. It also attempts to estimate the position of the coastline 2000 years ago.

Sea level in Southampton has risen at a rate of about 1 to 2mm per year (and is rising even more rapidly now). The Christchurch Harbour side of the headland would have been very different with lower sea level and this part of the harbour might have been narrower. The alluvium and salt marshes, however, would have been less developed at that time. What is very likely is that important archaeological remains lie buried under the alluvium and the salt marsh north of the headland.