Hengistbury Head Geology: Strata and Features

Boscombe Sands

The Base of Hengistbury Head is built on a base of compressed yellow sand interspersed with layers of shingle. The overall colour of this soft rock varies from a whitish grey through to a mauve-brown colour. This colour variation is governed by the amount of organic material present in the rock and also on the moisture content. The sand grains making up the Boscombe Sands are generally rounded or sub angular grains of quartz. This type of formation is typical of material deposited on a sea beach. The shingle beds in the lower parts of this layer consist of well rolled flints which show signs of being battered by wave action. These flints are usually white or grey. The sandstone is devoid of large animal fossils because, being laid down on a shallow tidal beach or sand bank, there was no possibility of the animal remains being entombed in the sand and forming fossils. Some of the pitted flints contain small amounts of chalky deposits and they probably came from erosion of the nearby chalk ridge. The upper layers of the bed contains layers of bleached pebbles which were probably washed down from the surrounding hills to a shallow estuary.

Boscombe Sands (the bottom stratum) is clearly visible here.

The lower levels of the Boscombe Sands contain minute fossils of the burrows of small crustations. Occasionally fossils of leaves and other vegetable material have been found but due to the friable nature of the Boscombe sands they are difficult if not impossible to extract.

The upper levels of the Boscombe Sands indicate that the previous sea beach environment was suffering under continuous flooding and submergence and the tidal and wave action effects which give the lower parts of this strata its disordered structure were becoming less dominant. Instead, without the massive influence of wave and tide the last few centimetres of this strata take on a more ordered structure with a finer grained sediment.

The Boscombe sands were created during the Eocene time period after the creation of the chalk ridge from Old Harry rock to the needles (Mesozoic time scale). The creation of the Boscombe Sands date back to between 40 to 60 million years. It is interesting to speculate that, as this was essentially a prehistoric beach, that the layers of flints were probably laid down by wave action, possibly during storms or other catastrophic events. While the composition and age of these materials is unimaginably vast we can still see unique short term events in their structure like the flint layers.

On the beach near the Double Dykes, a large clay slab lies under the river gravel and in the Boscombe sands. As the Boscombe Sands were formed on a sea beach, such a clay slab is out of place. These clay slabs are usually evidence of ponds and lagoons. This clay slab probably formed the base of the pond at some later date. It has probably ended up where it is due to erosion of the area in which it lay. Like the Iron Stone Doggers it has ended up being dumped on the beach while everything around it was washed away.

	The clay slab on the beach

The Boscombe Sands are the lowest strata of Hengistbury head and due to the dip of 3 degrees eastward the stratum disappears below the level of the beach about half way along Hengistbury Head.

The top of the Boscombe Sands is separated from the Lower Hengistbury Beds by a layer of black flints. This flint layer would indicate a sea invasion that signaled a sea level rise for at least part of the time period during which the Lower Hengistbury Beds were laid down. The deposits of this new strata are a maximum of 4 metres in thickness and are mainly glauconitic sandy clay. (Glauconite is a mineral with a greenish/olive colouring)

The type of formation indicates that it was formed as part of river estuary or delta with surrounding bogs, marches and swamps. The presence of Glauconite would also back this up. Glauconite is formed from an interaction of fine clumped clay particle with salt water. A river estuary would provide the ideal environment for this reaction to take place between in flowing sea water and the debris brought down to the estuary by the river.

This stratum is littered with the fossilised remains of prehistoric tropical and subtropical plants and ferns and occasional lignitic layers can be found as well. Lignite is a woody textured mineral derived from vegetation and is a bit a like a half way house between peat and coal. It can be burnt and is often used as fuel, although the deposits at the head are not extensive enough to have provided later human habitants with a fuel source.

The Hengistbury Head Beds used to extend to Alum Bay in the Isle of Wight which has similar structures to that found on Hengistbury Head. Elsewhere the Hengistbury Head Beds can be found in the bottom strata of the cliffs off Highcliffe and also in nearby St Catherine's Point.

Another view of the strata. Notice the Iron Doggers on the beach and
embedded in parallel lines in the Upper Hengistbury Beds

The Upper Hengistbury Beds are the most prominent of the stratum. They are approximately 15 metres thick and are a creamy brown colour. The most striking feature of this layer is the existence within it of large ironstone concretions, known as ironstone doggers. These vary in size but are usually at least 1 metre across with some being 2 metres. The Ironstones lie in almost parallel lines, there being between 3 and 5 layers along the cliff edge. These Ironstone doggers have been the main defence against erosion for the last several thousand years. The normal slow erosion that had taken place had caused the Ironstones to form a natural reef off shore. The boulders had also lined the beach protecting the base of the promontory. Their removal in the 1690's and the 1850's has been the main cause of the terrible erosion problems found at Hengistbury Head since then.

The Ironstone boulders were formed in a stagnant bog environment. Iron compounds were washed down by the river and formed a layer on the bed of an estuary. As the sea level fell the layer of iron compounds congregated into clumps and chemical reaction with a reducing agent (possibly Carbon from decayed vegetation) left the iron nodules we have today. The layering took place on many occasions and some of the doggers have layers like an onion. Within these layers have been found fossilised sharks teeth, vertebrae and unidentified vegetable matter. These concretions or Doggers were probably shifted from their point of creation to their locations with the Upper Hengistbury beds by flooding and storm. Below the bottom layer of doggers the Hengistbury Beds show signs of fossilised worm and crustacean burrows similar to those found in the Boscombe Sands.

The Upper Hengistbury Beds also appear east of Hengistbury head in the lower stratum of the cliffs at Highcliffe. However there appear to be no Doggers in this area. A similar structure can be found at St Catherine's point, about 3 kilometres inland fro Hengistbury Head. Here the stratum is only a metre or two from the top of the promontory. I understand that some ironstones have been found embedded in it.

The Strata at the easterly end of the Head. Note that the
Boscombe Sands have disappeared under the beach and the 3 layers of
Ironstone Doggers embedded in the Upper Hengistbury Beds

The Highcliffe Beds mark the top layer of the Eocene stratum on Hengistbury head. Like the Boscombe Sands this layer appears to have been laid down on a sea beach and has a similar structure to that of the Boscombe Sands. Small areas of clay can be found in this layer. These are from the river erosion of clay patches that were previously the base of small lagoons or ponds. The clumps of clay were washed and rolled down by a river and ended up being washed up onto the beach by wave action. The clay lumps then became incorporated into the general make up of the stratum. This layer also shows the typical sea beach feature of having bands of pebbles, each band probably indicating a large storm or short sudden incident. The Highcliffe Beds get their name for their more prominent location in the cliffs at Highcliffe, where they form the mid band between the Hengistbury beds and the Barton Sands.

The Bright cream coloured stratum at the top in this
and the last picture are the Highcliffe Beds. Notice the Doggers on the beach,
before 1850 the whole beach would have been similarly paved
with them

Due to the proximity of the nearby river systems a great deal of river deposits exist on and around Hengistbury Head. These river deposits consist mainly of river gravel's and are some of the newest of the deposits at the head.

During the last million years a succession of ice ages have been interspersed by warmer sub Mediterranean style weather. During the cold periods large quantities of water became locked up in ice and consequently the sea level dropped. Due to the lowering sea level, rivers flowing into the sea had to cut down into their beds to reach the new lower level, this left their former river gravel's suspended above the new river bed in a terrace. The new river course flooded during the warmer periods when the ice melted and raised the sea level. The process repeated when the next ice age started. Depending on the duration and the severity of the cold periods the grading (cutting down) of the valley floor varied. The net result is a series of river terraces in a river valley. Three such terraces exist on Hengistbury head and there is also the newer river gravel's from the former River Stour channel and the existing river system.

Just West of the Double Dykes is the former bed of the River Stour. After the River Avon and Stour eroded their dividing ridge and joined in a confluence about 1 kilometre north west of the hengistbury Head the old Stour bed, which used to route the river to the south of Hengistbury head was initially abandoned and then adopted by the river Bourne. Today the river Bourne flows through the middle of Bournemouth and enters the sea is a south-easterly direction. During the last ice age, when the area now occupied by Poole bay was dry tundra, the river Bourne probably continued further south and then hooked round and flowed into the confluence of the rivers Avon and Stour from a north easterly direction.

The abandoned river bed west of the Double Dykes is probably the product of two rivers, each flowing in the opposite direction to the other. This river bed shows the typical formations expected from a river bed. The bed consists of sub-angular flints mixed in with coarse sand. The texture is similar to that of builders concrete aggregate before mixing in the cement. The whole mass is a sandy orange in colour. These river deposits, mainly from the Stour and Bourne date back about 10,000 years or less

The gravel rich bed of the old Stour/Bourne river system
where it ends on the beach west of the Double Dykes

The three river terraces on Hengistbury Head have been named after similar structures found near London although there is no direct correlation.

The oldest river deposits on Hengistbury Head are on the top of Warren Hill. These gravel's are known as the Boyne Hill Terrace gravel's. This layer of gravel's is probably of the order of 200,000 years old

The set of gravel's known as the Lower Taplow terrace converges with the Boyne Terrace at Warren Hill but mainly occupies the eastern side of Warren Hill.

The newest set of gravel's that actually sit on the head are the second lower Taplow terrace. These sit about half way down the head on the northern escarpment. These gravel's have been disturbed and covered by the spoil from the old open cast mine from the Hengistbury Mining Company. This area is now known as The Batters.

The majority of the topsoil on both the head itself and on the land surrounding it is made up from windblown sand and soil and has a depth of about 1 metre although areas, such as the dip in front of and behind the most eastern of the double dykes the depth is up to three metres.

The other major new deposits exist at the lowest levels and are the recent deposits from the river Stour and Avon the areas with these deposits are surround Barn Bight, the Saltings and the rest of the northern coastline of Hengistbury head.

While Hengistbury Head has suffered massive erosion in recent times, there has also been deposition of materials on both the south easterly beach and the northerly harbour shore line. The deposits on the northerly shore line of Hengistbury Head are traditional river deposits leading to the building of salt marches and reed beds. These are most prominent to the west of the Double Dykes where they form an area known as the Wickhams. Further East, in front of the quarry there has also been a similar building of new land. These areas are called the Salt Hurns and Rushy Piece. An interesting (though unrelated) item in this area is a deeper area of the harbour just off the Salt Hurns called Lob's Hole. Lob's Hole is caused by a fresh water spring that breaks out into the harbour bottom and is only visible as a spring at very low tides.

Barnbight seen from the Batters. The majority of the
land surrounding this inlet has been down by the
Avon/Stour river system in the last thousand years or so

The building of materials off the south-easterly aspect of the Hengistbury Head has been deliberately caused by the building of a set of groynes. The purpose of these groynes is to reverse the damage done by the removal of the Ironstones in the mid nineteenth century. The main build up of material has been to the West of the major breakwater built at the easterly extremity of the head in 1938.

The breakwater has caused the rebuilding of the beach at
the easterly end of Hengistbury Head

Here there has been a major deposit of sand and shingle from long shore drift and the beach now probably extends out to close to its position in the late 19th century. The beach close in to the head has turned into a sand dune structure with grasses growing and further stabilising the immediate area.

The growing sand dunes in the new beach area at the eastern end
of the Head testify to the success of the new beach defences

If you like natural environments in other parts of the world, have a look at the following sites


Backpacker Days: A celebration of backpacking in Australia and NZ

Isla Contoy Nature Reserve near Cancun in Mexico

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