Led by Ramues Gallois

On the day, as predicted, it was raining heavily and Ramues proposed a change to the schedule. Since this first meant a walk under the shelter of the trees along the cliff top, then along the beach, putting the wind and rain in our backs no one objected. After a climb up Weston Cliff the group gathered for an introduction to some of the general aspects of the area.

To the east the view was of Branscombe mouth and on to Beer Head (see the field trip report of November 28th, 2004 for a description of this area). To the west was the stretch of coast between Branscombe Mouth and Littlecombe Shoot that we were about to explore. This section of coast lies within the Dorset and East Devon Coast World Heritage Site. The stratigraphy of this section ranges from exposures of the Late Triassic Branscombe Mudstone with red rock gypsum veins to the Upper Cretaceous Seaford Chalk, which dies out laterally beyond Littlecombe Shoot. Consequently it was expected that we would find the high ground, composed of clay-with-flints drift deposits, lying unconformably on the Upper Greensand in the west but conformably on the chalk where we were starting.

The area is heavily faulted but against normal expectation the faults trend north-south. The strong rock in this area has an east-west oriented grain which corresponds to the direction of main strength of the rocks of Southern England. The faults are badly exposed but Ramues pointed out the numerous roughly N-S trending valleys that have resulted from deep erosion along the fault planes. In his opinion the faults are more ancient than the base rock. The base rocks were folded by the Variscan Orogeny and further to the north show folds and associated fault systems with an east-west trend. Ramues suggested that the faults here on the coast are part of a pre-Variscan fault system resulting from the distant tectonic collision between the African and European plates. The forward stress field could have created acute conjugate strike-slip faults one branch running N-S just reaching the British Isles. Later this fault system was reactivated causing the faults in the lower strata to expand into the upper younger strata, including the clay-with-flints highground that lies unconformably on the folded base rock.

Information on the fault systems is scarce since the overlying marine Murcia Mudstones Group is very thick. Even the topmost Branscombe Mudstone bed is over 200 m thick. But from seismographic information it can be deduced that the faults must have a westerly down throw of approximately 60-70 m each. This succession of faults is now considered a coherent system of step faulting on the eastern flank of a basin opening to the west.

From our vantage point we had good views to the west [Photo 1] and east [Photo 2] and Ramues used these to describe the huge landslides in the area. The high ground consists of competent strata that overlie clay-rich, less competent strata that easily absorb water and loose their strength. In some areas a one centimeter thick layer of clay is sufficient to act like grease, allowing the higher strata to slide down when the Foxmould Greensand, becomes waterlogged, weakens and collapses. The Foxmould Greensand has such a high carbonate content that it easily dissolves to an extent that it almost liquefies when waterlogged.

Between heavier rainfalls Ramues managed to show us a copy of an old painting as proof that the two limestone pillars now at the foot of the cliff at Beer Head had slid down the cliff in an upright position. Internally they still show the original planar sequence of bedding and sedimentary structures. And with help of the geological map of the area, held up in a convenient dry spell by two members of the group, Ramues pointed out how well Branscombe valley shows the results of valley erosion and what features could be expected.

On reaching our next stop the group proved a little unwieldy as members struggled to carry their lunch bags over the soggy ground. The viewpoint was a little cramped but we were able to look at the top of a cliff outcrop just beyond. Here a thin strata of white Upper Chalk lies conformably on top of an even thinner bed of grey Cenomanian limestone (slightly more than 0.5 m thick) [Photo 3]. This highly condensed rock is very strong and would make good building stone, though it has never been used as such. It is highly bioturbated and these structures were later viewed close up in some large boulders on the beach below. Ramues also pointed out a thin hard band just below this limestone bed as an example of a hardened (hardground) surface. Here it marks the top of the Upper Greensand and indicates a long passage of time, since hardgrounds form in sediment starved conditions: thus a long time may pass with hardly any sedimentation occurring.

The Upper Greensand is termed a glauconitic sandstone, so called because it contains fine pellets of the green mineral glauconite, which may give the rock a greenish grey to bright green colour. However, we mostly see the dull rusty brownish tinge it shows after weathering [Photo 4].

In this area the Chalk laterally dies out and is rather thin here. Following regional uplift in the late Cretaceous most of the relatively freshly formed Chalk was removed by erosion and dissolution.

The next intended stop did not work out very well. The bushes having grown and become too dense since Ramues was here last. However, his detailed knowledge of the area allowed him to take us another spot to see the Clay-with-Flints. There were no bushes to hinder us here but we had to master a rather steep, slippery and muddy slope up the cliff. This slowed progress of the group to such extent as to make Ramues burst out in a show of agility, running up-and-down past the baffled group members who were left to grope for the support of nearby branches. On the cliff edge Ramues pointed out what to look for. The Clay-with-Flints is highly variable and consists of heterogeneous red-brownish clay with numerous clasts, flint and/or chert, remains of a complex sequence of processes one of which was the erosion of the Chalk. Where we stood there was still some chalk under the Clay-with-Flints but further west it is absent. The clasts and flints have retained much of their sharpness showing the main process of chalk removal was dissolution of the chalk and other strata rather than mechanical erosion.

Moving on we suddenly encountered a small plateau almost devoid of undergrowth. Ramues explained this was Berry Camp [Photo 2], supposedly the site of an Iron Age hill fort, half of which has already been eaten away by the sea. Theoretically this could have helped in calculating the erosion rate of the cliffs, were it not for the fact that this type of settlement is known to have been about for a period covering 4000 to 500 BP. Hence any calculation would have a rather high error margin.

Coming to a surprisingly undulating spot of ground Ramues told us these were overgrown spoil heaps from past quarrying activities. The number and size of these grass covered hillocks are beyond what is considered normal for such a relatively small quarry. The Chalk hereabouts was used as an agricultural commodity and so chalk was presumed to have been the reason for quarrying. However on finding that the spoil is Chalk an enigma arose: what was being quarried? Ramues told us that when mapping the area for the BGS he wondered if some vein of precious materials could have existed here and so he approach local officials for more information. With the little information available he came to the conclusion that most likely it was the Bindon Sandstone that was quarried here. This sandstone is the uppermost Member of the Upper Greensand Formation and is an excellent chert free building stone. Similar stone was used in Exeter Cathedral, however, this came from the Salcombe Regis quarry. It seems probable that the quarry here must have supplied stone for local buildings including churches. Disappointingly however, Ramues was unable to stimulate much local interest and had to accept that this hypothesis would remain unproven if local people were reluctant to delve into their own local history.

From the cliffs of Littlecombe Shoot we spotted some beach houses which seemed to be rather inaccessible. The houses are connected with ancient rights that in the past allowed crofters to exploit the steep slopes and cliff faces for shooting Peregrines and other birds. The crofters also used what the sea had to offer and so could live in almost complete isolation. These rights still exist but provide no access to facilities from the cliff top. Some of the houses are now used as holiday cottages but others have permanent residents who have to go by boat to Sidmouth for their shopping.

On the way down a narrow path to the beach Ramues told us to look out for the Cretaceous-Jurassic unconformity; just one footstep would be enough to send us back in time some 80 million years, but most were too busy trying to master the slippery rock and mud to take much notice. However, this junction was indirectly brought to their attention as it marks the point where the path suddenly drops down even more steeply. A much worn blue rope along the final part of the path helped us to get onto the beach, where we took a brief break for a somewhat belated lunch.

During the morning the rain had cleared making us realize that most of us had neglected our stomachs as we had strained through the wind and rain so as not to miss a word of what Ramues offered. A little west of our lunch spot Ramues lead us to some large concretions of tufa that had fallen to the beach [Photo 5]. The lower cliffs in this area are formed of the Upper Murcia Mudstone Group. This is thickly coated with tufa deposited by seepages from the base of the overlying calcite-rich Upper Greensand Formation. The tufa is interesting material, even today it forms large tubers from complex networks of calcium-carbonate tubes. Ramues thinks the tubes result from encrustation of plant roots or algal stems and are left when the original organic core material disappears [Photo 6]. A few loose pieces on the beach were collected.

Other members of the party found very soft red or greenish-blue pieces of the Foxmould Clay on the beach. When dry the rock is quite hard, but the wet pieces easily crumble between our fingers, proof of how weak this rock will become when waterlogged. Pieces of all higher strata could be found on the beach in large or smaller boulders brought down during winter storms. Many interesting rock pieces were examined, Ramues answering questions and explaining what to look for.

Some huge red boulders with white stripy spots drew our attention [Photo 7]. These were once part of the Red Rock Gypsum Member of the Branscombe Mudstone Formation, found a little above beach level. The gypsum was mined for agricultural purposes in the past and remains of the processing mill could be seen until quite recently at the hotel at Branscombe Mouth. The gypsum formed as an evaporite deposit in desert like conditions in shallow basins or coastal plane lakes. The water becomes super-saturated in calcium partly from animal remains and organic sulphates, these combine as calcium sulphate in the form of aragonite and/or gypsum (gypsum is the hydrated form of aragonite and either precipitates simultaneously or forms afterwards from it). When the sea periodically flooded the basins mud was mixed with some of the gypsum causing the typical appearance of alternating layers of purer gypsum and the muddy red rock gypsum.

The red rock also shows many green spots. Ramues explained that desert conditions are rich in oxygen because the soil does not contain much decomposing plant material. Thus the iron present becomes fully oxidised to the red ferric oxide. Later in a wetter climate plant material is abundant and rotting creates acidic organic fluids which seep through and over the rock. These fluids may cause the ferric oxide to be reduced to ferrous oxide locally turning from red to green [Photo 8] .

Slabs of the Cenomanian Limestone similar to those seen earlier in the cliff above had also fallen to the beach and now we could have a closer look using cameras and hand-lenses to study the enormous richness of minute and larger fossils [Photo 9] . The fossil density is the result of this rock being formed under sediment starved conditions hence the seafloor could collect a lot of decayed creatures before sediments covered them up.

A boulder of Greensand also provided a good example of the hardgrounds seen earlier. One good example hand lots of burrows [Photo 10] and shell debris [Photo 11] and another boulder showed the unconformity between the Chalk and the Greensand [Photo 12]. A further Greensand boulder contained marker beds of chert [Photo 13].

Ramues explained that chert and flint are chemically the same, but are structurally different rocks. Flint forms only when almost pure calcium carbonate rock, built from the skeletons of billions of tiny algae, slowly dissolves. Silica penetrates the resulting pores and precipitates forming the flint, which is usually black. Chert forms in less pure carbonate rocks and precipitates directly from seawater on the seafloor when the pH locally becomes more acidic. It normally forms concretions but may also form layers (bedded chert). During this process it traps organic or inorganic impurities giving it widely varying colours.

Other 'beach treasures' consisted of chalk or limestone with echinoids and needles from these sea urchins, but alas none could be taken home. Opinion was divided on the authenticity of a 'Pudding Stone'. Ramues thought it was likely to be genuine since most clasts in it were sharp edged, but some suspected a local concrete mixer to be responsible for this beautiful coloured breccia [Photo 14]. Ramues was also exited to find some kind of sponge, which he announced he would come back and save in an official capacity.

Going further along the beach the thick Red Rock gypsum vein was encountered at beach level [Photo 15] and higher up some thin marker beds [Photo 17]. As we were now close to Branscombe Mouth the group started to fall apart and headed off for tea [Photo 16]. At the car park Janet, our Branch leader, rounded us up and Ramues was thanked for his excellent explanations and other stories. The group's gratitude was expressed with a well earned, heart warming, liquid token of appreciation.

If you would like to see the original report by Eric click this link Branscombe.pdf and you can download this file. Be warned it is 2Mb!

Click here for extra photos.

Words: Eric Buter

Photos: Eric Buter

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