A FIELD TRIP TO THE NORTH COAST "Evidence of Sea-Level Change preserved in the Pleistocene and Devonian sediments of " Led by Professor John Mather, Report by Tom Miller & David Maddocks 16-18 April 2004

A group of OUGS members and friends visited the north coast of Devon. We stayed in The Thatch and Billy Budd's hotels in the village of . The accommodation was friendly and comfortable. The food and wine were good and the weather forecast looked promising. A characteristically great weekend was to follow. It combined splendid locations with good technical geology.

The coast of North Devon and West Somerset from to Porlock is especially interesting in that it preserves a succession of raised Pleistocene beach deposits overlying a marine platform of Devonian/Carboniferous rocks ( Sands to Baggy Point) and some important coastal geomorphology (the Valley of the Rocks, shingle ridge, etc.).

Saturday Location 1 -

After breakfast in The Thatch (with no eggs!) and a brief introductory talk from John, we set off for Saunton Sands. The weather was excellent. The plan was to park below the Saunton Sands Hotel and walk the beach section from there westward to Down End, below Saunton Down. We set out across the wave -cut platform and reached our first point where the raised beach deposits could be seen resting on the platform.

The Group study the Raised Beach Deposit (Pleistocene) resting on the Wave-Cut Platform (Upper Devonian?)

Here, the beds of the wave-cut platform are dipping steeply and easily distinguishable from the sub-horizontally bedded younger raised beach deposits above. The wave-cut platform rocks are the Pilton Shale s, which are a transitional group joining the Upper Devonian and Lower Carboniferous systems in the region.

The Pilton Shales are hard and quite strongly folded. The Lower Cliff beach deposits (known as Sand Rock) are quite friable by comparison and contain small pebbles and shingle beds in a calcareous sandy matrix. The Sand Rock is controversial in age but is thought to date from the Ipswichian interglacial (125 ka), when a warmer spell, combined with a general eustatic high, raised the sea level above its present height.

John points out Shingle Beds and other Sedimentary Structures in the Sand Rock

The Sand Rock of the lower cliff has very even, laminar bedding, but is quite variable in thickness. There are occasional traces of plant rootlets on its upper surface, suggesting emergence and development of a land flora. The Sand Rock is superposed by a ‘Middle Cliff’ unit that contains much larger bedforms. These are interpreted (cautiously) as dune bedding (aeolian), ramped up against ancient cliffs (Pilton Shales) behind.

Dune Bedding in the Middle Cliff

Further along the beach we came across several points of interest including bands of crinoid debris in the Pilton Shales, indicating their environment of deposition as shallow marine. I was quite impressed with the discovery of ancient clusters of barnacles on blocks of the Pilton Shales. These are coming to light one by one as the Sand Rock is eroding away.

One of the most interesting features of this coastline is the line of erratic blocks and boulders that have come to rest on the marine platform. These rest on the Pilton Shales but are sealed in by the Sand Rock deposits. An estimate for the age of placement of these erratics is 450 ka BP, towards the end of the Anglian glaciation. Could they be drop- stones, or lag from eroded till? We discussed this at length but reached no firm conclusion.

A Porphyritic Microgranite erratic boulder sealed in by the Sand Rock, on the Pilton Shales

Sketch of the Saunton Sands / Saunton Down beach section

A fold in the Pilton Shales became visible in the section below Saunton Down. We looked at tension gashes and the axial plane cleavage with interest. This must have been Variscan front deformation. After this, we returned to the vehicles and drove to the National Trust car park on the north side of Croyde Bay.

Saturday Location 2 - Baggy Point

In the National Trust car park we stopped for lunch and considered the afternoon's activity. A walk up to Baggy Point, looking at further Pleistocene shoreline deposits and the Upper Devonian Baggy Sandstones.

We had, at times during the morning, glanced up at the Head (solifluction) deposits above the Middle Cliff but we did not look at them closely. After lunch, John led the group with some enthusiasm up the path towards Baggy Point to see the Head deposits more closely.

John Mather focuses attention on the glacial Head deposits. Baggy Point in the Background

Baggy point has a covering of glacial Head overlying the Baggy Sandstone beds beneath. The Head is a solifluction deposit and is thicker (banked up) on the flanks than on the top. It contains angular blocks of the Baggy Sandstone, orientated down-slope. Several periglacial ice distortion features can be seen in the thin Head deposits on the top of the headland, including ice wedges.

Up at Pencil Point we could see the Head deposits clearly banked up against the sandstone slope. Pencil Point gets its name from the curious way that the rock by the footpath crumbles into shards (like pencils). This is due to the nature of the jointing and the orientation of the pressure cleavage. John suggested that the curious interaction of bedding and cleavage here might give some clue as to the proximity of the axis of the fold we had looked at earlier in the Pilton Shales. One for the structural geologists in the group!

Head deposit, containing large angular blocks, banked up against the Baggy Sandstone

At Baggy Point there is a complex sequence of different sediments (sandstones and shales), recording many different environments (marine advances, retreats, lagoonal facies, freshwater deposits, etc.). We stood for a while here, enjoying the views to landward and seaward. We could see Lundy Island in the west, Pembroke on the horizon to the northwest and Morte Point to the north. North of Baggy Point, the raised beach and rock platform disappears, i.e. they are not present at and Morte Point. We made our way back to the vehicles and brought the day's excursion to a close.

Tom Miller

Sunday Location 1 - Valley of the Rocks

Forecast overnight rain fell, true to plan, and was still falling as we drove across but must have seen the OUGS coming as it stopped just as we arrived at the Exmoor National Park Picnic Area in the Valley of the Rocks.

The Valley of the Rocks is a dry valley extending southwards from Lynton for about 2 km or so, parallel with the coast, before ‘plunging over the cliff’ into the Bristol Channel at Wringcliffe Bay. At its northern end the East Lyn and West Lyn rivers meet, just before passing through into the sea. Each of these rivers has a steep channel in this vicinity and much erosion debris forms the Lynmouth delta that we see today. Although the Valley of the Rocks is 145 m above the Lyn rivers below, it seems logical to assume that they once flowed through it.

There are two main theories as to how the present situation came about. One suggests that the slowly eroded back towards a bend in the river which was then able to discharge over a cliff waterfall into the sea and subsequently rapid erosion took place. The other theory is a glacial one. An ice dam across the mouth of the Lyn river would cause the water to back up and find a new course, through what is now the dry Valley of the Rocks. Certainly the ice front in glacial times is known to have been in this vicinity.

The Valley of the Rocks

The valley is eroded into the Lynton Beds, Devonian shallow-sea sediments formed mainly of sandstones, but with occasional thin beds of clays, silts and limestone. The latter contain brachiopod and crinoid fossil debris. In places there is slickenside evidence of beds moving over one another. Some of the valley is partly filled with Head, local erosion debris.

The seaward side of the valley is topped by a number of coastal tors, which at the same time are high points on the coastal cliff. Each is an outcrop of upstanding solid rock with conspicuous joints. It was whilst on the top of Castle Rock, one of these tors, admiring the rocks, plants, goats and birds, that we saw the big yellow bird, the RAF Rescue helicopter, coming straight at us along the coast from the west. Just before Castle Rock it swerved inland up the dry valley, then turned on its side to go around the rock and through the tor gap back out over the sea. All this just feet off the ground and close enough to see the pimples on the pilot’s nose - and I thought it was the goats that kept the grass so short......

RAF Rescue helicopter

Sunday Location 2 - Porlock Weir

A delicious hot pasty lunch was taken en-route to Porlock Weir, our last location for the weekend. Here a substantial shingle ridge shields the salt marsh and habitation behind. The ridge has been starved of new pebbles for a couple of hundred years now so the sea occasionally breaks through. In the past these breaks have been repaired but the last break has been left to see how nature deals with the changing condit ions. Just seaward of this ridge, and only visible on the lowest tides, is a submarine forest, the remains of oak and alder some 7,800 to 5,000 years old – evidence of sea level rise since the end of glaciation.

A not-so-easy walk along the shingle ridge towards Hurlstone Point allowed us to see the Hangman sandstone and much more of that unconsolidated gunge, the Head. Not surprisingly, there are regular landslips along this section of the coast. It had been cloudy but dry all day, reasonable field trip weather, and fortune had been with us. When I got home to Newbury it was raining and the rain gauge contained 31 mm that had fallen that day......

David Maddocks

WH Branch is exceedingly grateful to John for leading the trip, and to Pauline Kirtley for organizing what was an exceedingly pleasant weekend for all concerned.