Chalk

UKAFH weekender at Folkestone and Samphire Hoe, Kent 14th and 15th October 2017

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The Folkestone foreshore

On Saturday 14th October UKAFH took a group out to the Warren, Folkestone for day one of the Kent weekender.  We were blessed with unseasonably warm weather as our group of 30 descended the (pleasantly dry) mud footpath down to the beach to begin our hunt through the Cretaceous period! Once on the beach, UKAFH leader Chris Tait briefed the group on the geology of the location and what we might expect to find.

The rocks at Folkestone represent the Albian stage of the lower Cretaceous, 110-105 mya.  Lower Greensand is found at the base of the cliff with Lower and Upper Gault clay resting conformably atop, however the clay slumps over the Lower Greensand and is eroded at sea level to release large volumes of fossils onto the beach in this highly productive locality for fossil hunting.  During the time these sediments were laid down the UK was at a more southerly latitude in the area of the modern day Mediterranean and a warm sea teeming with life covered the UK.  During this time sea levels were transgressing, with the Lower Greensand being deposited as and continued to erode, to be replaced with fine clay sediments once nearby land was completely submerged.

The Lower Greensand is less fossiliferous as the near-shore environment it represents was less suitable as a habitat but still contains excellent fossils such as ammonites; the Gault Clay, however, is packed with diverse fossils, some with exceptional preservation. Ammonites, belemnites and molluscs are common; nautilus, crabs, crinoids, fish remains, shark teeth and scaphopods can be found, along with rare finds of reptilia. Examples of all of these were found by members of our hunt group!

Phosphatised preservation is typical but quality is variable, with examples often fragmented or in nodules.  However many examples are preserved in superb detail in pyrite and those which are newly emerged from the clay can retain some or all of their nacrous shell.  Bivalves and molluscs which are newly exposed are often extremely fragile and are rarely collectable unless carefully removed along with the surrounding clay, but ammonites are more durable and make marvellous specimens to add to a collection.

Soon after we reached the beach heading towards Copt Point the finds were already plentiful.  Partial regular and heteromorph (partially uncoiled) ammonites and bivalves were quite common and finds increased as people “got their eye in”. Some of the group progressed quite quickly along the beach to inspect the slips of clay for freshly washed out fossils and check out the shingle between the large rocks and boulders on the foreshore. Others remained nearer the start of the beach, working methodically through the shingle by hand, with a trowel, or dry sieving, in search of smaller finds like shark teeth.

Several members of the group found shark teeth, with Isabelle finding the largest example.  At the other end of the scale, Sam found a small but scarce Acrodus shark tooth while sieving using a 3mm mesh. Sieving is a good technique to remove sand and search for small fossils which wash out higher up the beach because they are lighter. Sieving and shingle-searching up the beach yielded crabs, solitary corals, urchin spines, shark teeth and vertebrae as well as fish teeth, a turtle bone and the day’s star find, a swordfish tooth!

By the end of the day we had a really great selection of finds amongst the group!

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Leader Selfie

The following day we were greeted with yet another gloriously sunny autumn day, enhanced by the towering white cliffs of Dover above our meeting point as Samphire Hoe Country Park.  We had another full house of attendees and headed west along the beach to hunt for fossils amongst the chalk boulders on the foreshore.  Aidan Philpott, UKAFH Leader, explained the geology and identified local fossils to look for to the group. The lower chalk (also known as the grey chalk) at Samphire Hoe is from the Cenomanian stage of the Upper Cretaceous so yields fossils aged 100.5 – 93.9 Mya.

Common finds are brachiopods, bivalves and echinoids. Sponges, shark teeth, worm tubes, gastropods and fish can also be found and, rarely, ammonites. As well as beautifully preserved fossils within the chalk, some echinoids and shells and many sponges form flint casts which can be washed out of the chalk and found in the shingle.  Attractive pyrite crystals can also be found in the chalk.

Our first find of the lay was an eroded echinoid inside a flint block.  UKAFH leader Chris Tait then found a large section of clam.  These giant molluscs are mostly found broken into small pieces so this was a really nice find.  As the day progressed a good variety of finds were made including echinoids, shark teeth and brachipods.

Samphire Hoe isn’t the easiest location for fossil hunting as you need to scrutinise the loose chalk boulders on the foreshore carefully for signs of fossils and then extract them very carefully with a chisel to ensure they are not damaged.  You can also hammer the boulders to break the chalk up in search of fossils so work, tools and care are needed to have a good chance of finding fossils here.  Once extracted, however, preservation is usually excellent, with fine details clearly visible.

Cleaning chalk fossils is easy, requiring little more than dry brushing the remaining chalk matrix with a soft brush (a children’s toothbrush is ideal) to remove any chalk remaining on the fossil surface.  As chalk is very soft, you can easily remove larger amounts of remaining chalk with a blunt knife or craft knife until you approach the surface of the fossil and switch to brushing.

Finds were pleasing but not abundant, however everyone enjoyed the hunt and the sunshine day.

Chris, Aidan and Sam, your UKAFH leaders, would like to thank all of our members and attendees for joining us on our weekend fossil extravaganza and we look forward to seeing you all soon!

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FOSSIL HUNTING EVENTS

 

 

Fossil Hunt at Seaford 1st October 2017

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On Sunday 1st October, UKAFH ventured along the coastline of Seaford in East Sussex – a small town about 10 miles east of Brighton with towering white cliffs.

After a very fortunate summer meteorologically speaking, we had spent the week with a close eye on the remanence of hurricane Marie which crossed the Atlantic and now threatened our fossil hunt.

Fortune persisted however, as although a bit windy and the sea choppy, we began the day dry and mild.

We began with an in depth talk about the geology by UKAFH leader Daniel Slidel. Exposed in the towering white cliffs of Seaford is the Upper Chalk, a Cretaceous deposit (Santonian-Campanian) about 86-83 million years old. Formed from the tiny platelets of coccolithophores – phytoplankton that was abundant in the deep, warm sea that existed here. This striking sediment helped preserve the creatures dwelling on the sea floor, which included bivalves, sponges, corals, bryozoan and the echinoids (sea urchins) this stretch of coastline is famous for. Within the cliffs are horizontal bands of flints which are visible as far as the horizon allows.

After a short walk from the car park we descended some concrete steps onto the beach. The abundance of fossil echinoids was immediately noted as within the exposed bedrock on the foreshore were the tell-tale circular marks of weathered echinoids in situ. We could not extract these as the bedrock here is protected, however this gave us ambition as we traveled west towards loose boulders from which we could extract specimens. It was not long before beautiful echinoids were being found loose or extracted from boulders. There were two genre found, Echinocorys and Micraster. These were found in chalk boulders preserved with delicately thin calcite test – however the flints on the foreshore should not be overlooked either as more robust and often sea rolled specimens for found here too. Other finds on the day included small bivalves, shapely sponges, coral and bryozoan.

As the afternoon drew late the atmosphere became heavy with the approaching storm and the first rain fell as we ascended the concrete steps back to the car park. Looks like we did it again and avoided the worst of the weather!

Thank you to everyone who attended this fossil hunt. It really was a great and friendly group of people, it was a pleasure to guide you through the Cretaceous geological history of Seaford.

Further Reading.

Hampton, M.J., H.W. Bailey, L.T. Gallagher, R.N. Mortimore and C.J. Wood 2007. The biostratigraphy of Seaford Head, Sussex, southern England; an international reference section for the basal boundaries for the Santonian and Campanian Stages in chalk facies. Cretaceous Research, v. 28, no. 1, p. 46-60.

Fossil Hunt at Ringstead Bay, Dorset Sunday 9th July 2017

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Ringstead Bay

Ringstead Bay remains an almost forgotten gem along the Dorset Coast. Despite the obvious beauty of the location, the site has never been commercialised. Private ownership of much of the land has prevented companies, such as Pontins or Butlins, from exploiting the area in the past and Ringstead Bay remains much unaltered. The winding lane leading down to the bay is certainly not conducive to heavy traffic. Nowadays, the area is under the care and ownership of the National Trust and despite a fairly busy scene around the beach café and car park on Sunday, the rest of the large bay was characteristically mostly deserted, even on a blisteringly hot summer day, as we found out! A few hundred yards away from the slipway and the peace and tranquillity of Ringstead Bay becomes apparent. A brief talk, by Steve, set a backdrop to the location; the geology, the fossils and the Jurassic coast.

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The group gathers for an introductory session.
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The hunt begins
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Some of the group, with the Isle of Portland in the distance.

To the west of the bay, the harder rocks of the Corallian sequence form the headland of Bran Point, composed mostly of the Osmington Oolite Series and packed full of the large cockle Myophorella clavellata, which are also evident in the reefs and rock pools at low tide. To the east, the majestic Chalk cliffs of White Nothe form a prominent cliff. In between, the tall, slumped and badly weathered cliffs within the bay are entirely composed of Kimmeridge Clay, which is where our hunt began.

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Ringstead Bay, viewed towards the west
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View across the bay, looking west.

The Kimmeridge Clay here is 151 to 156 million years old and is composed of much degraded mudstone, formed on the Jurassic sea bottom when England once bathed in a sub-tropical climate. In fact, it was probably much the same as the day of our hunt, where the sun shone fiercely and the sky and sea were blue and mostly cloudless.

After an introductory chat, explaining the geology of this part of the Jurassic Coast, the party descended to beach level and the hunt was on. Ringstead Bay suffers much the same as other locations at the height of summer; poor erosion from tides and rain and from tourists. The tourists love fossil hunting in Dorset and consequently the more usual specimens found at Ringstead Bay were in short supply. Local collectors wait for the autumn and winter storms to help stir up the coast replenish supplies of fossils. However, the recent lack of good storms, even over the winter, has also meant fewer cliff falls and fresh material being exposed. The Kimmeridge Clay here was very dry and finding fossils within the hard clay body was not easy. However, the first fossil – a piece of the oyster, Deltoideum delta- was found by 6-year old Sophie de Candole. This was followed by several large complete oysters of the same being picked up by most members of the group.

Pieces of hard shale, which could be easily split by hand, exposed several ammonites and various shells. The largest ammonite remained in an immovable boulder on the beach but Aaron Roberts, aged 11, found a nice smaller specimen. The Sandsfoot Formation, which marks the junction between the older Corallian rocks with the overlying, younger Kimmeridge Clay was exposed in the cliff  and formerly known as the Ringstead Coral Bed. Broken ‘chunks’ on the beach revealed little in the way of fossils, although 12-year old Cem Izzet found a good-sized bivalve, named Ctenostreon proboscideum. whose large strongly-ribbed shell is unmistakable and easily recognised.

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Halfway along the bay, high in the cliffs, are the visible strata of the Portland Limestone Formation and capped by the basal part of the Cretaceous Purbeck Formation. The folded and faulted rocks, form a spectacular unconformity, with the Chalk dipping to the right and the Portland and Purbeck rocks dipping towards the right.

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The day was certainly a success. The majority of the group found various fossils and were very pleased with the outcomes. Coupled with the glorious weather, it was a wonderful day out and encouraged those who had never been to the location before, to return. Many thanks to everyone who came along and UKAFH look forward to seeing you on another hunt.

Steve Snowball