The Jurassic Coast (or is it?) with the Geologists’ Affiliation
Since 2012, the Geologists’ Affiliation (GA) has placed on annual subject journeys to the Dorset coast led by Prof John CW Cope (of the Nationwide Museum Wales), who’s creator of the definitive Subject Information No 22. The second version was revealed in April 2016 and is reviewed on the web page reverse. The truth is, the journeys had been began to have a good time the publication of the primary version of the information.
The Dorset Coast is commonly equated with the ‘Jurassic Coast’ when, the truth is, the geology stretches from the topmost Triassic, close to the Devon border, by Jurassic and Cretaceous successions, to Eocene deposits at Studland. For this and different causes, it attracts novice geologists in giant numbers. John’s information offers important data together with descriptions of the succession and sensible steerage about entry. What’s lacking are the entertaining tales that John Cope can present and the context supplied by exploring inland a bit.
Day 1 – Saturday (1 October)
For our fifth subject assembly, we met up in Lyme Regis (within the automotive park subsequent to the newly-restored home initially owned by John Fowles – see under) – a city to stir the center of any geologist. Our mission for the weekend was to take a look at the unconformity under the Cretaceous, because it oversteps the older Jurassic and Triassic strata progressively in a westerly course. En route, we noticed the instability of the cliffs and suffered the identical ourselves, as we scrambled over the boulders and shingle.
On this event, we didn’t go to the Charmouth Heritage Coast Centre or seek for pyritised ammonites on the seashore. As an alternative, we walked west from the Cobb, passing by Chippel Bay to Pinhay Bay. The seashore lies under the well-known landslides – the Undercliff – that function within the John Fowles’ guide, The French Lieutenant’s Girl, and the movie of the identical title. It’s now a nature reserve. Underfoot had been the equally well-known ammonite pavements within the Jurassic Blue Lias formation, slowly revealed by the falling tide. The sorting of the ammonites by dimension suggests deposition over time onto a tough floor, slightly than in a catastrophic occasion, though that didn’t clarify the superposition of smaller examples on bigger. The ammonites fall into two dimension groupings of mature types, suggesting sexual dimorphism.
A little bit of engineering geology confirmed us the impact of quarrying on the Lias and the useless makes an attempt to stabilise the cliffs. (Keep protected when you go to – the perfect fossils are on the seashore.) The arduous bands have been proven to be major slightly than secondary (diagenetic) options by the presence of hint fossils, and are interpreted as the implications of local weather variation pressured by the Milankovitch cycles. (Quiz query time: what are the cycles? Reply: orbital eccentricity, axial tilt and precession.) Palaeoclimatic proof gathered over final three many years has demonstrated the consequences of those processes on our local weather at this time and geoarchaeology has proven the impression on human societies.
The Blue Lias is a very widespread facies. The truth is, GA members explored its outcrop in Southern Germany journey in 2016. Nothing at this time comes shut. It has a famously different fauna, however typically hidden by diagenetic modifications. For instance, the shells of bivalves shaped of aragonite are solely preserved if silicified, as in some sections in South Wales. In England, now you can see the variety displayed at The Etches Assortment – Museum of Jurassic Marine Life lately opened within the village of Kimmeridge because of Lottery funding.
Finally, we got here to the junction between the Blue Lias and the Langport Member (beforehand referred to as the White Lias) in Pinhay Bay. The junction was wavy, however the color change from white to blue distinct. This doubtless represents a big ‘disconformity’ or hole within the sequence with none change within the angle of the bedding planes. The floor of the junction has the attributes of a tough floor. Though the earliest Jurassic ammonites, Psiloceras erugatum and Neophyllites, aren’t identified from Dorset, the Basal beds of the Blue Lias (the Preplanorbis Beds) might be their equal, so the Langport Beds/Blue Lias boundary in Dorset will not be far faraway from the internationally recognised base of the Jurassic.
After lunch (throughout which the one bathe of the weekend fell), we ventured into Devon at Seaton Gap close to Beer, avoiding the latest landslips alongside the way in which. Right here, the yellow-green and white Cretaceous straight overlays the acquainted purple Devon cliffs of the Triassic. No Jurassic or Gault was to be seen (they had been coated by shingle), slightly, huge cherty Greensand and Chalk beds made up the higher a part of the cliffs. The bay was divided by a fault with a 200m throw, the japanese limb of a delicate syncline that brings the Chalk all the way down to the seashore. The Chalk contained many biogenic nodules, shaped as hint fossils certain by natural materials after which repeatedly reworked to create a pudding-like texture.
In some way, the wave motion had produced practically spherical pebbles, which had been treacherous to stroll on. A number of the chalk boulders had additionally acquired “Beatles” wigs product of that inexperienced seaweed universally recognised as a slip hazard. As we struggled again to our automobiles and the comforts of Lyme, we had been rewarded with a spectacular rainbow over Axmouth.
Day 2 – Sunday (2 October)
The next day, we made our strategy to Haven Cliff to the east of Axmouth to take a look at the desert rocks of the Triassic succession, from the purple Mercia Mudstone by the inexperienced Blue Anchor Formation, to the gray and white of the Penarth Group. The considered sabkha salt flats and fields of desert sand dunes in Devon was entertaining. The eventual advance of the ocean produced the Langport Member of the Penarth Group, beforehand referred to as the White Lias, which we had seen at Pinhay Bay.
The start of the stroll was nice, as we dodged Sunday sailors and the odd paddle boarder. The pebbles on the seashore this time had been arduous work and the cliffs multi-coloured as marketed.
Within the afternoon, we made our strategy to the seashore at Branscombe, a favorite place regardless of the costly automotive park. The view alongside the seashore to the east is dominated by the towers and turrets attributable to the large 1790 landslip. The underlying Triassic rocks are stuffed with gypsum and (inland) halite, each of that are very cell. The Mercia Mudstone types an impermeable layer that traps water flowing downwards by the overlying Cretaceous rocks, producing a really unstable base. Prof Cope counselled preserving effectively away from the foot of the cliff, though the homeowners of the various very costly seashore huts (we’re speaking £250,000) don’t appear to care that there’s a sturdy probability that they may find yourself in items on the backside of the cliff. I spent many completely happy days there in my youth, so I have to be fortunate.
The landslip could be very spectacular. The 200m-high cliff has rotated and thrust roughly the identical distance out to sea, leaving a nature reserve of tough floor between it and the undisturbed cliff face. We got here throughout apiarists learning the native bees burrowing into the comfortable sediment, and plenty of different species have discovered niches there.
Beer Head reveals off the chalk succession effectively, with its newly recognised divisions, notably the Holywell Nodular Chalk. Chalk geology has developed enormously over the previous couple of years, particularly, because of the work of ex-GA president, Prof Rory Mortimore (who additionally runs subject journeys for the affiliation). The identification of sponge reefs alive at this time on the western Canadian continental shelf offers a clue to the formation of the in depth horizons of flint.
We braved the undercliff on the way in which again, ducking between the Mercedes and BMWs – Devon isn’t prefer it was. We had been beneath statement by a paraglider driving the thermals till we bid a fond farewell within the automotive park.
We left effectively happy after one other terrific journey with the redoubtable Professor Cope in glorious climate. The subsequent journey, hopefully in September 2017, will enterprise into the Devonian and Triassic round Torquay to have a good time the immanent publication of his information to the geology of Devon.
All the pictures on this article had been taken by Harry Jones. In case you are excited about studying extra concerning the GA together with the sector journeys it arranges, or need to be a part of the affiliation, see: http://www.geologistsassociation.org.uk/.
Concerning the creator
Mervyn Jones is a long-time member of the GA. He studied geology at St Catharine’s Faculty at Cambridge College and is at the moment learning for an MSc in geoarchaeology at Studying College. Like William Smith, the daddy of stritigraphy, he’s each a surveyor and geologist!
Buy Fossils, Crystals, Tools
Subscribe to Deposits
Join Fossil Hunts
UK Fossil Locations