Jurassic Coast (or is it?) with the Geologists’ Association
Mervyn Jones (UK)
Since 2012, the Geologists’ Association (GA) has put on annual field trips to the Dorset coast led by Prof John CW Cope (of the National Museum Wales), who is author of the definitive Field Guide No 22. The second edition was published in April 2016 (Geology of the Dorset Coast (2nd ed)). In fact, the trips were started to celebrate the publication of the first edition of the guide.
The Dorset Coast is often equated with the ‘Jurassic Coast’ when, in fact, the geology stretches from the topmost Triassic, near the Devon border, through Jurassic and Cretaceous successions, to Eocene deposits at Studland. For this and other reasons, it attracts amateur geologists in large numbers. John’s guide provides essential information including descriptions of the succession and practical guidance about access. What’s missing are the entertaining stories that John Cope can provide and the context provided by exploring inland a bit.
Day 1 – Saturday (1 October)
For our fifth field meeting, we met up in Lyme Regis (in the car park next to the newly-restored house originally owned by John Fowles – see below) – a town to stir the heart of any geologist. Our mission for the weekend was to look at the unconformity below the Cretaceous, as it oversteps the older Jurassic and Triassic strata progressively in a westerly direction. En route, we observed the instability of the cliffs and suffered the same ourselves, as we scrambled over the boulders and shingle.
On this occasion, we didn’t visit the Charmouth Heritage Coast Centre or search for pyritised ammonites on the beach. Instead, we walked west from the Cobb, passing through Chippel Bay to Pinhay Bay. The beach lies below the famous landslides – the Undercliff – that feature in the John Fowles’ book, The French Lieutenant’s Woman, and the film of the same name. It is now a nature reserve. Underfoot were the equally famous ammonite pavements in the Jurassic Blue Lias formation, slowly revealed by the falling tide. The sorting of the ammonites by size suggests deposition over time onto a hard ground, rather than in a catastrophic event, although that didn’t explain the superposition of smaller examples on larger. The ammonites fall into two size groupings of mature forms, suggesting sexual dimorphism.
A bit of engineering geology showed us the effect of quarrying on the Lias and the vain attempts to stabilise the cliffs. (Stay safe if you visit – the best fossils are on the beach.) The hard bands have been shown to be primary rather than secondary (diagenetic) features by the presence of trace fossils, and are interpreted as the consequences of climate variation forced by the Milankovitch cycles. (Quiz question time: what are the cycles? Answer: orbital eccentricity, axial tilt and precession.) Palaeoclimatic evidence gathered over last three decades has demonstrated the effects of these processes on our climate today and geoarchaeology has shown the impact on human societies.
The Blue Lias is an extraordinarily widespread facies. In fact, GA members explored its outcrop in Southern Germany trip in 2016. Nothing today comes close. It has a famously varied fauna, but often hidden by diagenetic changes. For example, the shells of bivalves formed of aragonite are only preserved if silicified, as in some sections in South Wales. In England, you can now see the diversity displayed at The Etches Collection – Museum of Jurassic Marine Life recently opened in the village of Kimmeridge as a result of Lottery funding.
Eventually, we came to the junction between the Blue Lias and the Langport Member (previously known as the White Lias) in Pinhay Bay. The junction was wavy, but the colour change from white to blue distinct. This likely represents a significant ‘disconformity’ or gap in the sequence without any change in the angle of the bedding planes. The surface of the junction has the attributes of a hard ground. Although the earliest Jurassic ammonites, Psiloceras erugatum and Neophyllites, are not known from Dorset, the Basal beds of the Blue Lias (the Preplanorbis Beds) may well be their equivalent, so the Langport Beds/Blue Lias boundary in Dorset is not far removed from the internationally recognised base of the Jurassic.
After lunch (during which the only shower of the weekend fell), we ventured into Devon at Seaton Hole near Beer, avoiding the recent landslips along the way. Here, the yellow-green and white Cretaceous directly overlays the familiar red Devon cliffs of the Triassic. No Jurassic or Gault was to be seen (they were covered by shingle), rather, massive cherty Greensand and Chalk beds made up the upper part of the cliffs. The bay was divided by a fault with a 200m throw, the eastern limb of a gentle syncline that brings the Chalk down to the beach. The Chalk contained many biogenic nodules, formed as trace fossils bound by organic material and then repeatedly reworked to create a pudding-like texture.
Somehow, the wave action had produced nearly spherical pebbles, which were treacherous to walk on. Some of the chalk boulders had also acquired “Beatles” wigs made of that green seaweed universally recognised as a slip hazard. As we struggled back to our cars and the comforts of Lyme, we were rewarded with a spectacular rainbow over Axmouth.
Day 2 – Sunday (2 October)
The following day, we made our way to Haven Cliff to the east of Axmouth to look at the desert rocks of the Triassic succession, from the red Mercia Mudstone through the green Blue Anchor Formation, to the grey and white of the Penarth Group. The thought of sabkha salt flats and fields of desert sand dunes in Devon was entertaining. The eventual advance of the sea produced the Langport Member of the Penarth Group, previously known as the White Lias, which we had seen at Pinhay Bay.
The beginning of the walk was pleasant, as we dodged Sunday sailors and the odd paddle boarder. The pebbles on the beach this time were hard work and the cliffs multi-coloured as advertised.
In the afternoon, we made our way to the beach at Branscombe, a favourite place despite the expensive car park. The view along the beach to the east is dominated by the towers and turrets caused by the huge 1790 landslip. The underlying Triassic rocks are full of gypsum and (inland) halite, both of which are very mobile. The Mercia Mudstone forms an impermeable layer that traps water flowing downwards through the overlying Cretaceous rocks, producing a very unstable base. Prof Cope counselled keeping well clear of the foot of the cliff, although the owners of the many very expensive beach huts (we are talking £250,000) don’t seem to care that there is a strong chance that they will end up in pieces at the bottom of the cliff. I spent many happy days there in my youth, so I must be lucky.
The landslip is very impressive. The 200m-high cliff has rotated and thrust roughly the same distance out to sea, leaving a nature reserve of rough ground between it and the undisturbed cliff face. We came across apiarists studying the local bees burrowing into the soft sediment, and many other species have found niches there.
Beer Head shows off the chalk succession well, with its newly recognised divisions, notably the Holywell Nodular Chalk. Chalk geology has developed enormously over the last few years, in particular, as a result of the work of ex-GA president, Prof Rory Mortimore (who also runs field trips for the association). The identification of sponge reefs alive today on the western Canadian continental shelf provides a clue to the formation of the extensive horizons of flint.
We braved the undercliff on the way back, ducking between the Mercedes and BMWs – Devon isn’t like it used to be. We were under observation by a paraglider riding the thermals until we bid a fond farewell in the car park.
We left well satisfied after another terrific trip with the redoubtable Professor Cope in excellent weather. The next trip, hopefully in September 2017, will venture into the Devonian and Triassic around Torquay to celebrate the immanent publication of his guide to the geology of Devon.
All the photographs in this article were taken by Harry Jones. If you are interested in learning more about the GA including the field trips it arranges, or want to join the association, see: http://www.geologistsassociation.org.uk/.
About the author
Mervyn Jones is a long-time member of the GA. He studied geology at St Catharine’s College at Cambridge University and is currently studying for an MSc in geoarchaeology at Reading University. Like William Smith, the father of stritigraphy, he is both a surveyor and geologist!