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 and is reviewed on the page opposite. 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.