RGSC’s JURASSIC-COAST FIELD TRIP 2026
RGSC’s Spring 2026 field event took place on the Dorset coast over the weekend 18th-19th April, with 23 attendees. We visited Lyme Regis, Charmouth, West Bay, and Burton Bradstock.
The event Leader was Stephen Hesselbo, Professor of Geology and Deputy Head of Earth & Environmental Sciences at Camborne School of Mines, University of Exeter. Stephen is, inter alia, a recognised expert on the geology and stratigraphy of the Jurassic Coast, having researched and studied the area in great detail for many years, including authoring/co-authoring at least seven publications on the subject between 1992 and 2019. A copy of his information handout is available on our website.
Background Over the course of the weekend we visited spectacular and historically important exposures of Jurassic rocks and formations that accumulated in the region of massive subsidence known as the “Wessex Basin”. The object was to visit geological features such as fault-zones, rock outcrops and foreshore ‘fallen blocks, in order to understand the various processes that created the basin, and subsequent infill, over a period of some 250 million years, from its origin in the Permian, through the Jurassic and Cretaceous, to the Paleogene. The Wessex Basin started as a system of ‘extensional’ basins after the Late Paleozoic Variscan Orogeny, with a ‘structural grain’ of east-west faults, followed by continued development of ‘continental rift systems’. Subsequent sedimentary fill depositions resulted from river/estuarine, wind-driven, and lagoon/lake settings in a semi-arid climate, plus later marine deposits, which resulted in a wide variety of mudstone deposits of organic, carbonate, silt, and clay mineral content, also silty and sandy shoreface deposits. There is also evidence of basin inversion through Alpine Orogeny.
A further topic of interest, without visits, was to understand how the interaction of Jurassic marine shale deposits, with fault planes and ‘fault blocks’, resulted in ‘Lias hydrocarbon kitchens’ at depth and also oil and gas reservoirs in sandstone strata. These resulted in a number of oil seepage features in the Lulworth Cove area, whilst on the Isle of Purbeck is the well-known Wytch Farm oil field (see ‘Sunday pm’).
Field activities: Saturday a.m. – Lyme Regis – the event began with Stephen taking us through the detailed geology of the Wessex Basin, including hydrocarbon deposits. We then moved west across nearby Monmouth Beach which led to Seven Rok Point. Along the beach we frequently stopped to examine (a recurring theme) the lithology and origin of large ‘fallen blocks’ of stone, that had arrived at beach-level during progressive slip of the cliffs behind (fig 2), comprising formations of Black Ven Marl, Shales-With-Beef, and Blue Lias (‘-Beef’ due to appearance). There were also numerous cobble- to boulder-sized ammonites (see right), the novelty of which drew much attention. At the end of Monmouth Beach we stopped to look at an apparent limestone ‘plateau’ but which was composed of the classic Blue Lias-limestone-marl ‘rhythms’ which have yielded many of the most important fossil finds from this area. 
These interbeds have been shown to reflect orbitally forced climate changes (relating to changing tilt and orbit patterns of the earth) and provide the basis for a precise chronology (analogous to tree-rings but over a much longer timescale). Within the ‘sandwich’ was a thin layer of softish dark Lias shale which we were invited to rub between the fingers to recognise a faintly bituminous smell.
A planned lunchbreak at Seven Rock Point, and subsequent walk to Pinhay Bay, unfortunately had to be abandoned due to time/tide constraints. This section would have shown us the important transitions between the Triassic and the Jurassic formations which coincided with the break-up of the supercontinent Pangea, one of the ‘big five’ mass extinctions, and the emplacement/formation of the large Central Atlantic igneous province.
We would also have seen an unusual development of mass flow deposits as the basin founded into deeper water.
In the afternoon we moved on to Charmouth beach where the group engaged in a hunt for pyritized fossils, with mixed results – numerous broken lengths of ‘belemnite’, some pyrite ‘coagulations’, and a few ammonites. An interesting topic discussed here was the earlier presence at Monmouth Beach of numerous pieces of what looked like flint; Stephen explained it was chert, formed from Cretaceous sea sponges that had broken down leaving a residue of silica which had been subjected to pressure from later deposition.
Sunday a.m. we visited Eype’s Mouth where after some parking-payment games we examined a three-dimensional exposure of the Eype’s Mouth Fault, a substantial synsedimentary fault (see: https://www.youtube.com/watch?v=RuL4Ct-tX2c), one of a set that were active throughout the Jurassic and defined the Wessex Basin rift system. We explored the sedimentological and palaeontological evidence for activity on this fault including in large fallen blocks on the beach.
A quick lunch was taken in West Bay, followed by an assault-like scaling of the pebble ridge sea-defences to view the spectacular (virtually sheer) cliffs of yellow Bridport Sand Formation (a silty-sandstone laid down in shallow marine conditions) topped by Inferior Oolite.
After lunch we moved on to Burton Bradstock where we had a closer look at the Bridport Sandstone Formation, which has been an important reservoir in the Wytch Farm oilfield (under Purbeck and Bournemouth Bay); it has been the most productive onshore oil field in Europe, but is now very depleted. Also at Burton Bradstock we saw evidence for another synsedimentary fault – the Bride Fault – and an unusual fossil fauna associated with it (nautilus and a species of the ammonite Garantiana).
The weekend was highly successful – even the weather stayed fine – and gave us, as planned, the fascinating insight into the complex geological events that together created the Wessex Basin.
A spectacular journey, and I will from now on always view the Jurassic Coast in a new light.