Mervyn Jones (UK)
GA field meeting on 6 and 7 April 2019
This field meeting was the second following the publication of Prof Cope’s GA Guide No 73, Geology of the South Devon Coast (reviewed in Issue 51 of Deposits), the companion to GA Guide No 22, Geology of the Dorset Coast (reviewed in Issue 50).
Our mission for the weekend was to complete our examination of the complex Devonian succession from Torbay to the western limit of Lyme Bay at Start Point and then beyond; farther into South Devon. Of great interest was the marine Devonian, first described by Adam Sedgwick, assisted by Roderick Impey Murchison, who finally realised that these facies were contemporaneous with the familiar Old Red Sandstone found north of the Bristol Channel.
The area has much to offer enthusiasts of structural geology because the Devonian strata have been tectonised by the closure of the Rheic Ocean during the Variscan orogeny. The story has only been unravelled in the last 50 years as follows. First, sediments filled a series of basins caused by crustal extension; the basement beneath the Devonian rocks may well be a massif of Precambrian mica-schist, inferred from the copious amounts of mica and the occasional xenolith in the rocks above. Then, from the Early Carboniferous, continental collision caused a series of major thrust structures and metamorphic zones that progressively moved northward. As a consequence, any Carboniferous rocks that were deposited in the Torbay area were rapidly stripped off. The marine Devonian was then covered by terrestrial Permo-Triassic.
The first day was to be spent at the southern end of Torbay, a source of the famous Devonshire Marbles, described in Gordon Walkden’s two-volume GA Guide No 72 (Devonshire Marbles: their Geology, History and Uses, reviewed in Issue 46 of Deposits). This comprehensive work of scholarship has a useful section describing the fossils that we were to see.
We met up at Berry Head Quarry in Brixham, the south-western gateway to the English Riviera Global Geopark. The enormous quarry has impressive exposures of reef limestone deposited on the Brixham high, itself formed from extrusive igneous rocks at the edge of the South Devon Basin. The weathered red shales close to the shore revealed many stretched crinoid stems with the ossicles dislocated and preserved en echelon (Fig 1). The reef building organisms were principally stromatoporoids, an ancient group related to sclero-sponges. The limestones here are largely uniform and massive due to recrystallisation.
The impressive faces of the quarry reveal vertical “neptunian dykes” of Permian red bed material (Fig. 2). A neptunian dyke is a sedimentary deposit cutting through strata (much like an igneous dyke), formed by the filing of an underwater fissure. However, Neptune had nothing to do with it because, as quarrymen reported, the dykes often failed to reach the surface. Rather, they are interpreted as fissures created in the limestone by tension. Initially, they were water filled, encouraging the crystallisation of calcite as dog’s tooth spar. They were then filled with sediments washed in by occasional storms on the otherwise arid Permian land surface.
Our next location was on the coast to the immediate west of Berry Head at Shoalstone, where there is a fine seawater-fed swimming pool (well ‘fine’ on a hot summer’s day). The foreshore revealed swarms of neptunian dykes in the limestone, accompanied by mineralisation along the margins formed when the fissures first opened. Researchers have studied how these intersect, concluding that the older swarm runs east-west, revealing north-south tension at the time, whilst the younger run north-south, suggesting an east-west tensional regime. To the group, the patterns did not appear quite so clear.
Our next locality was at Goodrington beach, a jolly family resort. Ignoring these delights, we marched across the basal Permian Torbay breccia, dated by its paleo-magnetic properties, which lies unconformably on the Brixham Limestone (Fig. 3). Substantial boulders of fossiliferous Devonian limestone were washed down by flash floods from higher ground across a very uneven landscape to leave a wadi deposit similar to those found in desert regions. Prof Cope reminisced about nearly falling victim to such a flood, but in the Alps and a little more recently.
Across the beach, we encountered our first tectonic structures of the day – large (5m-scale) antiform folds in limestones and siltstones, in which the cleavage ran parallel to the bedding. The cliffs bear witness to later tectonism in the form of minor faulting bringing the Upper Devonian Saltern Cove Formation down to beach level. The rocks contain Goniatites and other marine fossils, as well as occasional volcanic tuffs and a dolerite sill.
We started the second day at Blackpool Beach, south of the Dart river, to begin our exploration of the highly deformed rocks of the Looe basin. The sediments are Devonian in age, but buried more deeply than those of the same age around Torbay, resulting in a higher grade of metamorphism and considerably more folding; or as Prof Cope put it, they have been “mucked about”. Here, they are highly cleaved and altered greenish grey shales exhibiting the sheen of mica on the surface, characteristic of phyllites. Unfortunately, the cleavage planes run parallel to the coast road, which has a habit of slipping down the cliff as a result. The engineering geologists amongst the group debated the merits of rock anchors or grouting to remedy the situation.
Our next stop was at Slapton Ley at the end of Torcross Sands. Over lunch, we enjoyed sitting on the tombola and observing the wildlife in the brackish lake behind. The memorial to the 749 American servicemen killed in an exercise in preparation for D-Day was very touching, particularly in the year of the 50th anniversary of the invasion.
The weather was rather against us, as strong onshore winds kept the tide from receding, although it did bring a large flock of seagulls and a seal close to the shore to feed on a swarm of starfish ripped up from the bottom (Fig. 4).
When we did make it to the outcrops on the beach South of Torcross, we were able to see the cleavage refracted through the sandy bands intercalated in the metamorphosed silts (Fig. 5). So intense is the cleavage that we could see strong lineation where it intersected with the bedding, plunging westwards at more than 20°. Intruded into these beds were igneous rocks, also metamorphosed to leave pseudomorphs of the original plagioclase replaced by calcite – enough to confuse any geologist with a bottle of acid to hand. Scrambling over the rocks we met a ‘Disneyland’ of intrusions, baked zones, alterations and mineralisation, resulting in mind-bending patterns in the rocks (Fig. 6).
Finally, we repaired to Hope Cove, a busy little resort where tomorrow’s engineers were busy making dams and spillways in the sand. Our task was to try and unpick the several phases of folding that have deformed the rocks so that they resemble stair rods packed tightly together. We identified at least two-fold directions, at right angles to one another (Fig. 7). When we took time to look up at the horizon, we could see very clearly the 60m platform, where the land surface was planed flat by the Pliocene seas.
Halfway across the beach, we crossed a fault, leaving the Meadfoot Group behind and reaching the lowest Devonian part-marine Dartmouth Group, surely the most deeply buried and “messed about” Devonian rocks of all. The cleavage was very well defined, picked out in red by pyrite that had been mobilised by pressure solution. Prof Cope set us the task of finding the bedding and this time I was lucky. A wiggly band of coarser sediment bent back on itself showed just how much deformation there had been (Fig. 8).
We left well satisfied after another terrific trip with Professor Cope. This was the seventh of the series; the eighth is scheduled for 21 and 22 March 2020 to West Dorset. We also hope Prof Cope will extend Guide 73 to cover Start Point and beyond, and to illustrate bedding and cleavage in the lively way he did on the beach!
About the author
Mervyn Jones has been a member of the Geologists’ Association for 11 years. He studied geology at St Catharine’s College, Cambridge and completed an MSc in Geoarchaeology at Reading University in 2018. Like William Smith, the father of stratigraphy, he is both a surveyor and geologist.
All photographs are by Harry Jones.
A version of this article appeared in the September 2019 edition of the Magazine of the Geologists’ Association.