Harrow and Hillingdon Geological Society
The group met with our President Bob Symes at Friday lunchtime Killerton Park National Trust property near Exeter at the start of what was to be a warm long weekend with unbroken sunshine. After lunch, Bob introduced us to the building materials of the former stable block (now cafe and shops) before walking to the chapel and a quarry within the grounds. We were on the side of Killerton Hill, a volcanic plug of Permian age. The building stones showed a variety of rock types from the lavas which were erupted probably as fissure eruptions on to the desert floor of the time. The crust was under tension at a late stage in the Variscan Orogeny when the northern and southern continents of Laurussia and Gondwana respectively came together to form the supercontinent of Pangaea. The lavas are potassium-rich basalts, lamprophyres and minettes which were well displayed in the buildings and the quarry. They were erupted about 290 Ma ago not long (in geological terms) before the emplacement of the SW England (Cornubian) granites. Following a cup of tea we drove to the hotel in Exeter.
On Saturday we explored the Triassic coastline of East Devon, going up the succession from west to east. First stop was Exmouth and Orcombe Point, the start of the Jurassic Coast. Here, Bob introduced us to Roger LeVoir, an expert on the Triassic in Devon. The tide was in, but we still able to see, from steps, current bedded sandstones, and mudstones, of the Exmouth Mudstone and Sandstone Formation. This unit belongs to the early Triassic Aylesbeare Group. Prior to this, we walked to the top of the headland to a recently constructed obelisk marking the start of the Jurassic Coast World Heritage Site. It was built of stones representing the main formations that can be seen between here and Swanage at the eastern end.
We then drove to Budleigh Salterton, where we examined the well-known Budleigh Salterton Pebble Beds. These were deposited by a powerful river flowing north as braided streams across the desert from what is now Brittany to the position of the Irish Sea. Most of the pebbles are of quartzite which may have come deposits of Ordovician age (nearly 200 Ma older than the Triassic) which occur in Brittany. The pebble beds are overlain by the Otter Sandstone, which displays weathering of a honeycomb appearance. Both formations belong to the Sherwood Sandstone Group.
We had lunch in Sidmouth, then we visited the town museum where Bob is curator. It is well worth a visit. We were shown local specimens of reptilian bones which are fairly rare. We then examined the cliffs to the west of the town, still within the Otter Sandstone with Roger and Bob. In these cliffs, near Jacobs Ladder at the western end of the promenade, there was evidence of root formation, showing that the land had some vegetation cover at the time. Here we were met by Keith Collier, member of the Sussex Mineral and Lapidary Society and the local Sidmouth Mineral Appreciation Group (SMAG). He took us further along the beach to where nodules containing baryte crystals come out of the cliffs, especially after winter storms. We then walked back to the museum, where Bob kindly provided us with coffee. On the way, we looked at the recently opened Jurassic Coast Interpretation Centre on the seafront; this focussed on the Sidmouth area, and was set up by the Museum. It was an excellent addition. After we left the museum, we went to Keith Collier's house to see his display of minerals. This was a truly spectacular display and the minerals were superbly lit. This was a fitting end to an excellent day.
On Sunday, we returned to Sidmouth, this time to explore the hill above the town to the east - Salcombe Hill. Here and to the east, the Triassic rocks are capped with the Cretaceous Upper Greensand (UGS) and also with chalk further east still. All the way from Exmouth, the Triassic rocks have been gently tilting eastwards, but east of Sidmouth the Cretaceous rocks are more or less horizontal, onlapping on to the Triassic, and east of Seaton, the Jurassic also. This constitutes and area-wide unconformity with a time gap exceeding 100 Ma in the west. From Salcombe Hill, there were superb views in both directions, and inland. The Upper Greensand caps many of the hills in the area and could be picked out by a change in vegetation from fields to mixed woodland and some open heath. Scattered around there were large boulders of conglomerate, formed mainly of flints and cherts cemented hard by silica. These were silcretes, probably formed in a similar way to, for example, sarsens and hertfordshire puddingstone further east in S. England. These were remnants of possibly Cenozoic cover and were cemented by silica-rich water percolating through under sub-tropical conditions.
We then went on to Branscombe Mouth, where we saw gypsiferous mudstones belonging to the Triassic Mercia Mudstone Group. These were capped with UGS and Chalk which thickens to the east. We could see Hooken Cliff (mainly in the Chalk), the site of a large landslip in the early 19th century involving quite a few acres of land splitting away within a couple of days. We finished the day at Beer Caves, a network of underground quarries in the Beer Stone, a 5 or 6 metre thick layer of hard gritty limestone within the Chalk made high quality building material which can be found in churches a long way from the area as well as in many local buildings. Quarrying began in Roman times and continued until the early 20th century. The network of tunnels covers several acres. The very interesting tour lasted an hour or so but it was 8.5 deg. C and very humid underground making it feel quite cold. The stone was easier to work underground because it was soft and there were areas set aside for this. The stone hardened considerably once it was out in the open.
On Monday morning, a number of participants had to leave us. The remainder visited a couple of local sites in Exeter before going off to Beer. We went to the long-disused Heavitree Quarry within walking distance of the hotel. In the middle of a 1970s housing estate, it displays early Permian breccias (the Heavitree Breccia of the Exeter Group). The clasts were derived fairly locally and were made up of a variety of rock types of both Dartmoor and local volcanics provenance. They were deposited as desert alluvial fans, sheet gravels and wadi deposits from nearby uplands probably by flash flooding. Then we went to another quarry a couple of miles away - Bishop Court Quarry. A disused section of Permian dune-bedded sands faces the Toys-R-Us car park; these belong to the same Exeter Group. The dunes probably bordered the stony basin represented by the breccias seen earlier. We then went on to Beer, the main location of the day. We spent a couple of hours on the beach, looking at the chalk and UGS in some detail. The lower part of the chalk is a condensed sequence resting on the UGS. From the beach we could see the cliffs of Seaton and east from Axmouth, mainly of late Triassic Mercia Mudstones and grey Penarth Group rocks coming in towards Pinhay Bay, heralding a change to marine conditions. At Beer, the Triassic was well below beach level but is brought up again at Seaton Hole by a fault which has a 60m throw.
We dispersed at about 3.30pm after cups of tea, ending a superb trip, and our sincere thanks go to Bob Symes, Roger LeVoir and Keith Collier for helping to make it such a success.