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Selected Articles from Issue Number 6/5 October 1999
The
Albury Downs Geology Trail Pictures of this trip can be found in the Photo Gallery This was a field trip on 27th June, led by Iain Fletcher. And he volunteered to write it up too! (Hopefully this doesn't set a precedent.... he wrote). The trail forms a convenient introduction to some of the Cretaceous rocks of Surrey. It starts at the large car park at Newlands Corner (TQ 043 492), 5km east of Guildford, and the full trail is 8km, returning to the starting point The first exposure was in the woods immediately north of the car park, the Headley Formation, former known as Netley Heath Deposits. Since it is a clayey sand and gravel, the question is: 'how did it get here?' The presence of gravel shows it isn't wind-lain, and the glaciers didn't reach beyond London. The answer is that it's essentially a relict deposit. The gravel is derived from Chalk flints, as well as from the now-eroded Lower Greensand and Tertiary deposits in this area. Marine fossils 2 million years old have been found in Netley Heath Deposits, 4km to the east of here. So this area has risen, from around sea-level, 170m in that time. From the woods, we walked south of the car park to a viewpoint overlooking the Vale of Holmesdale, for an overview (literally) of the geology. The view illustrated the relationship between geology and geomorphology, with the more erodible Gault Clay along the base of the scarp slope in the immediate foreground, and then beyond it the Folkestone Sand Formation, which formed the narrow ridge of wooded hills due to its more resistant capping of iron- cemented "carstone". Just beyond these low hills lies the village of Albury, sitting in a vale cut into the fine grained sands of the Hythe Formation (which also forms the long wooded slope running up to the skyline). The first of the Cretaceous exposures, down the chalk scarp slope to the south-east, was in the 'sludge pit', an exposure of Middle Chalk. Chalk isn't all the same ("oh yes it is"). "Oh no it isn't", as was pointed out here. White chalk, rather than green-grey, and no flints meant it was Middle Chalk, and the knobbly appearance indicated the Holywell Nodular Chalk, just above the Lower Chalk. There were few fossils to be found. Opposite the main face was a blocky cream-coloured exposure: a periglacial solifluction deposit (Chalk sludge from uphill). Subtle changes in the slope of the ground revealed the next part of the story of the rocks. Some green-grey Lower Chalk was found in the banks of the lane just beyond the sludge pit, and a little further on the harder, blocky Upper Greensand was found as well. In winter the Upper Greensand is more obvious, forming a low rocky ridge in the adjacent field. The underlying Gault Clay was not visible at all. One mystery: just how can the 88m thickness of Gault fit into 50m 'outcrop' width, as mapped? Answer: faulting. The next exposure was in an old sand pit, virtually hidden from view on the right of the track. The pit face showed a 6m height of iron-stained cross-bedded Folkestone Sand, with large scale cross-bedding dipping south-eastwards on the southern side of the pit. Contorted veins of black iron-cemented sandstone ('carstone') were also obvious. Bivalve fragments have been found here, in the carstone veins, and there were trace fossils (vertical burrows) in the south-eastern corner of the face. By now, having had a fairly intensive introduction to the geology, time was getting on and the pub in Albury beckoned! Which, as it turned out, was good timing, because we were in shelter from the one (heavy) shower of the day. Without the hint of a mutiny (fortunately) we carried on, walking up Blackheath Lane south of Albury to see the fine-grained fairly nondescript sand of the Hythe Formation well exposed in the sides of the 'sunken' road. The contrast with the overlying Bargate Member (of the Sandgate Formation) was quite obvious: the Bargates were an impressively current-bedded sandstone. A subtlety about the Bargates is that the calcareous cement and some of the fossils it contains were derived from Jurassic rocks. These rocks were exposed and eroded on the Lower Cretaceous coast not far to the north of this location. From here we walked to the crest of the ridge and then east, through the pine wood. This type of land use gave a clue to the underlying geology: Folkestone Sand again, since it makes poor land for pasture. On meeting another track, we turned north, downhill towards Albury. With an east-west strike to the geology, we were now descending through the same sequence as in Blackheath Lane, although it was less impressive. However the face of Hythe Sand at the bottom of the track showed current bedding and much evidence of burrowing. From here on there was much less geology to be seen, so it was more a pleasant walk down to the road and towards Silent Pool. A short diversion across the fields brought us to Albury Sand Quarry, presently both a source of building sand and a landfill site. There was no access to the site, being Sunday. Then we walked north, across the concealed Gault Clay and across the road, to the Silent Pool locality. Here at Sherbourne farm there are two ponds fed by a spring from the Chalk at the northern end. The lower one, Sherbourne Pond, lies mainly on Gault Clay, while its northern end and the stream connecting the two ponds lies on Upper Greensand. Blocks of the Upper Greensand can be found in the stream bed: a pale grey siltstone which weathers to brown and is harder than the Chalk. However, the difference between them is not obvious, and we didn't try to distinguish them on this occasion. Most of the northern pond (the Silent Pool) lies on Lower Chalk, which was seen at the northern end of the pond. It dips 25°N. Leaving the tranquillity of the Silent Pool and moving up-hill towards the Chalk escarpment, on the right was a path-side exposure of Chalk, 10m by 1m, with some bivalve fragments. As in the Chalk exposure near the start of the Trail, there was no obvious sign of bedding. Also as before, it was white, flintless and with a rubbly appearance, so it was possibly the Holywell Nodular Chalk again. We continued
up-hill, and at the tree-line turned right, walking a short distance to the pill-box and
the remains of a brick-built limekiln. This was a good location to study the
"geology", or rather, what can be deduced from the scenery. The younger
rocks (the Chalk) were to the left and the older ones (the Lower Greensand Group) to the
right. The grassy slope stretching out in front was relatively steep, with
undulations possibly due to variation in Chalk strength. To the right, before
reaching the road, the slope was flatter and the soil darker because this marked the zone
of Gault Clay. The Upper Greensand did not make a prominent topographic feature.
South of the road the soil was dark brown, supporting conifers: Folkestone
Sand Formation. One interesting
piece of news: Surrey RIGS Group has just been awarded a sizeable grant to produce a Trail
leaflet and site interpretation boards. Our own London branch John Daniels Memorial
Fund will also help financially. So the Trail will be enhanced, increasing the
appreciation of geology. Fairlight
Cove Pictures of this trip can be found in the Photo Gallery A happy group of 18 London Branch and South East Branch members including some 'first timers' met for lunch at 'The Smuggler' on a bright sunny Sunday in September for an afternoon on the beach with David Scarboro. Fortified with food and drink we assembled on the beach for a walk from Pett Level to Goldbury Point, a distance of about 2½ kilometres. David began the trip with some advice about the dangers of this stretch of coast including cliff falls, slippery rocks and the danger of being cut off by the tide. We were to encounter another by the end of the day, of which more anon! The exposures in the cliffs here are of the Ashdown Beds, near the base of the early Cretaceous. A drowned forest on the foreshore here, about 5,000 years old, reminds us that rising sea levels are not a modern invention. Walking westwards we came to the Cliff End faults, small normal faults forming an easily discernible graben. The rocks exposed in the cliffs here are Cliff End Sandstone, a local arenaceous lower division of the Wadhurst Clay, consisting of fluvial cross-stratified and channelled silts and fine sands. At the top of the cliff is the Cliff End Bone Bed containing the teeth of fish, reptiles and early mammals. Moving west along the foreshore we moved down the succession, the beds dipping gently to the east, and the junction between the Cliff End Sandstone and the underlying Ashdown Beds was revealed. The junction is marked by a shale band with iron carbonate nodules, and the environment of deposition is interpreted as brackish to fresh water, perhaps marking a waning of the strength of the river system. Some exotic rocks were seen here, Larvikite from Norway being used as sea defences. The houses overhanging the cliff seem to indicate that they are not working very well! David showed us a highly disturbed bed here, which he suggested might have been caused by dinosaurs milling around a watering or grazing place. A new term to most of us was introduced, dinoturbation! (Funny, my spell checker doesn't seem to recognise this one!). Continuing our
ambulations to the west we came to the Haddock's Reversed Fault with a downthrow of some
60m. to the west. Sedimentary dykes (Neptunean dykes) and dinosaur footprints exist on the
foreshore here, but are only revealed when winter storms remove the overlying sands. From here onwards there were many clues to the environment of deposition of the exposed rocks. The fine siltstone overbank deposits coarsen upwards into the Haddock's Rough Unit with considerable lateral variation and several erosion surfaces (point bar units?). Exposed on the foreshore was the Dinosaur Footprint Bed with footprint tracks and tail drags from a number of species of dinosaur evident (no eye of faith needed here!). The larger of these are thought to be those of iguanodon. Fragments of tree fern trunks can be found in blocks of finer grained lagoonal sediments here, although none have been found in-situ. It was about this time that our leader became concerned about a hazard, which we had not anticipated. Large black clouds with sheets of lightning issuing from their base had appeared out to sea, and they were heading our way! A quick vote decided that we would continue to the last exposure (brave or foolish?). Crossing another major reverse fault, the Fairlight Cove Reverse Fault, we found ourselves moving from the fluvial Ashdown Sands onto the finer Lower Ashdown lagoonal sediments near the centre of the Wealden anticline. Some low rank coals can be found in these beds, and complex siderite structures thought to be root infills were seen. We now had enough evidence to reconstruct the depositional environment of the early Wealden. A picture emerges of a low lying plain traversed by meandering rivers and with ephemeral lakes and lagoons. Herds of herbiverous dinosaurs roam across the plain dining on horsetails and tree ferns and being dined upon by the occasional carnivore. Changes in sediment grain size and rate of sedimentation can be attributed to the waxing and waning of the river system driven either by sea level changes to the south or changes on the London Platform to the north, topography or climate. Those black clouds had now caught us up, and a hasty retreat from the beach was made. Fortunately the pub was still open! Many thanks to David
for a very interesting and thought provoking day. If you wish to get a better appreciation
of the site do visit the London Branch web site where there are some excellent photographs
from Paul Hetherington and Lesley Owens. |
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