Chapel Hill Field Guide

Chapel Hill, Torquay Richard Blagden, Martin Broadbent, Annie Kitto, Gina Little and Trevor Ryder October 2015 GEOLOGY OF CHAPEL HILL, TORQUAY !1 Table of Contents 1. Introduction 2 2. Site Overview 4 3. Visitor Information 4 4. Devonian Limestone 4 5. Permian Conglomerate 7 6. The Sticklepath Fault 9 7. Summary 9 8. Additional Photographs 10 References 11 1. Introduction This field guide has been put together by a small group of current and former undergraduate Earth Science students from the Open University. In June 2015 we visited Chapel Hill on an OU Geological Society field trip to explore the site and practice our field skills, and this guide has been put together as a result of our work. Unless referenced otherwise, the work here is the result of primary research and we welcome any feedback that would improve it. Please note that no risk assessment is provided and if you wish to use this guide to explore the site you will need to make your own common-sense assessment of the risks involved. Richard Blagden (Editor) [email protected] GEOLOGY OF CHAPEL HILL, TORQUAY !2 Figure 1.1: Sketch map of Chapel Hill GEOLOGY OF CHAPEL HILL, TORQUAY !3 2. Site Overview Chapel Hill (OS grid reference SX9065) is a small geological site situated about 1km from the centre of Torquay. The English Riviera was designated as a Global Geopark in 2007 (English Riviera Geopark, 2015a). Chapel Hill is one of its geosites, and is also designated as a County Geological Site (CGS) and a Regionally Important Geological Site (RIGS) (English Riviera Geopark, 2015b). It is noted for its Devonian limestone and Permian breccia and for the relationship between the two (English Riviera Geopark, 2015b), and is known locally for the c. 13/14th century chapel, St Michael’s, that tops the limestone (Crowson, 2015). It is a small urban site measuring around 400m by 100m along a north-south axis, and consists of steep-sided woodland. Recently Torbay Council has begun a three-year management plan of thinning non-native trees to improve access to the geology and views from the chapel (Herald Express, 2015). Very little has been written about Chapel Hill in the geological literature and the exact position of the limestone within the local stratigraphy is open to conjecture. The British Geological Survey map (BGS 2004) marks the locality as part of the Nordon Formation, more specifically one of two possible strata either side of the Marldon Limestone Member. We did not feel that it would be time-efficient, or within our skill set, to attempt to tie the limestone down to a narrower time period than this. The limestone itself has been extensively quarried on its western side. The source of the Permian breccia which abut the western side of the limestones is clearer. The Sticklepath Fault runs underneath Torre railway station immediately to the west of the site (BGS 2004) and this has moved breccia that was originally several kilometres to the south-east alongside these limestones. It is believed that the contact is fault-controlled (English Riviera Geopark, 2015b). However, our survey of the site did not reveal anywhere we could definitively see the contact between the two rock types so we were unable to confirm this. 3. Visitor Information Although the busy A3022 passes directly to the west, the closest parking is on the opposite side of the site, near the NE entrance on Barton Road and other nearby streets. Torre railway station is 50m from the south-west entrance to the site and near the NW entrance there is a bus stop for the no 12 service, which runs from Newton Abbot-Torquay-Paignton-Brixham regularly through the daytime. There are no public toilets nearby; cafes and shops are about 1km from the site in Torre. 4. Devonian Limestone We started work at the northern end of the quarry (face 1) and were able to identify a bedding plane by the presence of stromatoporoids. From here we worked upwards through the succession and recorded details of six surfaces in total. The succession is massively-bedded and rather featureless and bedding planes were identified for the most part by coincidence of angle of dip. Although the rocks had recently been cleared of vegetation the surfaces were well-weathered (more so towards the southern end) and some hammering was needed to GEOLOGY OF CHAPEL HILL, TORQUAY !4 reveal the detail. Hand lenses were used to identify fossils. Table 1 summarises each of the beds studied. Bed Vertical distance Dip Notes (from bed 1)* 1 N/A 60º SW Stomatoporoids and corals on surface. Rocks at top dipping 60º, possibly by fault action. Permian material at base. 2 5m Variable, No fossils evident 50-75º 3 2m 70º SW Pink limestone with abundant corals 4 2m 60º SW Abundant corals, possible fault in corner between faces 3 and 4 5 Unknown 60º SW No fossils - well-faulted, rubbly surface, well weathered 6 Unknown 60-70º SW No fossils * Measurement perpendicular to the bedding plane, i.e. directly upwards through the strata. In general we found most bedding surfaces dipping at 60-70º to the south-west, which was a close approximation to the RIGS citation (English Riviera Geopark, 2015b). However at least two faces were clearly dipping south and some of the higher parts, inaccessible to us, appeared to abut the SW-dipping faces. Quarrying has removed much of the vital evidence but we suspect that more than one phase of deformation took place as in places the S-dipping beds appeared to overprint the SW. Figure 4.1: Sketch of the northern quarry faces, northernmost to the far left. Beds 5 and 6 continue to the south. See table 1 for explanation. GEOLOGY OF CHAPEL HILL, TORQUAY !5 At first glance the limestone appeared to be dull, grey and featureless, but judicious hammering revealed more details. Face three in particular was a striking pink colour and figure 4.2 shows a rugose coral extracted from bed 1. Figure 4.2: Rugose coral, from limestone face 1 at Chapel Hill. One interesting feature was the appearance of a reddish conglomerate material at the base of face 1, and this is discussed at length in section 6. The ruin of a lime kiln can be seen just between the northern and southern quarries and it would be interesting to research the story of the industry at Chapel Hill, with the railway station in such close proximity. GEOLOGY OF CHAPEL HILL, TORQUAY !6 5. Permian Conglomerate The exposure of Permian Strata at Chapel Hill Woods is located in a road cut by the side of Newton Road which marks the western boundary of the site. The length of this exposure is approximately 200m. Figure 5.1: Roadside cutting of the Permian rocks (Section 2 in the text). The general nature of the lithology of these rocks is a series of conglomerates or breccias dipping at approximately 9º in a direction apparently towards the north-west, although this may not represent the true dip (figure 5.1). The breccias are interbedded with lenses of finer material, most commonly of fine to medium sand. The thickness of beds shows considerable variation from 10 – 50 cm and, in places, poorly-defined graded bedding (fining upwards) may be visible. The majority of clasts in the breccias consist of Devonian limestones (up to 20 cm in diameter) which contain frequent fossil fragments including corals and brachiopods. Other, less common, clasts included quartz and very dark coloured, often black, fine-grained crystalline volcanics. The matrix of the breccias varies from silt to fine sand. The sand grains examined by hand lens were mostly angular and glassy. This lithology would be consistent with deposition in an alluvial fan environment. The original aim had been to construct a generalised graphic log from the base of the exposure (at its southern end) to the younger units (at the northern end) by tracing the topmost beds in one location and following them to where they “disappeared” into the pavement, from which point the log could be continued upwards. However, this proved impractical, as many “beds” are laterally discontinuous lenses. Therefore, only three sections are described here in more detail. GEOLOGY OF CHAPEL HILL, TORQUAY !7 The oldest exposure is located at the southern end of the road cut, and the following description is of a section located close to the lower road direction sign approximately 30 metres from the southernmost end of the exposure (section 1). The lowest unit in this section comprises approximately 110cm of matrix-supported breccia with clasts up to 20 cm in diameter. The clasts are evenly distributed throughout the unit and the matrix is predominantly of silt size. As throughout the exposure, the clasts are predominantly of Devonian limestones with some quartz and dark volcanic rocks. Above this layer is a unit of variable thickness (typically about 50 cm) with a coarser matrix (fine to medium grained angular sand) and more flattened clasts including some red shales and mudstones which are possibly re-worked local Permian sediments. Although more detailed analysis is required, there is a visual impression that these flattened clasts may show signs of imbrication indicating a Palaeocurrent direction towards the north-west, in a similar direction to the dip. This layer possibly represents a small temporary channel on the surface of a larger alluvial fan. Above this unit is a variable thickness (80 – 210cm) of the bedded breccias described in the second paragraph (above) which form the dominant lithology of the whole exposure. Section 2 is approximately in the centre of the exposure and is shown in figure 5.1.

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