The Geology of Castle Rock, Johnstone Terrace, Edinburgh Geology and Landscape Scotland Programme Commissioned Report CR/13/031 BRITISH GEOLOGICAL SURVEY GEOLOGY AND LANDSCAPE SCOTLAND PROGRAMME COMMISSIONED REPORT CR/13/031 The Geology of Castle Rock, Johnstone Terrace, Edinburgh M Krabbendam and E A Callaghan The National Grid and other Ordnance Survey data © Crown Copyright and database rights 2012. Ordnance Survey Licence No. 100021290. Keywords Castle Rock, Ballagan Formation, basalt. Front cover Castle Rock section above Johnston Terrace, Edinburgh Bibliographical reference KRABBENDAM, M, CALLAGHAN, E A. 2013. The Geology of Castle Rock, Johnstone Terrace, Edinburgh. British Geological Survey Commissioned Report, CR/13/031. 27pp. Copyright in materials derived from the British Geological Survey’s work is owned by the Natural Environment Research Council (NERC) and/or the authority that commissioned the work. 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This report was commissioned by Scottish Natural Heritage (SNH) in conjunction with Historic Scotland (HS), in advance of some work to stabilise part of the section and so partly obscure it. Acknowledgements Staff from Historic Scotland, especially Ian Armstrong and Stewart Mackenzie, are thanked for facilitating access to the section and rope supervisor Stuart Hogarth and his assistant Chris Ross are thanked for their assistance to the authors for roped access to the site. Tim Kearsey, David Millward and Mike Browne (BGS) are thanked for discussions and insights into the Ballagan Formation. Professor John Underhill (University of Edinburgh) is thanked for discussions concerning faulting. i CR/13/031; Version 0.1 Last modified: 2013/08/16 13:54 Contents Foreword ......................................................................................................................................... i Acknowledgements ......................................................................................................................... i Contents .......................................................................................................................................... ii Summary ....................................................................................................................................... iv 1 Introduction ............................................................................................................................ 1 1.1 Geological setting ........................................................................................................... 1 2 Geological Description of Section ......................................................................................... 2 2.1 Lithology ..................................................................................................................................... 4 2.1.1 Basalt – Lower Carboniferous intrusive igneous rocks ............................................................... 4 2.1.2 Ballagan Formation – Lower Carboniferous sedimentary rocks ................................................. 5 2.2 Structure: Faulting ....................................................................................................................... 7 2.2.1 Overall structure .......................................................................................................................... 8 2.2.2 Main fault .................................................................................................................................... 8 2.2.3 Anastomosing fault zone ............................................................................................................. 8 3 Conclusions ............................................................................................................................. 9 Appendix ...................................................................................................................................... 10 Glossary ........................................................................................................................................ 19 ii CR/13/031; Version 0.1 Last modified: 2013/08/16 13:54 FIGURES Figure 1 : Overview of geological setting of Castle Rock. ............................................................. 2 Figure 2: Overview of section above Johnston Terrace, annotated with geology. ......................... 4 Figure 3: Sedimentary log of part of Ballagan Formation. ............................................................. 5 Figure 4: Sketch diagram showing overview of fault zone. ............................................................ 7 Figure 5: Basalt within 50 cm of faulted contact, note vesicles and lack of damage (fractures). 10 Figure 6: Basalt; main fault just off photo to the right. Narrow zone of fault-parallel fracture cleavage. ........................................................................................................................................ 10 Figure 7: Top of section, this is part of area to be netted. Sandstone and mudstone beds of Ballagan Formation. ...................................................................................................................... 11 Figure 8: Sandstone bed with cross-bedding. ................................................................................ 11 Figure 9: Sandstone bed with cross-bedding with possible water escape structures. ................... 12 Figure 10: Bedding plane between two sandstone beds. ............................................................... 12 Figure 11: Sandstone bed (c.2 m thick) with cross-bedding and erosional, channelized base. Pebbly at base (see Figure 13). ...................................................................................................... 13 Figure 12: Detail of base of erosional channel in sandstone (c. 2 m thick). Pale grey pebbles are derived from calcareous nodules; black clasts are probably volcanic in origin. ........................... 13 Figure 13: Bedding plane: top of sandstone bed (tilted block). .................................................... 14 Figure 14: Irregular, knobbly surface of enigmatic origin. ........................................................... 14 Figure 15: Subvertical beds of mudstone and sandstone (left), younging to the left. Mudstone layer rich in calcareous, pedogenic nodules, left of hammer. ....................................................... 15 Figure 16: Main fault plane exposed at base of section. Basalt. Fault plane marked by steep fault grooves, c. 1 cm deep. ................................................................................................................... 15 Figure 17: Main fault plane exposed at base of section. ..............................................................