Dumbarton Rock
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Agglomerate Dumbarton Rock - Where is it? This grey flattish mass of rock is • Castle Road, Dumbarton, G82 1JJ GEOLOGICAL FEATURES a tuff formed from volcanic ash, indicating the proximity to the By road… OF DUMBARTON ROCK vent of the volcano. You should In Dumbarton off the A82 at Barloan roundabout. Follow the be able to see the different brown tourist signs for Dumbarton Heritage Sites and then grain sizes from fine ash to follow the Dumbarton Castle signs. larger sizes from underlying rock fragments caught up in the By rail… The nearest station is Dumbarton East. Exiting the station at eruption. Glasgow Road, turn right, and then left down Victoria Street and then Castle Road, following the signs for ‘Dumbarton Castle’ which takes you to Dumbarton Rock. Walking distance is approximately 1km/0.6mls. Alternatively, follow the signs Shale, Concretions and Calcite Veins ‘The Castle Circular Path’ which takesis a slightly slightly longer longer walking walking route. At the back of the bay area route. some of the vegetation has Fold fallen away to expose the Access and Acknowledgements underlying beds of shale. Shale is a fine grained Access by kind permission of Dumbarton Football Club Limited, Shale sedimentary rock formed such access taken to be fully at the users own risk. by compaction of clay or NO ACCESS will be available on match days and NO parking silt. It is very soft and flaky. on the site will be permitted. Photograph A The geological features in the bay area are subject to tidal You will also notice that influences and this may cause seaweed, sand, stones and there are pale rounded stones set into the shale. These marine debris to conceal or further expose the features. are carbonate concretions. After the clay and silt were Strathclyde Geoconservation Group gratefully acknowledge buried and started to turn to rock, water flowing along Photograph courtesy of Ghyll McCallum at Levengrove Dental Care certain layers precipitated calcite (calcium carbonate) to the cooperationcooperation and and financial financial assistance assistance of ofWest West Dunbartonshire Dunbartonshire form concretions which often joined to form a complete Council without whom the production of this leaflet would not layer. This shale/carbonate association is called have been possible. cementstone as these are the prime ingredients for Strathclyde Geoconservation Group gratefully acknowledge the making cement. There are also veins of the mineral cooperation and financial assistance of Historic Scotland in the calcite which formed when hot (the heat coming from production of this leaflet. the volcano) water saturated with calcium carbonate was Thanks to C. Davies, Killearn, for artistic interpretations of forced into cracks and eventually diagrams and Ghyll McCallum, Levengrove Dental Care, Dumbarton, for the front cover photo. A Geological Trail built up to form these veins. This leaflet was conceived by Strathclyde Geoconservation Photograph A shows a fold of Group (formerly known as Strathclyde RIGS Group_Group) whichwhich cementstone with shale under- is a sub-committee of the Geological Society of Glasgow. neath and more concretions of To know more about the Society and geoconservation, go to the website www.geologyglasgow.org.uk. cementstone and calcite veins. Calcite Photograph B shows an Veins Cementstone exposure of cementstone and concretions calcite veins in parallel. Photograph B Setting the Scene Schematic map of Dumbarton Rock The Sheared Blocks of Basalt In the Early Carboniferous period, about 330-340 million P Parking About 10,000 years ago years ago, there was widespread volcanic activity in the Access path vast sheets of ice Glasgow area. Evidence of this can still be seen today in via Dumbarton FC area covered an area of the lavas of the Kilpatrick Hills, Campsie Fells and Scotland as far south as Renfrewshire Heights. One of the volcanoes which 1 Locality 1; Columnar Jointing the Loch Lomond area. poured out lava was at Dumbarton Rock. Today, all that As the temperature remains is the basalt plug at the root of the long-extinct 2 Locality 2; warmed the ice melted volcano. Bay Area also showing rapidly, in terms of Basalt Blocks geological time, causing the land to readjust to the Figure 1 shows a cross section of what the volcano would loss of weight of the ice; it did so by fault movement have looked like and the underlying, older rocks which are causing earthquakes. These earthquakes caused the shales with carbonate concretions (cementstones), and huge basalt blocks to break off from the main park of underlying them, sandstones of the late Devonian period the Rock leaving a sheer face and the blocks strewn about 350 million years ago. The distance from Locality 1 to Locality 2 is approximately 300m. below. The lava is basalt, which is a dark coloured, fine grained Locality 1 - Columnar Jointing The large fallen basalt block, featured in the photo, volcanic rock. sitting at the water’s edge, has much of its surface As the basaltic lava cooled its weathered so that you don’t see the original surface, Basalt volume decreased causing but you might find a ‘fresh’ surface which will show cracks to form. The surface you dark colour and very fine grained texture. Shales with concretions cooled first so the cracks formed at right angles to the cooling Sandstone surface dividing the basalt into Sandstone columnar segments. Prime Next to the large fallen basalt Rising magma Basalt block examples of columnar jointing are block are three smaller Figure 1 Basalt plug Fingal’s Cave on the Isle of Staffa blocks of red sandstone. and the Giant’s Causeway in These are examples of the Northern Ireland. underlying Upper Old Red Sandstone of the Devonian period. These rocks are Present Day Locality 2 - Bay Area steeply dipping (i.e. they Erosion Level would originally have been Sandstone laid down horizontally but are now turned by ~ 80 degrees.) Figure 2 This disruption was caused As the magma chamber (the source of the lava from deep by the fracture and subsidence of the vent as within the Earth) empties, the flow of lava in the vent illustrated in Figure 2. ceases, accompanied by fracturing and subsidence. Eventually the lava cools and solidifies filling the vent to There are other pieces of sandstone in the bay area form what we call a ‘plug’. During the eruptions some of which were caught up in the melee. Can you spot the underlying rocks are broken up and become trapped them? in the lava. After hundreds of millions of years, the area of the volcano erodes to expose the remains of the plug and some of the underlying rocks, which is shown in Figure 2..