Post by John on Jun 26, 2008 20:02:14 GMT -5
COAL MEASURES OF THE RIVER WEAR GORGE AT DURHAM, ENGLAND
byG. A. L. JOHNSON1 and GORDON RICHARDSON2SUMMARY
Eleven stratigraphical boreholes put down near to the River Wear incised meander gorge at Durham City are described. The resulting interpretation of the stratigraphy of the region shows that the Middle Coal Measures (Upper Carboniferous, Westphalian) succession between the Hutton Coal and the Maudlin Coal is exposed. Two significant fault belts affect the rocks of the gorge. The north-east trending Prebends’ Bridge Fault, with a downthrow south of more than 1 5m, cuts the gorge obliquely and Tom’s Fault, which downthrows east 7.5m, runs almost north and south near to the axis of the meander. The fault lines disrupt the sequence of strata in the sides of the gorge and in particular, the Low Main Post sandstone and underlying coal seam which are recognized at the top of the gorge beneath Durham Cathedral, are replaced by the Maudlin sandstone and coal seam on the south side of the Prebends’ Bridge Fault. Contrary to the accepted interpretation, first published by Holmes (1928), the conspicuous sandstone and underlying coal seam that form the lip of the gorge are discontinuous. The marked lack of exposure of the 10m thick Low Main Post sandstone is proved to be caused by widespread medieval quarrying for building stone.
INTRODUCTION
Durham City has its origins in an ancient fortified settlement on a tight bend of the River Wear cut deep to form an incised meander gorge. The site is a natural stronghold, a promontory protected by the river and gorge on three sides so that only the narrow salient on the north had to be protected by fortifications (Figs 1 and 2). The Saxons developed a monastic settlement on the site in 995 and built a stone church to shelter the remains of St Cuthbert, a much revered 7th century Northumbrian Saint. After the Norman conquest the promontory was further fortified by Durham Castle on the north and a city wall was constructed round the lip of the gorge. Within the defended area, Durham Cathedral was built between 1093 and 1133 as the permanent resting place of St Cuthbert’s Shrine. The steep wooded banks of the river surrounding the promontory, though modified by quarrying, have survived to the present day and now form a unique amenity of river-side walks almost ringing the centre of the city.
The gorge is cut in bedrock Coal Measures (Upper Carboniferous) strata which are exposed, though much concealed by debris fill and vegetation. The general succession from the bottom is interbedded shale, sandstone and thin coal seams, overlain by a persistent coal seam that underlies the thick sandstone which forms the top of the gorge. The coal seam beneath the upper sandstone varies in thickness, but it is a remarkably constant feature round the gorge. The sequence was investigated by Holmes (1928) who identified the lower coal seams as the Brass Thill coals, the upper coal seam as the Low Main and the persistent sandstone, that forms the lip of the gorge, as the Low Main Post. The succession in the gorge is complicated by faulting and by conflicting evidence from adjacent abandoned colliery workings. Application of subsurface colliery data caused Hindson and Hopkins (1947) correctly to introduce a fault of some I 15m (50 ft) downthrow south on the north side of Prebends’ Bridge and they revised Holmes’s interpretation by calling the sandstone immediately below the Cathedral not the Low Main Post, but the sandstone above the Upper Brass Thill Coal. This interpretation was further discussed by Dunham and Hopkins (1958). During a re-survey of the eastern part of the gorge, F. A. Francis attempted to reconcile the exposures in the gorge with the colliery data and he proposed that the upper coal seam in the gorge was the Maudlin, the seam above the Low Main (Geological Survey one-inch scale Sheet 27, New Series, published 1965). In the memoir for Sheet 27, however, Francis revises his interpretation and states that the upper coal seam in the gorge is the Low Main in agreement with Holmes (Smith and Francis, 1967, p.70). New subsurface data from construction work at Hatfield College on the east side of the gorge was described by Whitworth (1972). The succession was proved and provided evidence for a north-south fault, here called Tom’s Fault after Dr
1: Department of Geological Sciences, University of Durham, South Road, Durham, DH 1 3LE.
2: British Geological Survey, Keyworth, Nottingham. NG 12 5GG. (Now at 67 Shaftesbury Avenue, Roundhay, Leeds. LS8 1DR)
Link to the site. www.dur.ac.uk/es.internal/virtearth/Continents/europe/uk/durham/Durham_Geology/GALJPaper_Pages/page84.htm
byG. A. L. JOHNSON1 and GORDON RICHARDSON2SUMMARY
Eleven stratigraphical boreholes put down near to the River Wear incised meander gorge at Durham City are described. The resulting interpretation of the stratigraphy of the region shows that the Middle Coal Measures (Upper Carboniferous, Westphalian) succession between the Hutton Coal and the Maudlin Coal is exposed. Two significant fault belts affect the rocks of the gorge. The north-east trending Prebends’ Bridge Fault, with a downthrow south of more than 1 5m, cuts the gorge obliquely and Tom’s Fault, which downthrows east 7.5m, runs almost north and south near to the axis of the meander. The fault lines disrupt the sequence of strata in the sides of the gorge and in particular, the Low Main Post sandstone and underlying coal seam which are recognized at the top of the gorge beneath Durham Cathedral, are replaced by the Maudlin sandstone and coal seam on the south side of the Prebends’ Bridge Fault. Contrary to the accepted interpretation, first published by Holmes (1928), the conspicuous sandstone and underlying coal seam that form the lip of the gorge are discontinuous. The marked lack of exposure of the 10m thick Low Main Post sandstone is proved to be caused by widespread medieval quarrying for building stone.
INTRODUCTION
Durham City has its origins in an ancient fortified settlement on a tight bend of the River Wear cut deep to form an incised meander gorge. The site is a natural stronghold, a promontory protected by the river and gorge on three sides so that only the narrow salient on the north had to be protected by fortifications (Figs 1 and 2). The Saxons developed a monastic settlement on the site in 995 and built a stone church to shelter the remains of St Cuthbert, a much revered 7th century Northumbrian Saint. After the Norman conquest the promontory was further fortified by Durham Castle on the north and a city wall was constructed round the lip of the gorge. Within the defended area, Durham Cathedral was built between 1093 and 1133 as the permanent resting place of St Cuthbert’s Shrine. The steep wooded banks of the river surrounding the promontory, though modified by quarrying, have survived to the present day and now form a unique amenity of river-side walks almost ringing the centre of the city.
The gorge is cut in bedrock Coal Measures (Upper Carboniferous) strata which are exposed, though much concealed by debris fill and vegetation. The general succession from the bottom is interbedded shale, sandstone and thin coal seams, overlain by a persistent coal seam that underlies the thick sandstone which forms the top of the gorge. The coal seam beneath the upper sandstone varies in thickness, but it is a remarkably constant feature round the gorge. The sequence was investigated by Holmes (1928) who identified the lower coal seams as the Brass Thill coals, the upper coal seam as the Low Main and the persistent sandstone, that forms the lip of the gorge, as the Low Main Post. The succession in the gorge is complicated by faulting and by conflicting evidence from adjacent abandoned colliery workings. Application of subsurface colliery data caused Hindson and Hopkins (1947) correctly to introduce a fault of some I 15m (50 ft) downthrow south on the north side of Prebends’ Bridge and they revised Holmes’s interpretation by calling the sandstone immediately below the Cathedral not the Low Main Post, but the sandstone above the Upper Brass Thill Coal. This interpretation was further discussed by Dunham and Hopkins (1958). During a re-survey of the eastern part of the gorge, F. A. Francis attempted to reconcile the exposures in the gorge with the colliery data and he proposed that the upper coal seam in the gorge was the Maudlin, the seam above the Low Main (Geological Survey one-inch scale Sheet 27, New Series, published 1965). In the memoir for Sheet 27, however, Francis revises his interpretation and states that the upper coal seam in the gorge is the Low Main in agreement with Holmes (Smith and Francis, 1967, p.70). New subsurface data from construction work at Hatfield College on the east side of the gorge was described by Whitworth (1972). The succession was proved and provided evidence for a north-south fault, here called Tom’s Fault after Dr
1: Department of Geological Sciences, University of Durham, South Road, Durham, DH 1 3LE.
2: British Geological Survey, Keyworth, Nottingham. NG 12 5GG. (Now at 67 Shaftesbury Avenue, Roundhay, Leeds. LS8 1DR)
Link to the site. www.dur.ac.uk/es.internal/virtearth/Continents/europe/uk/durham/Durham_Geology/GALJPaper_Pages/page84.htm