Stephenson, D., Loughlin, S.C., Millward, D., Waters, C.N. & Williamson, I.T. 2003. Carboniferous and Permian Igneous Rocks of Great Britain North of the Variscan Front. Geological Conservation Review Series, No. 27, JNCC, Peterborough, ISBN 1 86107 497 2.
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S.C. Loughlin
In the Roman Wall region, between Newcastle-upon-Tyne and Carlisle, the Great Whin Sill forms a spectacular north-facing scarp that extends for over 25 km to the north of the River South Tyne. The scarp forms a natural barrier that provided an ideal site for the construction of a substantial length of Hadrian's Wall and several important Roman forts
The Steel Rigg to Sewingshields Crags GCR site extends from the car park at Steel Rigg
The geology of the area was described briefly by Wallis (1769), Winch (1817) and Tate (1867, 1868) in their accounts of the geology of Northumberland. Baked sedimentary rocks above the sill in the Roman Wall area were important features in the summary of evidence by Topley and Lebour (1877) that finally established the intrusive origin of the sill. The Geological Survey mapped central and south Northumberland in the 1870s and six-inch sheets 106NE and 106SE were published in 1881. Smythe (1930a) described the transgression of the sill at this site in his extensive study of the geochemistry of the Whin Sill-complex. Further descriptions of the site and a detailed account of the Carboniferous rocks into which the sill is intruded were produced by Johnson (1959). A revision survey was completed by the Geological Survey in 1975 and 1:50 000 Sheet 13 (Bellingham) was published in 1980, together with a memoir (Frost and Holliday, 1980). The results of detailed ground magnetic surveys that clarify the structure of the sill at depth were described by Cornwell and Evans (1986). Descriptions are included in field guides of the area, including those by Jones (in Scrutton, 1995) and Johnson (1997).
The north-facing scarp formed by the sill in this area is arguably the most impressive landscape feature of the whole Whin Sill-complex
Several transgressions occur close together within this GCR site. They appear to take place along small faults, which are visible in places as truncations in the sedimentary rock outcrops above the sill (Johnson, 1959), but are commonly hidden beneath thick drift deposits. In the western part of the site the sill is intruded into the Shotto Wood Limestone (Scar Limestone of Johnson, 1959). The sill and limestone form a composite dip-slope for several kilometres, along which vegetation on the differing rock-types contrasts sharply in colour and variety (Tate, 1868; Frost and Holliday, 1980). The sill escarpment is heavily indented, probably as a result of small faults that have fractured and shattered the dolerite. The most westerly indentation
The sill at this site is composed of homogenous quartz-dolerite with thin chilled margins at the top and base (less than 0.5 m). The thickness of the sill varies from 20 m to 50 m. It shows well-developed columnar jointing, particularly at Sewingshields Crags
Alteration of the sill typically includes chloritization along joint planes and there are also some good examples of pectolitization. Pectolite (a zeolite) occurs at several places in the Whin Sill-complex but is particularly abundant in the vicinity of the Roman Wall. It generally takes the form of thin veins associated with calcite and also occurs in amygdales (Smythe, 1930a). These are surrounded by an aureole up to 1.5 cm wide of light-green altered dolerite in which further clusters of pectolite crystals can be observed. In other places amygdales comprise quartz and calcite.
The origin of the Whin Sill-complex was debated at length early in the 19th century. Early workers considered that it represented lava flows (e.g. Phillips, 1836; Hutton, 1838) but Tate (1867, 1868) recognized the evidence for intrusion in the vicinity of the Roman Wall, based on the metamorphism of the overlying strata. In their definitive discussion on the intrusive nature of the Whin Sill-complex, Topley and Lebour (1877) cited Sewingshields Crags as one of the better places to see evidence of contact metamorphism at the upper contact.
Since drift deposits at this site commonly cover areas crucial to the understanding of the sill transgression and its relationship to faulting, magnetic surveys have been used to work out the structure of the sill at depth. Strong magnetic anomalies occur along outcrops of the sill and also down-dip, where the intrusions can be detected at depths of up to several hundred metres below the surface. The Roman Wall area reveals numerous complicated anomalies which have been investigated by several groups (e.g. Summers et al., 1982; Cornwell and Evans, 1986). These studies reveal dearly the segmented nature of the sill and, in a study of the Hexham area, Cornwell and Evans identified many magnetic lineaments, interpreted as faults or joints that are not necessarily visible at the surface. Based on magnetic evidence within this GCR site, the main offset at Busy Gap is seen to coincide with a NW-trending lineament that swings to the ESE about 4 km south of the outcrop. A major offset in the sill at Limestone Corner, 10 km to the east of the site, is controlled by a similar structure (Cornwell and Evans, 1986).
The relationship of the sill to faulting in the region has been of interest to many authors. In the adjacent area east of the River North Tyne, Randall (1959) observed transgressions and faulting but was unable to ascertain the exact age relationship, although he considered that the faults could not be younger than the intrusion. Johnson (1959) considered the transgressive steps at this GCR site to be fault-controlled. The predominant fault trend in the region is ENE, with a secondary ESE trend. These have been interpreted as conjugate shears formed during a period of E–W-trending compression, after the main north–south Variscan compression, but before intrusion of the Whin Sill-complex (Frost and Holliday, 1980). However, Frost and Holliday also suggested that the faulting and intrusion could be contemporaneous. Cornwell and Evans (1986) interpreted the segmented nature of the sill in this region as evidence of fault and/or joint control on the form of the intrusion. They were unable to determine the age of faulting from magnetic evidence but considered it probable that a major component was in existence at the time of intrusion. The minor indentations in the scarp suggest some faulting after emplacement of the sill and the largest indentation acted as a water channel which drained a lake to the north of the sill at the end of the last glaciation. Crag Lough is a remnant of this glacial lake, which formed in an ice-scoured basin (Johnson, 1997).
The St Oswald's Chapel Dyke, a member of the dyke-swarm associated with the sill-complex
The Steel Rigg to Sewingshields Crags GCR site demonstrates very well the dramatic effect that the Great Whin Sill has on the landscape, here forming a substantial north-facing scarp upon which Hadrian's Wall and several Roman forts were built. The scarp is offset at several places as a result of transgression of the sill between different levels within the sedimentary succession of the Visean Liddesdale Group. These transgressions have been the subject of much discussion, as the role of faulting is not always clear. Geophysical methods tend to suggest that some of the transgressions are fault-controlled and that magma moved along pre-existing fault zones. Small later faults cut the sill forming minor indentations in the outcrop; one of these acted as a channel when water drained from a glacial lake on the north side of the scarp at the end of the last glaciation.
The site was important during the early debate on the origins of the Whin Sill as it provides abundant evidence of the contact metamorphism of overlying sedimentary rocks, thus proving that it is an intrusion and not a lava flow. Other features of this site, typical of sills in general, include a large raft of baked limestone near the base of the sill and well-developed columnar jointing.