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
Longhoughton Quarry, 4 km north-east of Alnwick in Northumberland, is located at the western end of a number of quarries collectively known as 'Howlet Hill Quarry', which were formerly worked for sets and road metal
The area was first surveyed between 1860 and 1864 by the Geological Survey and, following revision of the six-inch maps in the 1920s, was published at the one-inch scale in 1930 as Sheet 6 (Alnwick). The quarries were described by Carruthers et al. (1930) in the memoir that accompanied the published map, and the timing of faulting and intrusion in this area was discussed by Jones et al. (1980), Turner et al. (1995) and Chadwick et al. (1995).
A 20 m-high face at the north end of Longhoughton Quarry exposes the Great Whin Sill, which is intruded into sandstones and mudstones just below the Great Limestone. The overlying strata are thermally metamorphosed, and rafts of the overlying sandstones and mudstones, incorporated into the upper parts of the sill and prominent in this quarry, have been recrystallized. The chilled, fine-grained to glassy upper margin of the sill is exposed all along the eastern side of the workings. Just below the upper margin is a zone in which bands of small amygdales occur. The main part of the sill at this site is typical of the Great Whin Sill and has well-developed columnar jointing. It comprises homogeneous quartz-dolerite containing plagioclase, clinopyroxene, magnetite-ilmenite, quartz, orthopyroxene and small amounts of biotite, hornblende and carbonate. There is no evidence of a pegmatitic zone or segregation veins. A small fault at the western side of the quarry has exposed the basal margin and underlying indurated sandy mudstones. The basal contact can also be seen 300 m north-east of the summit of Howlet Hill, where thermally metamorphosed sandstones and mudstones are exposed at the base of a quartz-dolerite crag.
Just 0.5 km to the west of this GCR site the sill transgresses upwards through sandstones and mudstones and is intruded fully into the Great Limestone. A further 1 km to the west is an isolated outcrop of quartz-dolerite that was intruded above the Great Limestone.
Immediately to the south of the site, the sill cuts across the E–W-trending Longhoughton Fault with no apparent offset. The fault has undergone over 1.5 km of lateral movement (Carruthers et al., 1930) although the Great Limestone outcrops on either side of the fault happen to coincide
Tate (1868) studied many outcrops of the Great Whin Sill throughout Northumberland and although Longhoughton Quarry was not so extensive at that time, he described the dolerite at nearby Ratcheugh Quarry as 'porphyritic' with large feldspar crystals scattered through the outcrop. His studies provided abundant evidence for the intrusive nature of the Great Whin Sill and led to a general acceptance. In particular, chilling of the upper margin of the sill, incorporation of rafts of the overlying sedimentary rock and thermal metamorphism of the overlying strata, all evidence of intrusion, are demonstrated in spectacular fashion at this GCR site.
The relationship of the Great Whin Sill to the Longhoughton Fault here shows that the sill postdates the main fault movement. The sill cuts directly across the fault plane with no offset. Nevertheless, horizontal slickensides and shearing on some vertical faces in the quarries imply that there was also some late-stage strike-slip movement along related fractures after emplacement of the sill (Jones et al., 1980). This is valuable evidence to add to that from elsewhere that E–W-trending faulting occurred before intrusion, such as quartz-dolerite dykes intruded locally along faults (see Wydon GCR site report) and transgression of the sill along fault planes (see Steel Rigg to Sewingshields Crags GCR site report).
The Longhoughton Quarry GCR site shows abundant features that prove the intrusive origin of the Great Whin Sill, such as the metamorphism of overlying strata, chilling of the upper margin and rafts incorporated from the overlying sedimentary strata. In addition, the relationship of the sill to the nearby Longhoughton Fault provides evidence relating to the sequence of events in northern England in Late Carboniferous times. The intrusion of the sill clearly post-dates the main movement on the E–W-trending fault, but shearing of the quartz-dolerite and the presence of horizontal slicken-sides on vertical rock faces show that there was also some strike-slip (lateral) movement after emplacement of the sill.