Editing Lower Carboniferous of Bewcastle and Gilsland - an excursion

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=== Locality 4, Irthing Gorge ===
 
=== Locality 4, Irthing Gorge ===
  
The Upper Border Group rocks to the north of Gilsland form part of the gently dipping eastern limb of the Bewcastle '''Anticline'''. A succession extending from below the Millerhill Limestone to above the Leahill Limestone is well exposed in the River Irthing between Gilsland and Shankend [NY 6877 7654]. This section is repeatedly cut by '''faults''' of the Gilsland Fault Belt. ''Some of these localities are on M.O.D. land; heed the warnings posted along the road.''
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The Upper Border Group rocks to the north of Gilsland form part of the gently dipping eastern limb of the Bewcastle Anticline. A succession extending from below the Miller-hill Limestone to above the Leahill Limestone is well exposed in the River Irthing between Gilsland and Shankend [NY 6877 7654]. This section is repeatedly cut by faults of the Gilsland Fault Belt. ''Some of these localities are on M.O.D. land; heed the warnings posted along the road.''
  
==== Locality 4A, Gilsland Gorge [NY 635 680] ====
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==== Locality 4A, Gilsland Gorge [NY 635 680] ====
  
Though the sections in the gorge are largely inaccessible they allow the opportunity to view the dominantly '''arenaceous''', middle part of the Upper Border Group. Descend the steep path to the north of the car park of the Gilsland Spa Hotel. Follow signs to the Popping Stone, crossing two suspension bridges. The development of the gorge is controlled by a series of northwest '''striking''', normal faults. The lowest exposed beds comprise calcareous sandstones [NY 6350 6904], overlain by approximately 35 m of mainly arenaceous strata. A sequence of Crammel Linn, Green Grove and Cohering Sandstones is exposed along the northern side of the gorge, where individual beds show marked lateral thickness variation. In the section west of the Popping Stone [NY 6350 6812] the upper part of the Crammel Linn Sandstone is almost entirely replaced by shales.
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Though the sections in the gorge are largely inaccessible they allow the opportunity to view the dominantly arenaceous, middle part of the Upper Border Group. Descend the steep path to the north of the car park of the Gilsland Spa Hotel. Follow signs to the Popping Stone, crossing two suspension bridges. The development of the gorge is controlled by a series of northwest striking, normal faults. The lowest exposed beds comprise calcareous sandstones [NY 6350 6904], overlain by approximately 35 m of mainly arenaceous strata. A sequence of Crammel Linn, Green Grove and Cohering Sandstones is exposed along the northern side of the gorge, where individual beds show marked lateral thickness variation. In the section west of the Popping Stone [NY 6350 6812] the upper part of the Crammel Linn Sandstone is almost entirely replaced by shales.
  
==== Locality 4B, West of Forster's Hill [NY 6525 7030] ====
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==== Locality 4B, West of Forster's Hill [NY 6525 7030] ====
  
From the road [NY 6418 7060] follow the deep gully of the unnamed burn southeastwards to the River Irthing. At the confluence, a series of isolated outcrops expose the 2 m thick, shale dominated sequence which lies below the Spy Rigg Sandstone. The Throssburnfoot Coal can be found about 1 m below the sandstone. Shales beneath the coal contain an abundant fauna including the freshwater bivalve ''Naidites obesus ''and primitive '''mollusc''' ''Bellerophon ''sp. This level represents the locally developed Irthing Shell Bed'. Thin calcareous shales within the section yield occasional brachiopods, gastropods and '''nautiloids'''.
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From the road [NY 64187 060] follow the deep gully of the unnamed burn southeastwards to the River Irthing. At the confluence, a series of isolated outcrops expose the 2 m thick, shale dominated sequence which lies below the Spy Rigg Sandstone. The Throssburnfoot Coal can be found about 1 m below the sandstone. Shales beneath the coal contain an abundant fauna including the freshwater bivalve ''Naidites obesus ''and primitive mollusc ''Bellerophon ''sp. This level represents the locally developed Irthing Shell Bed'. Thin calcareous shales within the section yield occasional brachiopods, gastropods and nautiloids.
  
In cliff exposures 100 m further downstream, [NY 6515 7020] a pair of Tertiary, '''tholeiitic dykes''' 1–2 m thick incline to the southwest. Shales to the south are baked and '''cleaved'''. In the cliff a thin coal becomes anthracitic and cindery close to the intrusion. At 1–1.5 m from the southern contact and parallel to it are bands of brecciated and decomposed '''dolerite'''. Occasional vesicles up to 0.5 mm in diameter are filled with clay minerals. The Spy Rigg Sandstone is well developed in the opposite bank where it is affected by a series of small faults.
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In cliff exposures room further downstream, [NY 65157 020] a pair of Tertiary, tholeiitic dykes 1–2 m thick incline to the southwest. Shales to the south are baked and cleaved. In the cliff a thin coal becomes anthracitic and cindery close to the intrusion. At 1–1.5 m from the southern contact and parallel to it are bands of brecciated and decomposed dolerite. Occasional vesicles up to 0.5 mm in diameter are filled with clay minerals. The Spy Rigg Sandstone is well developed in the opposite bank where it is affected by a series of small faults.
  
 
== [[Northumbrian rocks and landscape: a field guide#Glossary|Glossary]] ==
 
== [[Northumbrian rocks and landscape: a field guide#Glossary|Glossary]] ==

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