Editing Magnesian Limestone between South Shields and Seaham - an excursion

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== Excursion details ==
 
== Excursion details ==
  
=== Locality 1 [NZ 384 667], Trow Point ('''S.S.S.I.''', no hammering), South Shields (1 hour) ===
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=== Locality 1 [NZ 384 667], Trow Point ('''S.S.S.I.''', no hammering), South Shields (1 hour) ===
  
Park in Trow Lea car park [NZ 383 667], near The Water's Edge. This complex exposure is summarized in [[:File:YGS_NORTROCK_FIG_13_4.jpg|Figure 13.4]].
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Park in Trow Lea car park [NZ 383 667], near The Water's Edge. This complex exposure is summarized in [[:File:YGS_NORTROCK_FIG_13_4.jpg|Figure 13.4]].
  
=== Locality 2 [NZ 385 641], Cleadon Park Quarry, northeast corner (30 mins) ===
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=== Locality 2 [NZ 385 641], Cleadon Park Quarry, northeast corner (30 mins) ===
  
Park in Quarry Lane, near the junction with Larch Avenue; the face is adjacent. Keep within 100 m of the road. This 3–5 m vertical face is in about the middle of the Concretionary Limestone Formation. Most of the rock is finely laminated unfossiliferous spherulitic limestone but a few thin graded beds are present and these contain moulds of the '''bivalves''' ''Liebea'' and ''Schizodus''. The spherulites were formed by the recrystallization of the rock whilst it was deeply buried, and are up to about 8 cm across. Some have been slightly rotated and fractured by dissolution. Small patches of buff powdery '''dolomite''' lie between many of the spherulites and, in the southeast corner of the face, all the limestone locally passes laterally into soft buff '''dolostone'''. These rocks were probably formed on the low–middle part of the basin-margin slope, in anoxic conditions under perhaps 120–200 m of stratified sea water. The laminites were built up of ?annual couplets of '''pelagic''' lime mud (winter) and '''phytoplankton''' (summer); the graded beds are probably '''turbidites''', composed of lime mud and lime silt that was originally deposited in oxygenated shallower water on the shelf or higher on the slope and redistributed into the basin by turbid suspension currents.
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Park in Quarry Lane, near the junction with Larch Avenue; the face is adjacent. Keep within 100 m of the road. This 3–5 m vertical face is in about the middle of the Concretionary Limestone Formation. Most of the rock is finely laminated unfossiliferous spherulitic limestone but a few thin graded beds are present and these contain moulds of the '''bivalves''' ''Liebea'' and ''Schizodus''. The spherulites were formed by the recrystallization of the rock whilst it was deeply buried, and are up to about 8 cm across. Some have been slightly rotated and fractured by dissolution. Small patches of buff powdery '''dolomite''' lie between many of the spherulites and, in the southeast corner of the face, all the limestone locally passes laterally into soft buff '''dolostone'''. These rocks were probably formed on the low–middle part of the basin-margin slope, in anoxic conditions under perhaps 120–200 m of stratified sea water. The laminites were built up of ?annual couplets of '''pelagic''' lime mud (winter) and '''phytoplankton''' (summer); the graded beds are probably '''turbidites''', composed of lime mud and lime silt that was originally deposited in oxygenated shallower water on the shelf or higher on the slope and redistributed into the basin by turbid suspension currents.
  
=== Locality 3 [NZ  398 651], northwest end of Marsden Bay, an S.S.S.I. (50 mins, hard hats essential) ===
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=== Locality 3 [NZ 398 651], northwest end of Marsden Bay, an S.S.S.I. (50 mins, hard hats essential) ===
  
Park at the northwest end of Marsden Lea car park [NZ 397 651] then take the steps to the beach, turning right at the bottom and walk southeast (no farther than the tall narrow stack). The cliffs here are formed mainly of 16–20 m of cream and buff fine-grained dolostone of the Concretionary Limestone Formation. Concentrate for the first 135 m on the general appearance of the rock face from a distance, for this is one of the best places in Britain for seeing the effects of foundering caused by the dissolution of underlying '''evaporites'''. All the strata have foundered by about the same amount (?60–100 m) but some parts have been let down gently and without much dislocation whereas others have had a more complex history of subsidence and are intensely fractured (''''breccia'''-gashes'). After the first 135 m inspect the rock in detail. As at Locality 2, it comprises a mixture of finely laminated and unlaminated rock, here mainly fine-grained buff dolostone but also includes unlaminated '''oolite'''; some of the unlaminated beds, including the oolite, contain moulds of ''Liebea, Permophorus ''and ''Schizodus, ''many are graded and some have tight folds and shear-planes caused by downslope slumping and sliding. The inferred depositional environment is as for Locality 2.
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Park at the northwest end of Marsden Lea car park [NZ 397 651] then take the steps to the beach, turning right at the bottom and walk southeast (no farther than the tall narrow stack). The cliffs here are formed mainly of 16–20 m of cream and buff fine-grained dolostone of the Concretionary Limestone Formation. Concentrate for the first 135 m on the general appearance of the rock face from a distance, for this is one of the best places in Britain for seeing the effects of foundering caused by the dissolution of underlying '''evaporites'''. All the strata have foundered by about the same amount (?60– oom) but some parts have been let down gently and without much dislocation whereas others have had a more complex history of subsidence and are intensely fractured (''''breccia'''-gashes'). After the first 135 m inspect the rock in detail. As at Locality 2, it comprises a mixture of finely laminated and unlaminated rock, here mainly fine-grained buff dolostone but also includes unlaminated '''oolite'''; some of the unlaminated beds, including the oolite, contain moulds of ''Liebea, Permophorus ''and ''Schizodus, ''many are graded and some have tight folds and shear-planes caused by downslope slumping and sliding. The inferred depositional environment is as for Locality 2.
  
 
=== Locality 4 [NZ 407 596], Roker promenade, Sunderland (40 mins) ===
 
=== Locality 4 [NZ 407 596], Roker promenade, Sunderland (40 mins) ===

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