Editing Magnesian Limestone between South Shields and Seaham - an excursion

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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 (?60–100 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.
 
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.
  
=== Locality 4 [NZ 407 596], Roker promenade, Sunderland (40 mins) ===
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=== Locality 4 [NZ 407 596], Roker promenade, Sunderland (40 mins) ===
  
Park in any of several east–west residential roads off the coast road and (from NZ 4068 5961) proceed down the steps to the beach, noting the large blocks of Concretionary Limestone beside the steps, probably from Fulwell quarries 2 km to the west. Turn right at the bottom, to inspect the c.8 m cliff of Roker Dolomite dolostone. This is unevenly bedded and '''dips''' gently southwards; it is cream and buff, mainly finely oolitic, soft and porous, and most of the '''ooliths''' have hollow centres. Tabular '''cross-lamination''' is present and thin beds of mud- or silt-grade dolostone drape broad low-amplitude ripples. Several disturbed beds up to 0.6 m thick, probably debris-flows, lie on scoured surfaces. Moulds of ''Liebea ''and ''Schizodus ''occur in some beds. These rocks were probably formed high on the basin-margin slope, in well-oxygenated water of moderate energy.
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Park in any of several east–west residential roads off the coast road and (from NZ 40685961) proceed down the steps to the beach, noting the large blocks of Concretionary Limestone beside the steps, probably from Fulwell quarries 2 km to the west. Turn right at the bottom, to inspect the c.8 m cliff of Roker Dolomite dolostone. This is unevenly bedded and dips gently southwards; it is cream and buff, mainly finely oolitic, soft and porous, and most of the ooliths have hollow centres. Tabular cross-lamination is present and thin beds of mud- or silt-grade dolostone drape broad low-amplitude ripples. Several disturbed beds up to 0.6 m thick, probably debris-flows, lie on scoured surfaces. Moulds of ''Liebea ''and ''Schizodus ''occur in some beds. These rocks were probably formed high on the basin-margin slope, in well-oxygenated water of moderate energy.
  
The famous 'Cannon-Ball Rocks' are the second main feature of interest at Roker. They form a rounded mass against the promenade just north of the steps and comprise a tightly-packed assemblage of subspherical calcite concretions with patches of inter-concretion fine-grained buff dolomite. The concretions are up to 0.25 m in diameter and most are concentrically laminated and partly coarsely radially crystalline.
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The famous 'Cannon-Ball Rocks' are the second main feature of interest at Roker. They form a rounded mass against the promenade just north of the steps and comprise a tightly-packed assemblage of subspherical calcite concretions with patches of inter-concretion fine-grained buff dolomite. The concretions are up to 0.25 m in diameter and most are concentrically laminated and partly coarsely radially crystalline.
  
=== Locality 5 [NZ 357 576], Castletown river cliff (except at high tide); (40 mins) ===
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=== Locality 5 [NZ 357 576], Castletown river cliff (except at high tide); (40 mins) ===
  
Wellingtons can be an advantage in approaching this exposure. Park in Sunderland Enterprise Park [NZ 3578 5672] and take the footpath signposted 'Hylton Riverside' to the south-southeast through a narrow wooded valley to the riverside. Here the exposure on your left comprises Yellow Sands (6 m+) resting unconformably on Upper Coal Measures sandstone (2 m+). This is the only good exposure of the unconformity in the Sunderland area; it is an almost plane erosion surface and represents a time gap of at least 40 Ma. The underlying sandstone, except for the uppermost 0.3 m, has been reddened by desert weathering and is the youngest permanently exposed Carboniferous stratum in northeast England. The Yellow Sands is a typical desert dune formation; it is weakly cemented (but with patchy well-cemented nodules) in a parallel-laminated coarse-grained basal unit (c.1 m thick) and strongly trough cross-bedded in the remainder where it is medium- to coarse-grained and almost incohesive. The sand is cut by several minor faults and fissures, some of which harbour downward-tapering brown clay probably squeezed down from the Marl Slate when the faults and fissures were created.
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Wellingtons can be an advantage in approaching this exposure. Park in Sunderland Enterprise Park [NZ 35785 672] and take the footpath signposted 'Hylton Riverside' to the south-southeast through a narrow wooded valley to the riverside. Here the exposure on your left comprises Yellow Sands (6 m+) resting unconformably on Upper Coal Measures sandstone (2 m+). This is the only good exposure of the unconformity in the Sunderland area; it is an almost plane erosion surface and represents a time gap of at least 40 Ma. The underlying sandstone, except for the uppermost 0.3 m, has been reddened by desert weathering and is the youngest permanently exposed Carboniferous stratum in northeast England. The Yellow Sands is a typical desert dune formation; it is weakly cemented (but with patchy well-cemented nodules) in a parallel-laminated coarse-grained basal unit (c.1 m thick) and strongly trough cross-bedded in the remainder where it is medium- to coarse-grained and almost incohesive. The sand is cut by several minor faults and fissures, some of which harbour downward-tapering brown clay probably squeezed down from the Marl Slate when the faults and fissures were created.
  
=== Locality 6 [NZ 391 545] Tunstall Hills S.S.S.I. (40 mins) ===
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=== Locality 6 [NZ 391 545] Tunstall Hills S.S.S.I. (40 mins) ===
  
Approach by the track from Tunstall Road [NZ 3895 5464], parking at [NZ 3916 5456 or [NZ 3912 5452]. The rock here is massive brown reef limestone of the Ford Formation and was formed near the seaward crest of the reef. It comprises a sparse framework of filter-feeding fanlike ''(Fenestella, Synocladia) ''and twiggy ''(Acanthocladia, Dyscritella) '''''bryozoans''' that were fixed to the substrate or to each other, and the remains of other marine organisms (mainly bivalves, '''brachiopods''' and '''gastropods''') that lived in the protected spaces between the bryozoans or were attached to them. High on the main face, twiggy bryozoans are thickly encrusted with concentrically finely laminated limestone that may be '''algal'''. Another feature of the main face is a number of steeply inclined contemporaneous tension cracks, some of which have been filled by laminated limestone whereas others have remained partly unfilled and have yielded coarse frosted wind-blown sand grains.
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Approach by the track from Tunstall Road [NZ 38955 464], parking at [NZ 3916 5456 or [NZ 3912 5452]. The rock here is massive brown reef limestone of the Ford Formation and was formed near the seaward crest of the reef. It comprises a sparse framework of filter-feeding fanlike ''(Fenestella, Synocladia) ''and twiggy ''(Acanthocladia, Dyscritella) ''bryozoans that were fixed to the substrate or to each other, and the remains of other marine organisms (mainly bivalves, brachiopods and gastropods) that lived in the protected spaces between the bryozoans or were attached to them. High on the main face, twiggy bryozoans are thickly encrusted with concentrically finely laminated limestone that may be algal. Another feature of the main face is a number of steeply inclined contemporaneous tension cracks, some of which have been filled by laminated limestone whereas others have remained partly unfilled and have yielded coarse frosted wind-blown sand grains.
  
The ridge extending southeastwards from here is the surface expression of the comparatively resistant reef rock; lower land to the east of the ridge corresponding with the basin which here was at least 60 m deep. The reef is more than 300 m wide, but its southwestern margin was removed during the last (i.e. late Devensian) ice age when Glacial Lake Wear overflowed southeastwards and cut the spectacular channel of Tunstall Hope.
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The ridge extending southeastwards from here is the surface expression of the comparatively resistant reef rock; lower land to the east of the ridge corresponding with the basin which here was at least 60 m deep. The reef is more than 300 m wide, but its southwestern margin was removed during the last (i.e. late Devensian) ice age when Glacial Lake Wear overflowed southeastwards and cut the spectacular channel of Tunstall Hope.
  
=== Locality 7 [NZ 43 49], Seaham S.S.S.I. (parts covered at highest tides) (60 mins, hard hats are advisable.) ===
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=== Locality 7 [NZ 43 49], Seaham S.S.S.I. (parts covered at highest tides) (60 mins, hard hats are advisable.) ===
  
Park in the car park [NZ 430 494] and walk via the northwest corner of North Dock [NZ 432 495] and the cobbled path northwards down to the beach. Follow the beach for c. 150 m to the north-northwest, noting the industrial debris, to cliffs [NZ 4306 4966]–[NZ 4303 3975] where the southeast-dipping sequence is shown in [[:File:YGS_NORTROCK_FIG_13_5.jpg|Figure 13.5]]. This is one of the best places in Britain for seeing an evaporite dissolution residue.
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Park in the car park [NZ 430 494] and walk via the northwest corner of North Dock [NZ 432 495] and the cobbled path northwards down to the beach. Follow the beach for c. 150 m to the north-northwest, noting the industrial debris, to cliffs [NZ 4306 4966]–[NZ 43033975] where the southeast-dipping sequence is shown in [[:File:YGS_NORTROCK_FIG_13_5.jpg|Figure 13.5]]. This is one of the best places in Britain for seeing an evaporite dissolution residue.
  
 
The harbour was cut into a headland of Seaham Formation limestones, which are well exposed in several large faces. Most of the limestones were originally fine-grained and thinly bedded, and many contain large numbers of ''Liebea'', ''Schizodus'' and ''Calcinema''; they are finely cross-bedded and rippled, and some are graded. Changes to some of the limestones, probably during deep burial, resulted in the creation of thick beds full of spectacular calcite concretions not unlike those at Locality 2 but without the distinctive fine lamination of the latter.
 
The harbour was cut into a headland of Seaham Formation limestones, which are well exposed in several large faces. Most of the limestones were originally fine-grained and thinly bedded, and many contain large numbers of ''Liebea'', ''Schizodus'' and ''Calcinema''; they are finely cross-bedded and rippled, and some are graded. Changes to some of the limestones, probably during deep burial, resulted in the creation of thick beds full of spectacular calcite concretions not unlike those at Locality 2 but without the distinctive fine lamination of the latter.
  
Seaham Formation (8 m+). Slightly dislocated thin- to thick-bedded mainly fine-grained buff and grey limestone with abundant ''Liebea'', ''Schizodus'' and ''Calcinema'' (a small stick-like ?alga) in some beds.
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Seaham Formation (8 m +). Slightly dislocated thin- to thick-bedded mainly fine-grained buff and grey limestone with abundant ''Liebea'', ''Schizodus'' and ''Calcinema'' (a small stick-like ?alga) in some beds.
  
SR 1–3: Seaham Residue (6–9 m). The insoluble remains of the Fordon Evaporite Formation, here otherwise dissolved. Comprises lower (SR1) and upper (SR3) units of heterogeneous calcareous clay and clayey limestone and a dislocated median unit (2 m) of white to buff oolitic limestone.
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SR 1–3: Seaham Residue (6–9 m). The insoluble remains of the Fordon Evaporite Formation, here otherwise dissolved. Comprises lower (SR1) and upper (SR3) units of heterogeneous calcareous clay and clayey limestone and a dislocated median unit (2m) of white to buff oolitic limestone.
  
 
Roker Dolomite Formation, top of. Cream and buff finely oolitic dolostone, partly fractured and altered to limestone.
 
Roker Dolomite Formation, top of. Cream and buff finely oolitic dolostone, partly fractured and altered to limestone.
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{{EWwalks}}
 
{{EWwalks}}
  
[[Category:Northumbrian rocks and landscape: a field guide ]]
 
 
[[Category:7. Northern England]]
 
[[Category:7. Northern England]]

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