Editing Carboniferous and Permian rocks between Tynemouth and Seaton Sluice - an excursion

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At low tide the next locality, Hartley Bay, can be reached by walking along the foreshore if time permits, but the cliff top path is quicker and easier.
 
At low tide the next locality, Hartley Bay, can be reached by walking along the foreshore if time permits, but the cliff top path is quicker and easier.
  
=== Locally 5, Hartley Bay [NZ 344 758] ===
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=== Locally 5, Hartley Bay [NZ 344 758] ===
  
 
As an alternative to the cliff top path between St Mary's Island and Collywell Bay, limited parking is available above Hartley Bay by the caravan club park at Old Hartley.
 
As an alternative to the cliff top path between St Mary's Island and Collywell Bay, limited parking is available above Hartley Bay by the caravan club park at Old Hartley.
  
Steps lead down to the beach at the southern end of the bay. The exposures in Hartley Bay have been relatively unaffected by faulting and include all the strata between the 1.7 m thick Northumberland Low Main Coal seam and the 1 m thick Five-Quarter seam ([[:File:YGS_NORTROCK_FIG_08_2.jpg|Figure 8.2]]). The Northumberland Low Main seam can be seen below the steps, and by the concrete support wall at the bottom of the steps where it is partly covered by cliff fall material. The seam can be traced at intervals along the base of the cliff where it is underlain by a pyrite-bearing seatearth and fine-grained sandstone. The sandstone contains ripple cross-lamination, with scattered burrows, '''bioturbation''' and shallow scour features. Rippled bedding surfaces and plant fossils occur in some of the fallen sandstone blocks at the foot of the cliff; the cliff face itself provides excellent exposures of two typical coarsening-upward coal-bearing sequences. These gradually decline to foreshore level approaching Crag Point, at the northern end of Hartley Bay, due to the shallow northeasterly dip. A 13 cm thick coal seam at the cliff base on the promontory immediately north of the steps in Hartley Bay, is overlain by a mussel band containing '''ostracodes''' and ''Spirorbis ''(worm tubes). About 1 m below the coal seam is a thin sandstone characterized by convolute laminations, attributed to reactivation of the nearby Ninety Fathom Fault.
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Steps lead down to the beach at the southern end of the bay. The exposures in Hartley Bay have been relatively unaffected by faulting and include all the strata between the 1.7 m thick Northumberland Low Main Coal seam and the t m thick Five-Quarter seam ([[:File:YGS_NORTROCK_FIG_08_2.jpg|Figure 8.2]]). The Northumberland Low Main seam can be seen below the steps, and by the concrete support wall at the bottom of the steps where it is partly covered by cliff fall material. The seam can be traced at intervals along the base of the cliff where it is underlain by a pyrite-bearing seatearth and fine-grained sandstone. The sandstone contains ripple cross-lamination, with scattered burrows, bioturbation and shallow scour features. Rippled bedding surfaces and plant fossils occur in some of the fallen sandstone blocks at the foot of the cliff; the cliff face itself provides excellent exposures of two typical coarsening-upward coal-bearing sequences. These gradually decline to foreshore level approaching Crag Point, at the northern end of Hartley Bay, due to the shallow northeasterly dip. A 13 cm thick coal seam at the cliff base on the promontory immediately north of the steps in Hartley Bay, is overlain by a mussel band containing ostracodes and ''Spirorbis ''(worm tubes). About 1 m below the coal seam is a thin sandstone characterized by convolute laminations, attributed to reactivation of the nearby Ninety Fathom Fault.
  
Continuing northwards along the foreshore toward Crag Point the wave cut sandstone platform contains some rectangular hollows about 1 m deep, now partially filled by wave reworked pebbles, cobbles and boulders. These hollows are thought to be the bottom of old bell-pits, dug from the cliff top during Napoleonic times to mine the thick Northumberland Low Main Coal seam beneath the sandstone. The cliff top must then have extended much further seaward. At the back of the bay a small sandbody about halfway up the cliff face contains well developed southerly dipping lateral accretion surfaces indicative of point bar deposition within a small meandering channel. Evidence of faulting can be seen in the lower part of the cliff approaching the northernmost end of Hartley Bay which is defined by the prominent, but unstable cliffs at Crag Point (Figure 8.1). The sharp contrast between sandstone and shale here is due to the Crag Point Fault, which downthrows strata 15 m to the north. The cliffs consist of two channel sandstones: a lower fine-grained sandstone characterized by wedge and trough cross-bedding with ripple cross-lamination and horizontal laminations, and an upper coarse-grained, predominantly planar cross-bedded sandstone ([[:File:YGS_NORTROCK_FIG_08_2.jpg|Figure 8.2]]) parts of which have collapsed making access around the point difficult. The contact between the sandstones is a well defined erosion surface overlain by a conglomerate containing small quartz pebbles, coal clasts and plant material.
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Continuing northwards along the foreshore toward Crag Point the wave cut sandstone platform contains some rectangular hollows about 1 m deep, now partially filled by wave reworked pebbles, cobbles and boulders. These hollows are thought to be the bottom of old bell-pits, dug from the cliff top during Napoleonic times to mine the thick Northumberland Low Main Coal seam beneath the sandstone. The cliff top must then have extended much further seaward. At the back of the bay a small sandbody about halfway up the cliff face contains well developed southerly dipping lateral accretion surfaces indicative of point bar deposition within a small meandering channel. Evidence of faulting can be seen in the lower part of the cliff approaching the northernmost end of Hartley Bay which is defined by the prominent, but unstable cliffs at Crag Point (Figure 8.1). The sharp contrast between sandstone and shale here is due to the Crag Point Fault, which downthrows strata 15 m to the north. The cliffs consist of two channel sandstones: a lower fine-grained sandstone characterized by wedge and trough cross-bedding with ripple cross-lamination and horizontal laminations, and an upper coarse-grained, predominantly planar cross-bedded sandstone ([[:File:YGS_NORTROCK_FIG_08_2.jpg|Figure 8.2]]) parts of which have collapsed making access around the point difficult. The contact between the sandstones is a well defined erosion surface overlain by a conglomerate containing small quartz pebbles, coal clasts and plant material.
  
=== Locality 6, Seaton Sluice [NZ 338 769] ===
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=== Locality 6, Seaton Sluice [NZ 338 769] ===
  
Turn off the A193 coast road [NZ 337 768] by the road bridge across Seaton Burn at Seaton Sluice to the car park in front of The King's Arms. Cross the wooden footbridge over 'the cut', a deep vertical trench cut through sandstone. Turn right down the footpath and proceed on to the old wharf on the north side of the cut, which is 274 x 9 by 16 m deep. The cut was excavated in 1761–64 by Thomas Delaval when Seaton Sluice was a busy industrial area and coal port, to provide a new harbour entrance, especially for coal barges. Two sandstones are exposed along the foreshore and in the cut, but northwards they dip into the subsurface. The lower, which has been correlated with the Lower Crag Point Sandstone, is a fine-grained, '''argillaceous''' sandstone characterized by lenticular bed geometries and predominantly wedge and trough shaped cross-bedding deposited by currents flowing to the southeast ([[:File:YGS_NORTROCK_FIG_08_2.jpg|Figure 8.2]]). The upper sandstone, in comparison, is much coarser-grained and more '''feldspathic''', with anomalous concentrations of '''garnet'''. It is characterized by sheet-like bed geometries, and structured internally by planar and less commonly trough cross-bedding arranged in cosets up to 50 cm thick. At the seaward end of the cut on the north side, troughs migrated down the lee faces of some larger scale bars or sandwaves. Aggradation of these bars, possibly as part of large sandflat complexes, can be compared with the development of medial and lateral sand bars forming today in sandy braided river systems. The base of the sandstone is defined by an erosion surface, locally overlain by small pebbles and granules of quartz, '''siderite''', coal clasts and plant material, which has removed all of the intervening Bensham Coal seam and associated strata from this part of the succession ([[:File:YGS_NORTROCK_FIG_08_2.jpg|Figure 8.2]]). The Upper Seaton Sluice Sandstone has been correlated with the Upper Crag Point Sandstone and unlike other channel sandbodies in the succession it was deposited by a braided river system flowing westwards, across rather than down the local and regional southerly palaeoslope. It is thought to have been deposited in response to a sudden lowering of depositional base level due to fault controlled uplift of the Fame '''Granite''' and older Carboniferous strata located not more than 80 km off the present day Northumberland coast.
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Turn off the A193 coast road [NZ 337 768] by the road bridge across Seaton Burn at Seaton Sluice to the car park in front of The King's Arms. Cross the wooden footbridge over 'the cut', a deep vertical trench cut through sandstone. Turn right down the footpath and proceed on to the old wharf on the north side of the cut, which is 274 x 9 by 16 m deep. The cut was excavated in 1761–64 by Thomas Delaval when Seaton Sluice was a busy industrial area and coal port, to provide a new harbour entrance, especially for coal barges. Two sandstones are exposed along the foreshore and in the cut, but northwards they dip into the subsurface. The lower, which has been correlated with the Lower Crag Point Sandstone, is a fine-grained, argillaceous sandstone characterized by lenticular bed geometries and predominantly wedge and trough shaped cross-bedding deposited by currents flowing to the southeast ([[:File:YGS_NORTROCK_FIG_08_2.jpg|Figure 8.2]]). The upper sandstone, in comparison, is much coarser-grained and more feldspathic, with anomalous concentrations of garnet. It is characterized by sheet-like bed geometries, and structured internally by planar and less commonly trough cross-bedding arranged in cosets up to 50 cm thick. At the seaward end of the cut on the north side, troughs migrated down the lee faces of some larger scale bars or sandwaves. Aggradation of these bars, possibly as part of large sandflat complexes, can be compared with the development of medial and lateral sand bars forming today in sandy braided river systems. The base of the sandstone is defined by an erosion surface, locally overlain by small pebbles and granules of quartz, siderite, coal clasts and plant material, which has removed all of the intervening Bensham Coal seam and associated strata from this part of the succession ([[:File:YGS_NORTROCK_FIG_08_2.jpg|Figure 8.2]]). The Upper Seaton Sluice Sandstone has been correlated with the Upper Crag Point Sandstone and unlike other channel sandbodies in the succession it was deposited by a braided river system flowing westwards, across rather than down the local and regional southerly palaeoslope. It is thought to have been deposited in response to a sudden lowering of depositional base level due to fault controlled uplift of the Fame Granite and older Carboniferous strata located not more than 80 km off the present day Northumberland coast.
  
 
== [[Northumbrian rocks and landscape: a field guide#Glossary|Glossary]] ==
 
== [[Northumbrian rocks and landscape: a field guide#Glossary|Glossary]] ==
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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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