Editing Magnesian Limestone between South Shields and Seaham - an excursion

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The highly varied rocks to be seen on this excursion were all formed during the last few million years of the Permian Period and comprise the Yellow Sands Formation and the internationally known and spectacular Magnesian Limestone. The sequence is shown in [[:File:YGS_NORTROCK_FIG_13_1.jpg|Figure 13.1]].
 
The highly varied rocks to be seen on this excursion were all formed during the last few million years of the Permian Period and comprise the Yellow Sands Formation and the internationally known and spectacular Magnesian Limestone. The sequence is shown in [[:File:YGS_NORTROCK_FIG_13_1.jpg|Figure 13.1]].
  
Most of the Permian Period in northwest Europe, including County Durham, was dominated by erosion, uplift and reddening of Carboniferous and earlier rocks that had been '''faulted''' and gently '''folded''' by the late Carboniferous '''Variscan''' earth movements. During this time, perhaps for 40 '''Ma''', the region drifted slowly northwards from the wet equatorial belt to the dry trade wind belt, where it formed part of one of the great deserts of world history. A mature desert land surface — a peneplain, now represented by the '''unconformity''' — and the patchy aeolian Yellow Sands (?360–355 Ma old), are all that remains of this prolonged episode.
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Most of the Permian Period in northwest Europe, including County Durham, was dominated by erosion, uplift and reddening of Carboniferous and earlier rocks that had been '''faulted''' and gently '''folded''' by the late Carboniferous '''Variscan''' earth movements. During this time, perhaps for 40 '''Ma''', the region drifted slowly northwards from the wet equatorial belt to the dry trade wind belt, where it formed part of one of the great deserts of world history. A mature desert land surface — a peneplain, now represented by the '''unconformity''' — and the patchy aeolian Yellow Sands (?360–355 Ma old), are all that remains of this prolonged episode.
  
Subsidence of a broad belt extending from the ancestral Pennines eastwards to Lithuania and Poland created a vast inland drainage basin during the desert phase. A dramatic change of scene late in the Permian period took place when the Boreal Ocean, perhaps following a '''glacioeustatic''' sea-level rise, broke in from the north, flooding the inland desert basin and instantly (in geological terms — perhaps 5 to 15 years) forming the tropical Zechstein Sea. The middle of this sea was probably initially 200–300 m deep, but was almost completely filled with salts by the end of the period.
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Subsidence of a broad belt extending from the ancestral Pennines eastwards to Lithuania and Poland created a vast inland drainage basin during the desert phase. A dramatic change of scene late in the Permian period took place when the Boreal Ocean, perhaps following a '''glacioeustatic''' sea-level rise, broke in from the north, flooding the inland desert basin and instantly (in geological terms — perhaps 5 to 15 years) forming the tropical Zechstein Sea. The middle of this sea was probably initially 200–300 m deep, but was almost completely filled with salts by the end of the period.
  
The thick and variably fossiliferous Magnesian Limestone of the Durham coastal cliffs was formed on the gentle shallow submarine slopes near the western margin of the Zechstein Sea during the last 5 to 7 Ma of the Permian. The sequence in the cliffs and adjoining inland areas is divided into five major '''carbonate''' formations that are grouped into three main cyclic units ([[:File:YGS_NORTROCK_FIG_13_1.jpg|Figure 13.1]]) separated by the insoluble residues of former salts ('''halite''' and '''anhydrite'''). The fracturing and foundering of the carbonate rocks resulting from the dissolution of these former salts is one of the three most spectacular features of the Durham coastal cliffs, the others being the striking evidence of downslope submarine slumping and sliding in two of the formations and the bewildering array of '''calcite concretions''' in the Concretionary Limestone Formation.
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The thick and variably fossiliferous Magnesian Limestone of the Durham coastal cliffs was formed on the gentle shallow submarine slopes near the western margin of the Zechstein Sea during the last 5 to 7 Ma of the Permian. The sequence in the cliffs and adjoining inland areas is divided into five major '''carbonate''' formations that are grouped into three main cyclic units ([[:File:YGS_NORTROCK_FIG_13_1.jpg|Figure 13.1]]) separated by the insoluble residues of former salts ('''halite''' and '''anhydrite'''). The fracturing and foundering of the carbonate rocks resulting from the dissolution of these former salts is one of the three most spectacular features of the Durham coastal cliffs, the others being the striking evidence of downslope submarine slumping and sliding in two of the formations and the bewildering array of '''calcite concretions''' in the Concretionary Limestone Formation.
  
The geographical distribution of the main formations of the Magnesian Limestone in northern coastal Durham is shown in [[:File:YGS_NORTROCK_FIG_13_2.jpg|Figure 13.2]], together with the approximate position of the recommended stops. The stratigraphical position of the rocks at the localities to be visited is shown on [[:File:YGS_NORTROCK_FIG_13_3.jpg|Figure 13.3]]. Each of the main formations is seen in at least one location, except the back-reef facies of the Ford Formation, and the Marl Slate which are omitted for logistical reasons. They may be studied at Ford Quarry [NZ 3630 5720] and Claxheugh Rock [NZ 3630 5760] respectively (hours required, hard hats).
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The geographical distribution of the main formations of the Magnesian Limestone in northern coastal Durham is shown in [[:File:YGS_NORTROCK_FIG_13_2.jpg|Figure 13.2]], together with the approximate position of the recommended stops. The stratigraphical position of the rocks at the localities to be visited is shown on [[:File:YGS_NORTROCK_FIG_13_3.jpg|Figure 13.3]]. Each of the main formations is seen in at least one location, except the back-reef facies of the Ford Formation, and the Marl Slate which are omitted for logistical reasons. They may be studied at Ford Quarry [NZ 36305 720] and Claxheugh Rock [NZ 36305 760] respectively (11/2 hours required, hard hats).
  
 
== Excursion details ==
 
== Excursion details ==

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