Editing Middle and Upper Carboniferous rocks (Millstone Grit and Coal Measures) of the Sheffield region - an excursion

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== Geological background ==
 
== Geological background ==
  
The upper part of the Millstone Grit to the southwest of Sheffield is dominated by thick sandstone units interbedded with shales. Occasional marine incursions and subaerial exposure of the delta top gave rise to thin marine bands and the development of coals. One of the sandstones, the Rivelin (or Chatsworth) Grit, forms characteristic scarp features such as Stanage Edge, Burbage Edge and Millstone Edge. The grit is of upper Namurian (Marsdenian) age and consists of an upper and lower leaf. It is bounded below by the Reticuloceras superbilingue Marine Band, and above by the Ringinglow Coal. The grit is a major sandbody which was laid down in a fluvial setting as part of the advancing delta system from the north. The sandstones of the Rivelin Grit are coarse grained, occasionally pebbly, and show excellent examples of cross-bedding on a variety of scales. Different facies are present within the grit, characterized by the presence or absence of pebbles and the scale and type of the cross-bedding. The shales below the lower leaf contain rare goniatites. Drifted plant remains are more common in both the sandstones and shales. The rocks generally dip very gently to the east, although there are local variations, but the considerable downcutting of Burbage Brook has created an inlier, exposing the beds underlying the Rivelin Grit along most of its course.
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The upper part of the Millstone Grit to the southwest of Sheffield is dominated by thick sandstone units interbedded with shales. Occasional marine incursions and subaerial exposure of the delta top gave rise to thin marine bands and the development of coals. One of the sandstones, the Rivelin (or Chatsworth) Grit, forms characteristic scarp features such as Stanage Edge, Burbage Edge and Millstone Edge. The grit is of upper Namurian (Marsdenian) age and consists of an upper and lower leaf. It is bounded below by the Reticuloceras superbilingue Marine Band, and above by the Ringinglow Coal. The grit is a major sandbody which was laid down in a '''fluvial''' setting as part of the advancing delta system from the north. The sandstones of the Rivelin Grit are coarse grained, occasionally pebbly, and show excellent examples of '''cross-bedding''' on a variety of scales. Different '''facies''' are present within the grit, characterized by the presence or absence of pebbles and the scale and type of the cross-bedding. The shales below the lower leaf contain rare '''goniatites'''. Drifted plant remains are more common in both the sandstones and shales. The rocks generally '''dip''' very gently to the east, although there are local variations, but the considerable downcutting of Burbage Brook has created an '''inlier''', exposing the beds underlying the Rivelin Grit along most of its course.
  
The Coal Measures overlie the Millstone Grit to the east and, owing to their similar lithological characteristics, give rise to comparable (though generally more subdued) topographic features. Bradgate Brickyard (Excursion ob) is located in urban Rotherham, and the gently dipping sequence of middle Coal Measures provides a convenient way to see a variety of rock types (sandstones, siltstones, shales, thin coals, seatearths), sedimentary structures (cross-bedding, slump structures, lamination) and fossils (plant fragments, bivalves, brachiopods and fish remains) within a typical Coal Measure sequence. The coarsening-up cycles within the Coal Measures reflect alternating regressive and transgressive events, and represent a deltaic/fluvial environment in which periods of emergence led to the growth of luxuriant forests.
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The Coal Measures overlie the Millstone Grit to the east and, owing to their similar '''lithological''' characteristics, give rise to comparable (though generally more subdued) topographic features. Bradgate Brickyard (Excursion 10b) is located in urban Rotherham, and the gently dipping sequence of middle Coal Measures provides a convenient way to see a variety of rock types (sandstones, siltstones, shales, thin coals, '''seatearths'''), sedimentary structures (cross-bedding, slump structures, lamination) and fossils (plant fragments, '''bivalves, brachiopods''' and fish remains) within a typical Coal Measure sequence. The coarsening-up cycles within the Coal Measures reflect alternating '''regressive''' and '''transgressive''' events, and represent a deltaic/fluvial environment in which periods of emergence led to the growth of luxuriant forests.
  
 
== Excursion details ==
 
== Excursion details ==
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=== Excursion 10a ===
 
=== Excursion 10a ===
  
From Upper Burbage Bridge ([[:File:YGS_YORKROCK_FIG_10_01.jpg|Figure 10.1]]), walk to the east and take the signed footpath through the gate in the wall. There is a fine view of the area from here with the main scarp of the upper leaf of the Rivelin Grit extending 2.5 km to the south. Note also the termination of the line of crags (by a '''fault''') to the west of Burbage Brook, and the prominent hills of Higger Tor and Carl Wark (see Localities 5 and 6). Follow the footpath south for approximately 100 m.
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From Upper Burbage Bridge ([[:File:YGS_YORKROCK_FIG_10_01.jpg|Figure 10.1]]), walk to the east and take the signed footpath through the gate in the wall. There is a fine view of the area from here with the main scarp of the upper leaf of the Rivelin Grit extending 2.5 km to the south. Note also the termination of the line of crags (by a fault) to the west of Burbage Brook, and the prominent hills of Higger Tor and Carl Wark (see Localities 5 and 6). Follow the footpath south for approximately ioo m.
  
 
==== Locality 1 [SK 262 829] ====
 
==== Locality 1 [SK 262 829] ====
  
Cross-bedded sandstones are well exposed along the scarp edge in this area ([[:File:YGS_YORKROCK_FIG_10_02.jpg|Figure 10.2]]). The rock is a typical 'grit' a coarse-grained '''feldspathic''' sandstone with common pebbly layers. The grains are subangular and consist of colourless '''quartz''', pink/ cream-coloured weathered feldspars, and occasional '''mica''' flakes. The rounded pebbles are dominantly of '''vein''' quartz. Cross-bedding is ubiquitous and '''foreset''' beds at this locality commonly indicate a transport direction towards the northwest. The rocks are well-jointed along two dominant directions (northwest–southeast and northeast–southwest).
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Cross-bedded sandstones are well exposed along the scarp edge in this area ([[:File:YGS_YORKROCK_FIG_10_02.jpg|Figure 10.2]]). The rock is a typical 'grit% a coarse-grained feldspathic sandstone with common pebbly layers. The grains are subangular and consist of colourless quartz, pink/ cream-coloured weathered feldspars, and occasional mica flakes. The rounded pebbles are dominantly of vein quartz. Cross-bedding is ubiquitous and foreset beds at this locality commonly indicate a transport direction towards the northwest. The rocks are well-jointed along two dominant directions (northwest–southeast and northeast–southwest).
  
Continue south along the sandy footpath (material derived from adjacent sandstones), noting marshy patches with rushes and mosses near the footpath where water passing through the permeable and porous sandstones meets the impermeable shales underneath (see Locality 4), and seeps out at the surface as springs. Also note the large-scale foreset beds in the scarp and the land surface strewn with boulders resulting from '''solifluction'''.
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Continue south along the sandy footpath (material derived from adjacent sandstones), noting marshy patches with rushes and mosses near the footpath where water passing through the permeable and porous sandstones meets the impermeable shales underneath (see Locality 4), and seeps out at the surface as springs. Also note the large-scale foreset beds in the scarp and the land surface strewn with boulders resulting from solifluction.
  
 
==== Locality 2 [S[SK 268 815] ====
 
==== Locality 2 [S[SK 268 815] ====

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