Editing Millstone Grit of Almscliff Crag and Harlow Car, near Harrogate - an excursion

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== Geological background ==
 
== Geological background ==
  
In early Carboniferous times the area was situated in a subsiding trough known as the Craven Basin, with an area of slower subsidence, the Askrigg Block, not far away to the north. The distinction between block and basin was maintained by periodic movements on the Craven '''Fault'''-Belt. The top part of a thick sequence of marine mudstones and '''turbiditic''' sandstones laid down in the basin is exposed in the core of the Harrogate Anticline.
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In early Carboniferous times the area was situated in a subsiding trough known as the Craven Basin, with an area of slower subsidence, the Askrigg Block, not far away to the north. The distinction between block and basin was maintained by periodic movements on the Craven Fault-Belt. The top part of a thick sequence of marine mudstones and turbiditic sandstones laid down in the basin is exposed in the core of the Harrogate Anticline.
  
Sedimentation continued into late Carboniferous times without a break, but the area of marine deposition was soon invaded from the northeast by a large river carrying coarse-grained sand, and the sequence rapidly became dominated by its deposits, the Millstone Grits. The section at Harlow Car exposes interbedded siltstones and sandstones, including some '''turbidites''', which are thought to have accumulated rapidly on a delta slope in front of the river mouth. The overlying Almscliff Grit is coarser-grained, and is believed to represent the deposits of the river channel itself. The advance of the river may have been accelerated by a drop in sea level.
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Sedimentation continued into late Carboniferous times without a break, but the area of marine deposition was soon invaded from the northeast by a large river carrying coarse-grained sand, and the sequence rapidly became dominated by its deposits, the Millstone Grits. The section at Harlow Car exposes interbedded siltstones and sandstones, including some turbidites, which are thought to have accumulated rapidly on a delta slope in front of the river mouth. The overlying Almscliff Grit is coarser-grained, and is believed to represent the deposits of the river channel itself. The advance of the river may have been accelerated by a drop in sea level.
  
 
The advance of the river was brought to an end by a rise in sea level, which caused the delta to retreat for an unknown distance back towards the source area. A thin bed of fossils, deposited in the shallow sea that occupied the area as the sea level rose, has been seen in a temporary section near Harlow Car, but is no longer visible. The fossils collected identify it as the Cravenoceras cowlingense Marine Band, and serve to correlate the Almscliff Grit with the Warley Wise Grit of areas farther west.
 
The advance of the river was brought to an end by a rise in sea level, which caused the delta to retreat for an unknown distance back towards the source area. A thin bed of fossils, deposited in the shallow sea that occupied the area as the sea level rose, has been seen in a temporary section near Harlow Car, but is no longer visible. The fossils collected identify it as the Cravenoceras cowlingense Marine Band, and serve to correlate the Almscliff Grit with the Warley Wise Grit of areas farther west.
  
The Almscliff Grit is visible in the Harlow Car area but is more easily examined at its type locality, Almscliff Crag. Here it is possible to work out the flow direction of the river, and to see how the overall shape of the river deposit has been affected locally by a gravity-induced slump, or growth fault (Chisholm, 1981). Repeated advances and retreats of the Millstone Grit river produced a sequence of alternating sandstones ('grits') and mudstones overlying the Almscliff Grit, and some of these beds can be identified in the features of the surrounding scenery.
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The Almscliff Grit is visible in the Harlow Car area but is more easily examined at its type locality, Almscliff Crag. Here it is possible to work out the flow direction of the river, and to see how the overall shape of the river deposit has been affected locally by a gravity-induced slump, or growth fault (Chisholm, 1981)
  
The deposits of '''till''' that were spread across the landscape by the Quaternary ice sheets tend to obscure the detail of topographical features related to bedrock outcrops, but the broader outlines are not affected: the harder gritstone beds normally form upstanding ridges and the interbedded mudstones form lower ground.
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Repeated advances and retreats of the Millstone Grit river produced a sequence of alternating sandstones ('grits') and mudstones overlying the Almscliff Grit, and some of these beds can be identified in the features of the surrounding scenery.
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The deposits of till that were spread across the landscape by the Quaternary ice sheets tend to obscure the detail of topographical features related to bedrock outcrops, but the broader outlines are not affected: the harder gritstone beds normally form upstanding ridges and the interbedded mudstones form lower ground.
  
 
== Excursion details ==
 
== Excursion details ==

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