Gritstone edges: Black Edge
The limestone plateau of the 'White Peak' is surrounded by wide expanses of moorland underlain by shales and sandstones deposited about 320 million years ago in the Upper Carboniferous age. This is the 'Dark Peak' area of the Peak District. The outcrops of the sandstones are marked by prominent escarpments -or gritstone 'edges' - which form features that can be traced across the landscape. The face of each edge may be up to 30 m in height. These provide excellent exposures of the sandstone, as well as good opportunities for rock climbing.
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The sandstones are prominently bedded, resulting from deposition in a fast flowing river in a delta system (see Sandstone deltas). The orientation of the bedding indicates that stream flow was usually from north to south.
The escarpment at Black Edge is underlain by shales and mudstones which erode easily by water percolating through the porous sandstones. This causes the cliff face to retreat and blocks to break away from the edge and tumble downslope. Most gritstone edges are characterised by block fields accumulating at the base of the slopes and in the past the blocks have often been worked for millstone wheels.
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