Sandstone deltas: Black Rock
Black Rock, a popular venue for rock climbing, is an imposing feature overlooking Cromford, Matlock and the Derwent Valley. It is a landslipped crag of massive, coarse grained sandstone or grit. In the past, these coarse sandstones were used to make millstones. Today, the millstone is the emblem of the Peak District National Park and gives its name to the distinctive 'Millstone Grit' rocks of the Pennines. These rocks form the 'Dark Peak' scenery of Derbyshire.
The sandstones were deposited by a great river system which built out southwards from the Scottish Highlands around 320 million years ago, flooding over the older limestones.
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The river formed a delta by depositing layers of sand and mud. This sediment-filled delta eventually subsided, this allowed the sea to flood back, after which the delta-building began again. This process was repeated many times. As a result, the Millstone Grit consists of a series of deltas piled one on top of another.
As well as grindstones, the Millstone Grit sandstone has also been quarried for building stone, paving flagstones, roofing slates, for walling stone, lintels and doorposts. Today only a few sandstone quarries remain in the Peak District, although demand is strong for the use of this material in both restoration and in new buildings.
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