Peak District geology
The core of the Peak District is mostly formed from pale grey, thickly-bedded limestones of Carboniferous age. The limestones form a 'sequence', or pile, of strata up to two kilometres thick, although only the uppermost 600m are exposed at the surface. They were deposited between 350 and 325 million years ago on a shallow 'platform' in a tropical sea which was mostly surrounded by much deeper water. Around the margins of the platform, a belt of reef limestones were developed. In the deeper water beyond the reefs in the south of the Peak District, a thicker sequence of darker, impure limestones and shales were deposited.
In a few places in the Peak District, the limestone sequence is interrupted by volcanic lava flows or by volcanic rocks which were 'intruded', or forced between the sediments.
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This volcanic activity occurred at the same time, or just after, the limestones were deposited. Elsewhere, the passage of water containing mineral salts has altered the composition of some limestones and deposited metal-rich mineral veins in others.
The limestones were overlain by a thick succession of Upper Carboniferous sandstones and shales deposited by a giant river delta system between 325 and 315 million years ago. Today the sandstone and shale sequence of the Millstone Grit has been eroded back to form the series of escarpments and moorlands which surround the limestone outcrop.
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