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What is gritstone and shale?

The Peak District looked very different over 300 million years ago! Like the Mississippi today, the Peak District was where a great river met the sea to form a massive delta.

Fingers of swampy land are divided by braided river channels bringing sands and muds from a northerly mountain range.  As the river slows to meet the sea, sands and muds fall to the river bed gradually building up alternating layers of mud and sands as the river channels move and shift.

The alternating sandstone and shale layers at Mam Tor, Castleton

Compaction of the sands and muds over many millions of years, resulted in the sands turning to gritstone and the muds becoming shale.  In the Peak District, these alternating deposits of gritstone and shale that reflect the shifting delta are known as the Millstone Grit series.  Fossil trilobites and brachiopods found in the shale indicate that these waters were full of life in Carboniferous times.

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