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Unconformity, Somerset

The two sets of rock layers seen in the photo below form a feature that geologists call an unconformity.

The layers of the lower part of the cliff are limestone, containing fossil evidence showing that they were laid down in tropical seas about 330 million years ago, in the Carboniferous period. These limestones were later tilted and uplifted to form mountains which, in turn, were eroded away over millions of years until, about 180 million years ago, they were left as low-lying islands in the Jurassic sea.

The eroded shores of these islands were gradually covered by the sea, which deposited sediments with different sorts of fossil sea-life to the older limestones. The line of contact between the two sets of rocks not only represents an ancient sea-shore – it also represents a time-gap of about 150 million years!

Unconformity, Vallis Vale, Somerset

Unconformity, Vallis Vale, Somerset
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