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Ages of rock

The Junction Bed where this ammonite was found is known as a ‘condensed sequence’ as it contains some four million years of geological time in less than one metre of rock. On the Yorkshire coast, the same time is represented by over 100 metres of black shale. Geologists know this because of the ammonites found in the rock. Hildoceras is found both here on the Dorset coast and on the Yorkshire coast. It lived for a short period of geological time representing perhaps a quarter to half a million years. So even though the rocks look completely different, we know that they formed at the same time but under different conditions, creating different types of rock. 

Find out more about ammonites on the molluscs pages.

Common name


Scientific name



Molluscs > Cephalopods


Upper Lias, Junction Bed

Time period

Lower Jurassic


180 million years

Where found

Thorncombe Beacon

Found by

Paul C Ensom


Dorset County Museum, Dorchester

Accession number