Geology of the AONB

Brief Summary

One hundred million years ago this Area was covered by a shallow sea. The Gault Clay and Upper Greensand that we see in the Vale of Wardour formed the sea bed. The skeletons of invertebrates that teemed in the warm waters slowly accreted to form the impure Lower Chalk strata and then the harder, purer Upper Chalk layers. It is this chalk rock that determines the landscape character of the AONB.

During the twenty million years of chalk accretion, the silicate skeletons of sponges were concentrated in voids and formed flint. This hard but frangible rock was extensively used by the very first humans to settle the Area and is still a characteristic building material in the villages.

During successive glaciations, when the porous chalk was frozen into an impermeable state, rivers followed faults in the rock and eroded the valleys and steep scarp slopes. The dry valleys and folded landform of the Downs date from this period. The rivers cut down through the chalk to expose the Greensand and Kimmeridge Clay of the Vale of Wardour and the Chilmark Stone that was used to build Salisbury Cathedral.

Geology map of the AONB PDF (350kb)