In the tertiary era, the Aquitaine basin held a warm, shallow sea with salt-water
lagoons and lakes. Over fifty million years, the compacted remains of evolving salt
water and fresh water creatures and plants mingled with the sand and clay to form
a variably compact rock (calcareous rock formation).
Layers thus formed, making up the tertiary substratum of the Médoc. In the quaternary
era, these limestone layers were to be greatly deformed and broken up by the formation
of the Pyrenees.