Continental bookends—the arrival and departure of the Jurassic Sea in southern England

Rocks laid down during the Jurassic Period (201–143 Ma) in southern England are almost exclusively marine, but are sandwiched between continental deposits of the Late Triassic and Early Cretaceous. The transitions from fully continental to marine then back to continental, in response to continent movements and sea level change, mirror each other in important respects, taking place via intermediate environments which, although separated by approximately 60 million years, show remarkable similarities in their rocks and faunas.