Landscape and Geology
Beneath the Estuary lies a foundation of solid rock. The Estuary marks a boundary between the older rocks of Wales and the Forerst of Dean and the younger limestones, deposited 350 million years ago in the Carboniferous period. This limestone forms the islands of Flat Holm and Steep Holm and the headlands of Brean Down and Sand Point. These limestones were formed from calcium-rich shells and skeletons of tiny animals which accumulated on the floor of a tropical ocean.
Later, as the continental plates shifted, the area became a land locked desert, and red rocks, eroded form sandstone mountains were laid down 220 million years ago.
210 million years ago the land mass was submerged again, shown by the black Rhaetic Shales, containing fish bones, scales and teeth.
180 millon years ago (MYA) the sea deepeded and further limestones and shales were deposited.
This complete sequence of rocks can be seen today in the cliffs at Aust and Westbury on Severn.
Sediments laid down in the Basin of the estuary since the Jurassic period (180 MYA), have been eroded away by the action of the river itself.
During the Ice ages (from 1.8 MYA to present), the sea level dropped to 50 meters below what it is today. Course gravels, scrapped form the land by massive glaciers, were carried downstream by fast moving rivers which carved deep channels as they flowed sea-ward. The main channel of the Severn now lies submerged at the bottom of the estuary and provides a deep water route for shipping.
During the last 15,000 years a warmer climate has returned and ther ice sheets retreated. HTe extensive flat plains surrounding the estuary flooded, and the hilltops of Flat Holm, Steep Holm and Brean Down became islands.
The estuary is therefore know as a 'drowned valley' type of estuary