The Brenne Explained: Geology

The area known as the Brenne was created in the Tertiary era about 40 million years ago, after the sea had advanced and retreated a number of times. It is a vast shallow bowl, part of the even bigger Paris Basin, collecting water coming off the Massif Central. Over time, clay has accumulated in the depression, creating an impervious base. In many places the deposits are rich in iron and have formed an intermittent layer of sandstone very resistant to erosion and known locally as 'buttons'. In the Miocene, about 20 million years ago, the sea encroached for the fourth and last time up the Loire Trough but was stopped by the low hills between Ligueil and Ciran. After this the Brenne became a vast wooded marsh, not yet cut by river valleys.

A small herd of Camargue horses (introduced) in the Brenne, seen from a bird hide.

Finally, a little over a million years ago, thanks to the Ice Ages, water courses cut deep valleys across the plain. The ice itself never reached the Brenne, but the moving sediment it caused resulted in the erosion of the softer soils, leaving the hard buttons emerging from the landscape. The new rivers left their own sandy deposits too and so the diverse mixture of soil types we see today was established.

The Creuse

(Note: Much of this information has been cribbed from 'Flore remarquarble du Parc naturel régional de la Brenne' by François Pinet. Any errors are the result of me either misunderstanding or mistranslating the French terminology.)

Susan