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.