The site is situated on top of a complex sequence of glaciofluvial and littoral deposits of Quaternary age.

The Ousse valley is separated from the Gave by littoral deposits dating from Ypresian to Miocene.

The littoral deposits consist of micritic mudstones and dolomitic mudstones that are thought to have been deposited in intertidal to supratidal environments.

I state: If we consider what must take place during the gradual rise of a group of islands, we shall have the currents endeavouring to cut down and deepen some shallow parts in the channels as they are successively brought near the surface, but tending from the opposition of tides to choke up others with littoral deposits.