The basal Hettangian-Sinemurian sequence is fully transgressive over basement rocks or Permo-Triassic sediments. At that time, the first open-marine sediments (yet rather poor in fossils) were being deposited in the Aquitaine Basin. The "Lias Transgression", as it is also called, started to encroach on the entire Aquitaine during the Sinemurian, characterised by calcareous-dolomitic, partially oolitic sediments. Despite smaller regressions during the Pliensbachian towards the end of the Lias and at the beginning of the Dogger the sea had onlapped the basement rocks of the Massif Central and the western Vendée (reaching today's limits) by 30 km. On the Aquitaine Plateau in the north, an interior shelf was constructed as far south as the line La Rochelle-Angoulême-Périgueux-Figeac. On this shelf the generally detritic transgression sediments of the Hettangian normally comprise a base conglomerate, arkoses, and fairly thick layers of sand- and mud-stones rich in plant material. The rest of the Hettangian is made up of marine sediments deposited in a restricted environment (lagoonal) evolving towards a lacustrine facies (green shales, coloured marls, dolomitic limestones and platy limestones rich in dwarf fossils, and evaporitic interlayers). The sediments of the Sinemurian are again fully marine and carry a pelagic fauna (soft banded limestones and hard lithographic limestones). At the end of the Sinemurian, a sudden regression occurred, forming hardgrounds. The second sequence of the Lias again is marine-transgressive and commences during the "Lotharingian"/Lower "Carixian". The sediments can be well dated by ammonites—(Arietites, Oxynoticeras, Deroceras, and "Uptonia jamesoni"). They are mainly calcareous and rich in quartz grains and pebbles of reworked Sinemurian. The Upper Carixian consists of very fossiliferous ("Aegoceras capricornu") marly limestone layers interlayered with grey marls. These are followed by ammonite-bearing ("Amaltheus margaritatus") and oyster-bearing ("Gryphaea cymbium") marls indicating a shelf environment open to the spreading Atlantic Ocean.