 During the Triassic, the Franconian part of these depressions was often part of the mainland, in the Jurassic it was covered for most of the time by a marginal sea of the western Tethys Ocean.At the time when the limestones and dolomites of the White Jura were being deposited, this sea was divided into sponge reefs and intervening lagoons.The reef bodies and the fine-grained lagoon limestones and marls are the material from which the majority of the Franconian Jura is composed today.