As the impurities increase in amount the clay rocks pass gradually into argillaceous sands and sandstones, argillaceous limestones and dolomites, shaly coals and clay ironstones.
The sea grew shallow, the deposits became conglomeratic and shaly, volcanic eruptions began, and the present folds of the Apennines were initiated.
The underlying geology is a gray shaly metamorphosed limestone, perhaps turbidite, interbedded with sandstone; sands and gravels fill the valleys.