was some 19° south of the equator and lay on the northern shore of an ocean basin that separated the continents of Laurasia and Gondwanaland. When these continents collided to form the single supercontinent of Pangaea, the sediments on the ocean floor were pushed up and over, while the molten rock below the surface was forced out. Today, these igneous intrusions and the red sandstones, visible in the neighbouring county of Devon, slope away from west to east, and are deeply buried beneath younger deposits in Dorset. Triassic Through most of this period, southern Britain experienced continental conditions, initially with deposition by large river systems forming the Sherwood Sandstone Group. This was succeeded by mudstones with evaporites (including locally thick deposits of halite) deposited in a desert environment, forming the Mercia Mudstone Group. A major change to marine conditions is shown by the mudstones with limestones and sandstones of the overlying Rhaetian age Penarth Group. No rocks from this period crop out in Dorset, but they are known to be present in the subsurface from hydrocarbon exploration drilling. Latest Triassic to Jurassic Around 204 Mya, Dorset, now 30°N, was under water, and the first ammonites (Psiloceras planorbis) appear among the shales and limestones that make up the lower Lias. Lower Lias Formed during the Late Rhaetian to Pliensbachian, in what was then a shallow marine environment, the lower Lias is composed of Blue Lias, Black Ven Marls, and Green Ammonite Beds (Charmouth Mudstone Formation). Mostly covered by lush vegetation, it forms the floor of Marshwood Vale in the west of Dorset and can be seen in stream beds, where the land has been excavated, and along the coast to the west of Seatown. The sides of the vale are mainly made from the clays and sands of the upper and lower Lias, while younger strata from the Cretaceous Period (145–66 Mya), crown the higher points. The Blue Lias is the lowest of the Liassic strata and where it is visible on the coast near Lyme Regis is layered with hard limestone and oil-rich shale. The iron pyrites in the clay can heat up when exposed to the air and occasionally ignite the shales. Around Lyme Regis, during the 18th and 19th centuries, the collection and sale of fossils became a popular occupation. Landslides and the excavation of the clays, used in cement production, exposed not only an abundance of ammonites