Walk through time at Larne
Sketch map of the Triassic and Jurassic strata exposed on the shore at Waterloo Bay, Larne.
The oldest rocks on the shore, a little north of the Leisure Centre, are brick-red mudstones (Mercia Mudstone Group) with scattered green spots and thin veins of gypsum. They were deposited on an arid plain with shallow seasonal lakes, rather like parts of central Australia today.
Below the cliff that overlooks the Promenade are greenish-grey siltstones in beds about half a metre thick (Collin Glen Formation). They were deposited in shallow lagoons, formed as rising sea levels began to flood across the land, but contain only a few microfossils. A little further north there is an abrupt change to almost black shales (Westbury Formation) with thin layers containing fossil shells, fish scales and teeth. This marks the point when the lagoons were finally inundated by the open sea.
At the top of the Westbury Formation, a little further north, there is an abrupt change to paler sandstones (Cotham Member) in which the layers through more than 4 metres of strata are intensely contorted. Since the layers above and below are undeformed, these sediments must have been violently shaken while they were still soft. They are evidence for an extraordinarily violent earthquake across the UK almost 200 million years ago.
North of the contorted strata is layer upon layer of dark grey mudstone with just a few thin limestones. The first few metres (the Langport Member) contain rather few fossils other than scattered bivalves, though an ichthyosaur skeleton was found here in 1999. Continuing north, and so into younger, the fossils become more common and diverse. The first ammonites appear just a metre or two south of the disused sewer pipe. This is where scientists believe the start of the Jurasssic Period should be defined.
For another simple description of the changing rock types and environments representated at Larne, visit this BBC page.