Sedimentary rocks mostly are deposited in near horizontal layers, with younger layers deposited on top of earlier ones. Each layer, or bed, of rock is separated from adjacent ones by bedding planes. Over the course of many millions of years earth movements may bend, break and tilt the originally horizontal layers. These 430 million year old sandstone layers at Portavogie, on the Ards peninsula, Co. Down, were tilted to nearly vertical by continental collision about 400 million years ago.

Fossils can still be found in tilted rocks such as these which have not been altered too greatly by heat and pressure.