What rocks to look for when fossil hunting

Fossils form when sediment buries an organism and then hardens. Igneous rock — granite, basalt — was molten, and metamorphic rock has been cooked and crushed. Neither preserves fossils in any useful way.

This is why the fossil map of Britain looks the way it does. The productive coasts are where soft sedimentary rocks meet the sea: Dorset, Yorkshire, the Isle of Wight, East Anglia, the Thames and Severn estuaries.

• Shale and mudstone — dark grey, splits into layers; the source of most Jurassic ammonites and belemnites
• Limestone — pale grey and hard, often crowded with shells, brachiopods, corals and crinoids
• Chalk — soft, white, and the source of flint sea urchins and sponges
• Clay — sticky and grey or brown; London Clay and Gault Clay hold superb shells, teeth and crustaceans
• Sandstone — variable, but Wealden sandstones carry plant material and dinosaur footprints

Many of the best British specimens come out of nodules: hard, rounded lumps of cemented sediment that formed around a decaying organism. On the Yorkshire coast and at Lyme Regis they litter the foreshore, and a nodule that is already split is the easiest place to start.

An unsplit nodule is a gamble. If you do split one, use a proper geological hammer, wear safety glasses and strike on the edge, never on your hand.

Granite, basalt, slate, flint without a fossil in it, and the smooth quartz pebbles that make up much of a British beach. Also ignore the temptation to hammer anything still attached to a cliff — it is unsafe, it damages the site, and the loose material at your feet is better anyway.