england fossils geology palaeontology

Heavy rains and storms in Dorset: Collecting fossils

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Tony and Anna Gill (UK)

The best time to look for fossils in Dorset is after heavy rain and winter storms. These conditions make the cliffs unstable and collapse. High winds produce rough seas, which wash the mud away, leaving the nodules that contain the fossils exposed on the beach. The beginning of November 2005 saw a period of heavy rain and strong winds. This stormy weather continued for several days and on 5 November gale force 10 winds, before a high tide, exposing new material (Fig. 1).

Fig. 1. A large landslip, approximately four hundred metres east of Charmouth, contains most of the best fossil horizons.

This slip is illustrated with the sea crashing into it (Fig. 2). The large stones in the picture below do not contain any fossils. If they did, they would not be there. The best place to look for smaller fossils is around these large stones. Some of the pyrite ammonites found here can be up to 25cm to 30cm in diameter.

Fig. 2. The storm crashing against the cliffs at Charmouth.

The flat stones, which are from the Obtusum Shales, sometimes contain the ammonite Asteroceras Planicosta and usually the smaller ammonites Promicroceras Planicosta (Fig. 3).

Fig. 3. Charmouth was the seabed in Jurassic times, some 195 million years ago.

The football shaped and sized, Stellare Nodules, when broken, contain calcite crystals. Occasionally though, an ammonite can be found inside the larger ones. These ammonites can be up to 50cm across, but unfortunately, most times, these fossils are badly crushed or distorted (Fig. 4).

Fig. 4. This photo shows Charmouth at low tide, and after the storm. It shows the Lower Lias reefs, which are usually covered in sand.

Other finds include slabs of crinoids, belemnites, ammonites preserved in iron pyrite or fools gold (Fig. 5).

Fig. 5. A picture of some common finds that can be collected without breaking any rocks or using a hammer.

Species of ammonites commonly found preserved in iron pyrite include; Promicroceras, Crucibiloceras, Eoduroceras and Oxynonticeras (Fig. 6). Iron pyrites are most often found in the loose shingle around the slip, after a storm. Ichthyosaur vertebrae are most commonly found at Charmouth, but occasionally parts of rib, paddle bones and, if you are very luckily, a piece of jaw with teeth attached can be collected.

Fig. 6. This small piece of limestone contains the ammonite, Promicroceras, which is preserved in calcite. The piece was found while searching through the shingle, beside the large stones. It was broken open with a geological hammer.

These beautifully preserved calcite ammonites in limestone nodules are known as flat stones. They can range in size from 2cm to 30cm. The nodules can be dinner plate size up to tabletop size. When found, we break them down with a hammer until an ammonite is found and then we take them back to the workshop and the rest of the stone is removed using air tools.

This is a good time to look for bones, mainly ichthyosaurs. Other less common fossils include fish such as; Dapedium, shark, and plesiosaur remains. There is not much time for looking for these fossils as the tide returns very quickly.

Watch out for mud flows there is always plenty of abandoned footwear after heavy rain, please Check the tide times.

There is a fossil collecting code in operation and a data base of important finds, for more information, go to: https://jurassiccoast.org/ and https://charmouth.org/.

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