Rocks And Fossils: Extracting Ammonites

Rocks and fossils: Micro-excavation of ammonite fossils from a rock matrix follows collecting expeditions for boulders in the Lias mud by the coastal road of Antrim, Ireland.

A wide variety of fossils can be found among the rocky and muddy cliffs on the Co Antrim coast of Ireland. Ammonites are probably the most visually appealing. Bivalve molluscs are very common. Gryphia, "devil's toenails" are popular with collectors. Belomnites are found in the hard white limestone. Some beautiful crinoids are to be found. Occasional large vertebrates are reported.

Heavy rain causes the cliffs to become waterlogged. The resultant mudslides are normally channeled under the coastal road, but in some years the landslips are so bad that the coastal road has to be closed for some weeks while reconstruction work takes place. In some places, reservoirs are built to collect the mudslides on the landward side of the road. It is then taken across the road and dumped into the sea when the reservoir is becoming full. The sea does on excellent job of washing away the mud to reveal whole intact fossils, broken fossils, and boulders containing fossils.

The best time for collecting is therefore the low tide of a spring tide following a period of storms, which was preceded by heavy rain. The collecting conditions are not safe. The mud is deep and clinging. The seaweed is slippery. The collecting area is not visible from the coastal road. The ideal collecting periods are cold, windy and wet. Safety equipment and a safety lookout are essential.

The best rocks and boulders to collect are those which are small enough to handle easily, soft enough to work, unfractured and with tantalising exposure of the extreme edge of a likely attractive fossil. By chance, it happens that rocks and small boulders bearing ammonites are the ones which most udually best meet these requirements.

The Lias clays in which the ammonites are deposited look an unattractive dirty grey colour and are generally rough to the touch.

The safety aspect of the excavation of the fossils from these rocks must be emphasised. A method of securely clamping the rock is required. It is necessary to use power tools to achieve good results. These generate a large amount of unpleasant dust when being operated. There are sometimes fragments of rock which fly off the workpiece. The work should be carried with suitable dust extraction, and wearing facemask and goggles.

The best tools for the types of rocks that are collected in this place are a small hand-held high-speed drill, with a set of collets to take different types of fittings. These should include a range of tools for the wide variety of hardness of the rocks being worked on. One specimen vary from being soft enough to scratch with a thumbnail at one side to being hard and dense on the other side. Rotary steel wire brushes are often a good start, though for some specimens a rotary brass wire brush does less damage. Tungsten carbide burrs, diamond burrs and a miniature diamond cutting wheel all all part of what is needed for successful micro-excavation of the fossils from the matrix.

A specialist hobby model shop might the best place to find this equipment In the UK a good supplier is Model Flight Accessories of Worth in Kent.

There are times when it is opportune to use a fine screwdriver from a watchmaker's tool kit as a chisel to break away some of the larger parts of the matrix. Again, safety is important here and goggles are essential. Subsequent use of those tools on watches would not be advised.

It is usually possible to extract exactly half the specimen, wasting what was on top, and using the bottom half of the matrix as an inbuilt display stand for the specimen. A steel wire brush is usually the best way of giving this a good polish. In some cases it is possible to expose two fossils in the same rock.

The resultant specimens have proved to be popular as gifts to like-minded enthusiasts.

The specimens that I have found most suitable for this extraction process are the ammonites Caloceras johnstoni and Schlotheimia extranodosa and the crinoid Isocrinus psilonoti. These identifications are tentative, and would need expert verification.

A totally different method of extraction was suggested to me during an Open Day at the Ulster Museum. This has also proved to be most successful. That is baking the rock at high temperature. This will tend to produce a fracture through the centre plane of an ammonite, the halves must then be carefully polished to produce presentable specimens.

The geology is that these specimens are in the Lower Lias. They are in the Hettangian stage at about the lowest two million years of the lower Jurassic. Lias formations are normally said to be within the Jurassic. The Geological Map of Northern Ireland shows them as belonging Penarth group which it shows entirely within the Upper Triassic. The Mercia mudstones of the middle and upper Triassic are frequent along this coast. The whole lot is overlaid by bassalts, which further round the coast provide the spectacle of the Giant's Causeway cliffs.

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