Fossils from the Marlstone rock and Junction bed (now known as the Beacon Limestone Formation) can be found on the Dorset coast between Seatown and Eype and also in the Fields on the higher ridges around Ilminster. Despite the closeness of these two sites the collecting is different at each of them. This partly reflects the nature of the exposure but is also a genuine difference in the beds. The ammonites from these beds are common and relatively inexpensive fossils in many fossil shops. Fossils of the same age can also be found on the Yorkshire coast at places like Whitby, Port Mulgrave and Sandsend.
The name Junction Bed has a long history. It was originally applied to the Toarcian limestones and clays we think of as the Junction Bed today, and also the underlying Marlstone, which was considered upper Pliensbachian in age. Howarth has now demonstrated that the top of the marlstone is in fact Toarcian in age, containing the ammonite Dactylioceras tenuicostatum. Around Ilminster the Toarcian rocks are now properly called the Barrington Member. This is to commemorate the exposures which used to exist around the village of Barrington, near Ilminster in Somerset. The Barrington Member together with the Marlstone Rock Member make up the Beacon Limestone Formation. Despite these changes most fossil collectors still know the rocks around Ilminster as the Junction Bed.
Geological Map of Ilminster and the surrounding area, provided by the British Geological Survey.
The Beacon Limestone Formation, found at Ilminster, Moolham, Herne Hill, and Hurcott, is where Junction Bed ammonites or ‘Ilminsters’ can be found. Please seek the landowners permission before raiding their fields - or you could be done for trespassing and theft. Many farmers charge per day of fossil hunting.
The coastal sections of the Junction Bed can be easily explored on the beach between Seatown and Eype, with the usual warnings about tides and cliffs. Inland exposures are only accessible with the permission of the landowner. This is most likely to be obtained when the fields have just been ploughed and contain no crops. It is also at these times that fossils are most easily found. Sadly, modern ploughing with tractors damages the ammonites more than the horse drawn ploughs of the past. The old ploughs were more likely to push the ammonites to one side. Modern powerful ploughs break through them. This is especially the case with larger ammonite species such as Harpoceras. Many broken coil sections of larger specimens can be found.
The beds around Ilminster were formerly exposed in small quarries which produced stone for building. The most famous of these was Strawberry Bank, renowned for the vertebrate fossils and insects it produced. In more recent times work on the A303 created new exposures of the junction bed in 1987.
The Junction Bed is an example of a condensed sequence and was deposited in fairly deep water. It is a mixture of clays and creamy limestones. Sediment was deposited very slowly, so the equivalent in age of the junction bed is represented by a far thicker sequence of rock in places like Yorkshire. We can tell this by the fact that in both places we find the same ammonite sequence. Because of the slow deposition ammonite shells had time to accumulate on the sea floor, so the rock can be crowded in their remains.
In some cases, ammonites of different subzones have been mixed by disturbance of sediment on the sea floor. Local erosion can also mean that ammonite zones can be entirely absent in some areas as the layer has been eroded away. The preservation in the Junction Bed is often inferior to that of the Yorkshire beds because ammonites were exposed for longer on the sea floor. Many specimens show indications of wear or have encrusting serpellid worms and oyster shells, some specimens have been eroded out on the seafloor and redeposited. For this reason, many of the ammonites are cut and polished to reveal the chambered interior of the shell. Specimens can, however, be well preserved and if prepared using modern equipment they can look stunning.
Amazing results can be acheived using an air pen, air abrasive, and acid preparation. (C) Chris Andrew
|A Hildoceras luscitanicum, distorted on during preservation has highlighted the chamber walls. This fossil would normally be cut and polished by commercial collectors, if collected at all. (C) Chris Andrew
|Phymotoceras robustum. This specimen shows how long the ammonite was sitting on the sea floor - had it been a more common species it might have been cut and polished rather than prepared due to the amount of worm tubes and pre-preservational damage. (C) Chris Andrew
Cutting and Polishing
When you come across a particularly calcite-rich ammonite from the Junction Bed of Ilminster, you know it will polish well. Sometimes, however, the chambers can be mud-filled, which will provide a less aesthetically pleasing finish. Fossils from the fields of Ilminster are usually found cut and polished in the shops; as this is generally considered the most appealing form of preparation and is certainly the most cost effective when done in bulk. This can be achieved by using a rock saw, and cutting the fossil down the centre. Most of the time, you’ll end up with only half the fossil as the centres of ammonites are so thin.
Cut and polished ammonites from Ilminster. Note the mud fill in some chambers and the crystalline fill of others. The skill in cutting ammonites is in making sure that the centre is intersected.(C) Chris Andrew
Commercial preparators will use vibrating laps, but the same finish can be achieved with various grades of sandpaper. Once you have a cut fossil, you’ll need to start abrading out the surface scratches. It is best to use coarse to ultra-fine grades of wet and dry sandpaper in succession, using a figure of eight motion with the fossil across the paper to keep the surface even. By using 200-grit, 400-grit, 800-grit and 1200-grit in turn, you should end up with a surface ready to take a polish. Regularly rinse the sludge off the fossil so that bits of grit don’t get worked in and create further scratches, and maintain the paper wetness until the surface is visibly very smooth. Then it is time to wash the fossil under running water, using a soft bristle brush to remove any remaining residue or dirt.
Create a polish by adding water to a small amount of aluminium oxide polishing compound to make a paste. A small amount can be placed onto a leather pad, which can be used to vigorously buff the surface of the fossil until it starts developing a glossy shine. Alternatively, you could use a buffing wheel Dremel attachment with the same paste. The aluminium oxide acts to heat up the very surface layer of the fossil, giving it a glass-like appearance. Calcite will always take a better polish than mud fill, but you can still get excellent results.
Mechanical and Acid Preparation
Most ammonites from Ilminster do not take kindly to using an air pen, but with enough care and skill they can look great. It’s usually worth penning down to a certain level and then using chemicals or air abrasive. There is seldom much of a contrast between the encrusting matrix, and the fossil within and so they are unlikely to ‘pop’. A more calcite rich ammonite will prepare better in general, but even then, it is not usually considered commercially viable.
|A Dactylioceratid prepared mechanically, using an air pen and a scalpel to get in between the individual ribs. (C) Chris Andrew
|Porpoceras vortex prepared using an air pen to get close to the fossil, followed by anr abrasive for the finishing touches. (C) Chrs Andrew
|Amazing results can be acheived using an air pen, air abrasive, and acid preparation. (C) Chris Andrew
|This Hildoceras was found largely exposed, but finished by using an air pen and brick acid. (C) Chris Andrew
However, a combination of air pen and air abrasive work with either aluminium oxide or dolomite powder can produce excellent results. The same rules especially apply with air abrasive as usual – overdoing it will ‘frost’ the ammonite and quite possibly destroy the detail. The matrix and the rock within are usually of similar hardness and so this method must be used conservatively and with due care. If you do not have an air abrasive set up, and you’re trying to get between the ribs of something like a Dactylioceras, it is worth using a scalpel or a pin vise.
A tip from Steve Snowball regarding chemical preparation. Potassium hydroxide flakes are a good way to finish the prepping. After cleaning off as much of the matrix as is possible, dampen the fossil with a plant sprayer (water), apply the flakes carefully (use tweezers), spray again and leave for few hours. They need to be checked regularly. The limestone is reduced to a sludge which can be rinsed off, then the fossil needs neutralising with white vinegar and then thoroughly rinsed once again. Marigold gloves are a must! Results on the calcite specimens are excellent but it works on the others too. With flakes, the application can be carefully directed at remaining matrix.
Alternate between these methods to achieve the desired result. Some fossils may take more mechanical preparation than others; some may react very well to air abrasive but not to pen work, and so on.