here is a deeper look into my collection of Early Carboniferous microfossils. This will be split into part 1 (here) where I discuss the process of finding and mounting specimens on pre-bought (professional) microfossil slides and part 2 where I look at finding cheaper alternatives and progressing the mounting of specimens.
First, finding (and collecting) the specimens. The microfossils I've collected are all from the Lower Carboniferous, specifically the Alston Formation (Yoredale Gp), from a variety of localities. While most of the sites are quarries, I've also found success with ghylls and small streams, with the fossiliferous bedrock in the bank. The defining feature of these sites is the presence of shale bands- and this is the bedrock from which the microfossils are found.
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| Faulds Brow Quarry, Faulds Brow and the shale band (belonging to the Scar Limestone Fm). |
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| Aughertree (Green How) Quarry, Aughertree, and the shale band (belonging to the Five Yard Limestone Fm). |
These shale bands are calcareous, sandwiched between massive limestone (or occasionally sandstones). Rich in fossils themselves, the shale bands are easily eroded and the debris is where microfossils can be found. Choosing the correct debris is important and dramatically increases microfossil yield. The top surface of the debris undergoes action by the wind, blowing away the finer clay, concentrating the microfossils. Also by finding areas on slopes where water has periodically trickled, washing away clay and accumulating the microfossils, you can find far more.
One of the best ways I've found to concentrate the microfossils, from this collected debris, (though this may be rather unconventional) is to use a gold pan. Instead using the pan for its intention of separating material through density, instead the pan is used to 'wash' the material. The first thing this does is remove any organics (moss, grass etc) and secondly it washed the fine clay material away- this clay can be a real nightmare later on in the process as it sticks to the microfossils, obscuring them. The pan can also be used to remove material based on size. Its also worth washing until there is no 'dirty water' washing away from the material, leaving just the grit, as it makes a real difference.
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| Panning the grit. |
Preparation and sorting
After the grit has been collected, the next step is drying. I do this by having a large tray, filled with a thin layer of grit, left overnight in the kitchen.
One optional step that can help is a brief vinegar bath. By soaking the grit in vinegar (for around 5 minutes until the bubbles stop rising) much of the fine material is dissolved, leaving a far purer grit. The downside to this method though is if you leave the grit in the vinegar for too long, any carbonate fossils will have their surfaces eaten away, loosing detail and damaging them- this is of course not an issue for phosphate based fossils such as conodont elements.
Once collected, washed and dried, the grit can then be sorted. This is a long and tedious process so it is important (and not health and safety 'gone mad') that you are positioned properly at the microscope, as to not strain your neck and back.
The best method for sorting that I've found is putting a A5 piece of paper on the microscope stage with a fold down the middle. Then put around quarter of a teaspoon of grit onto the paper before spreading it out. Then pick through it with tweezers first, to remove any larger fossils, and then use a damp cocktail stick to pick up the rest of the smaller, finer specimens. I usually transfer these into a large pot, so that when I tap them there is a bigger target I don't miss (and every so often pouring this into a 5ml specimen tube). After the grit is sorted pick up the paper and pour (along the crease) into a pot or bag.
Two notes here: firstly LABEL everything, even at the start. Eventually you'll end up with a dozen bags of grit with no clue what is sorted or unsorted or where they are from. Secondly microfossils refer to fossils that are generally between 0.001mm - 1mm in size. While many of my specimens fit this (foraminifera, ostracods, some small gastropods etc) many don't and should be referred to as mesofossils (small crinoid fragments, brachiopods and bivalves etc), although for ease they will continue to be grouped for the remainder of this post.
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| Sorted microfossils. |
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| Sorted microfossils. |
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| Sorted microfossils (note pin for scale). |
After I've sorted my microfossils out from the non-fossiliferous grit/matrix, I then group the fossils:
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| Ostracods. |
This requires the identification of specimens. Guided by the local BGS memoir's faunal list for the localities, and internet searches, some of the specimens can be put down to species level. However, most can only be broadly described. When first finding microfossils I created as guide to the specimens:
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| A guide to the microfossils of the Alston Fm: now somewhat outdated, with many specimens refined in their identification. |
Mounting the specimens
I've bought commercial microfossil slides from both a Dutch website [1] and a UK website [2]. Both of these I would highly recommend- I suspect that both companies source the slides from the same manufacturer. Additionally, despite the slides coming from the Neatherlands [1], they are at a similar price point when postage and packaging is included as it is calculated the weight of items that you buy.
Once the slides have been bought, the next step is glue. From what I can find it seems that the traditional glue used in mounting the specimens is tragacanth gum, so I bought some online and experimented.
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| Mixing the tragacanth gum. |
While the tragacanth gum did work, I found a few issues. Firstly it is difficult to find, with most products being CMC, the chemically synthesised version of the natural gum tragacanth- this is what I used. Secondly, when mixing it with water, the powder holds a large amount of liquid, resulting in the slide surface (if paper or card) buckling, although it does eventually dry. After mixed success I decided to look at a commercial microfossil glue. The only one I could find [3], that I bought alongside some more slides, works incredibly well.
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| Mounting the specimens. |
Continuing, next the specimens must be mounted. This was done by sticking a cocktail stick into the glue, putting a spot of the glue in the correct place and then placing the specimen (with tweezers or a wet cocktail stick) ontop of it. I personally did this by eye, without the aid of a microscope as can be difficult to position the specimens correctly. Instead, while the glue is wet, I moved the slide under the microscope and positioned the specimens more accurately (e.g. all ostracods hinges facing the same way).
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| Bryozoa I. |
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| Bryozoa II. |
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| Ostracods (1-7), foraminifera (17-23), unknown foraminifera-like organisms (22-37), conodont elements (49-50) |
As well as microfossils, I also experimented with using the slides to mount modern organisms- namely foraminifera and other organisms, collected from a Hebridean beach in 2019. This works just as well, however, one has to be particularly careful as they are far more delicate and are crushed if too much pressure is added by the cocktail stick (as seen in the centre of the 10 square below).
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| Foraminifera (1-12), ostracods (17-20) and bivalves and gastropods (25-28). Specimens from Killinallan, Isle of Islay, Inner Hebrides. |
After mounting the specimens, the final step is to add a label. I actually added two labels, one to the front, stating what it was, the formation and the location (with an 8-figure NGR), and one on the back with more detailed information, including the full stratigraphic information, as well as the age and detailed location. These were made by printing onto blank 'sticker paper' using a inkjet printer.
It is a really satisfying feeling completing the process from collection to mounting. These slides don't just act as a display of my microfossil, but as a record of specimens, somewhat similar to type specimens, that I can later refer back to, to compare specimens' morphologies. The next step is making my own slides as a cheaper alternative as well as being able to make bespoke sizes and apertures.
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