Extracting microfossils from Lower Chalk

Microfossils can be obtained from the Grey Chalk Subgroup (formerly the Lower Chalk, minus the Plenus Marls) with great ease. This rock has a high clay content in comparison with the whiter chalks and breaks down readily to reveal a rich fauna when exposed to successive cycles of wetting and drying. These chalk cliffs are often coated with a fine powdery substance, which is the product of sub-aerial weathering. This powder can easily be collected and is virtually ready to be examined for microfossils as it is.

If it is unsafe to collect from the cliff face itself, but accumulations of weathered and weathering material can usually be found a little distance away. When collecting from these, there are two approaches that can be adopted. The first is to collect large pieces of chalk that will need to be weathered artificially before microfossils can be obtained from them. The second is to sample chalk sediment that has broken down into a soil-like material. This material takes less processing than the large chalk blocks, but the microfossils obtained from it will be from various levels, rather than restricted to one level as in the case with blocks.

If large blocks have been collected, break them up when you get home to pieces of about 2.5cm3 cubed. Dry these naturally outside, or in an oven set to about 80oC, and then soak them in water for a day. After this time, much of the chalk should have broken down into a sludge. Pour this sludge into a container, being careful not to include any lumps of chalk that have not fully broken down.

The soil-like material found at the base of the cliffs should also be dried and soaked to break down any remaining rock fragments. Powder obtained directly from the cliff face will not need to be soaked before sieving.

The material is then wet sieved using 2mm, 250 micron and 150 micron sieves, which are available from UKGE. Be sure to remove the ‘u’ bend below the sink you are using and place buckets under the plug hole instead. Failure to do so will probably lead to blocked drains.

The residue remaining in the 250 micron and 150 micron sieves after sieving is then washed in a suitable vessel and boiled in a 10% solution of sodium carbonate. After about an hour of boiling, wet sieve the material again and then allow it to dry.

Within this material will be found a great quantity of forams, ostracods and other microfossils.

microfossils2Highly fossiliferous Lower Chalk microfossil residue.

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Lower Chalk microfossil residue packed with forams etc.

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Some Lower Chalk microfossils – forams and ostracods. Arrangement is 10mm across.

lower chalk microfossils
More lower chalk microfossils.

by Joe Shimmin

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