Extracting microfossils from Lower Chalk
by Joe Shimmin

Microfossils can be obtained from the Lower (Grey) Chalk with great ease. This rock has a high clay content in comparison with the white chalks and breaks down readily to reveal a rich fauna when exposed to successive cycles of wetting and drying.

Grey 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, accumulations of weathered and weathering material can usually be found a little distance away. When collecting from these accumulations 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 soil-like 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 about an inch 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 off the sludge into a container, being careful not to include any lumps of chalk that have not fully broken down.

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.

From here the material is wet sieved using 2mm, 250 micron and 150 micron sieves 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 into 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.

Microfossil Slides

Endecotts Test Sieves Available from UKGE

Highly fossiliferous Lower Chalk microfossil residue

Lower Chalk microfossil residue packed with forams etc

Some Lower Chalk microfossils - forams and ostracods. Arrangement is 10mm acros

More lower chalk microfossils