A Marine Clay from Burghead Bay
MFC Paper 21 by SM Ross and T Macfarlane
In May 1977 a large area of horizontally laminated olive green clay became exposed on the beach below high water mark 300m east of the Mid Fishing Station, Burghead Bay at NJ 066 646. This deposit is of great interest because with the rate of erosion on this part of the coast being between 1m and 2m per year, the clay has only recently been emerging from under a cover of 8-9m of sand and gravel deposited at a time when the sea level stood higher than at present
The clay measured 160m along the beach and 60m across, with the eastern and southern edges dipping out of sight under the sand at the change of slope of the beach. The exposure was 42m from the sand and gravel "cliff" on 26:5:77 and 20m to the seaward of the old coastal defences. No precise levelling was possible but the top surface of the clay was 2.69m below high water mark on that day, when the tide was one of 3.5m. The clay was still visible on 21:12:77 but the top surface was deeply furrowed by small channels in which pebbles washing up and down were cutting through the deposit. Salmon fishers recall encountering other areas of clay between this spot and Burghead. These lay under the sand and were located when stakes were driven in to support the nets.
The clay was dense and impervious and varied in thickness from 38 to 50cm, probably due to scouring. It contained many small shells, most of which were concentrated in layers near the top. All of these were extremely fragile, retaining their shape due to the support of the clay. Plant and seaweed remains of a brownish hue occurred on horizontal laminations and these planes showed a little white mica. Some solid stem material also occurred.
Below the clay there was a quick transition over a few cm to a grey silty sand of anaerobic aspect in which occurred splendid specimens of large bivalves in perfect state of preservation complete with periostracum.
Samples of the shells were sent to the Royal Scottish Museum and the Institute of Geological Sciences in Edinburgh for identification and included:
- Bittium reticulatum (da Costa)
- Abra Alba (Wood)
- Parvicardium exiguum (Gmelin)
- Parvicardium Scabrum (Philippi)
- Thyasira Flexuosa (Montagu)
- Gibbula cineraria (Linné)
- Littorina Littorea (Linné)
- Littorina saxatalis (Olivi)
- Lutraria lutraria (Linnaeus)
- Cerastoderma edule (Linnaeus)
- Spisula Sp.
- Lucinoma Borealis (Linné) ... from below clay.
All these shells occur in the Moray Firth today with the exception of Bittium reticulatum which is suggestive of a period when the climate was a little warmer than at present. Options for this are therefore 13000, 9500, 5000 and 500 BP. A preliminary comment on a clay sample sent to I.G.S. suggests the material may be of post glacial age but closer examination will be needed before this can be confirmed.
Should the clay prove to be post glacial in age, then it must have been over~run by sand and shingle spits moving along the coast from the north east during the time of the post glacial high sea level of some 4500 BP when the sea reached a height of 5m above its present level.
Another point of interest is that a few years ago one of us (TM) found a .large number of oyster shells on this beach but to the north east of this point at low water. Nearby were the remains of an old boat and it was thought at the time there might have been some connection and the matter was not pursued. Bands of marine clay are also found in the Loch of Spynie area and Lossiemouth Airfield under varying depths of surface deposits.
Acknowledgement: We wish to thank Dr. Sheila Smith of the R.S.M. and Dr. Douglas Graham of I.G.S. for identifying the shells.