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Caithness Field Club Bulletin
The Stratifigraphic Position of the
Old Red Sandstone of Sandside Bay
With the present interests in Sandside Bay I thought it desirable to produce a document as to its position in the stratigraphy of the Caithness Old Red Sandstone. Its position is of some concern because the British Geological Survey Map, Reay, Scotland Sheet 115(E), Solid Edition, maps it as being in the Latheron Subgroup (=Spittal Subgroup) of the Upper Caithness Flagstone Formation, which would appear to be inconsistent with its fossil fauna. The typical fish found there is Thursius macrolepidotus which, in 30 years of collecting, I have not recorded above the Lower Caithness Flagstone Formation. The relationships of the Subgroups of Donovan et al. (1974) and Donovan (1975) for the Upper Caithness Flagstone Formation are not always clear and some of the current mapping is obviously erroneous. The table on the geological map shows that Thursius macrolepidotus is found from the Clyth Subgroup right through to the top of the Mey Subgroup. I find that hard to believe. There are several fish beds exposed at West Sandside, two of which were plundered by foreign collectors several years ago. All the beds contained Thursius macrolepidotus. A fortuitous find in 1999 revealed a "new" Dipnoan (lung fish). Having a larger than normal posterior dorsal fin, it resembles Pentlandia; it is, however, more robust, the head is well ossified and both skull and scales are covered with cosmine. It has been my belief for many years that the less accessible fish beds at East Sandside were of the same age. This was in doubt since there seemed to be evidence of a large fault in the bay.
During the mapping of the fish beds at both east and west of the bay I have indicated them as: 1W, 2W etc. for the West side of the bay and 1E, 2E etc. for the East side of the bay. This does not mean that Fish Bed No 1 on the West side of the bay correlates with Fish Bed No 1 on the East side of the bay. There may, in fact, be no correlation between any of the beds.
The shore section at West Sandside is easier of access than East Sandside, though the fish beds North of the harbour are only accessible at low tide. Fish bed No 1W dips about 6deg. to the North North West. They are of a medium grey colour.
Fish beds Nos 2 & 3 W dip about 6deg. to the North North West. They are of a medium grey colour and were ripped out by illegal collectors a few years ago. They were sent about their business once discovered, but left behind sufficient material of Thursius macrolepidotus to ensure that they were recorded correctly. There is still plenty of evidence from loose blocks and it was from one of these that the "new" Dipnoan mentioned earlier was discovered.
Fish Bed No 4W is in the sea stack North of the harbour. The rocks are much redder than the surrounding rocks which might indicate that it is a faulted block. The fish bed dips at about 12deg. North North East. The laminite containing the fish fragments is light grey and about 1m in thickness. It was impossible to extract any material since collectors have excavated the fish bed back into the overhang of the beds above.
Fish Bed No 5W is in the reef to the North of the sea stack. A single specimen of Thursius macrolepidotus was collected from here many years ago; it was sent to the Royal Scottish Museum in Edinburgh.
Fish Bed No 1E dips at about 7deg. to the North. It is blue in colour and contains small Osteolepid scales (Thursius macrolepidotus?)
Fish bed No 2E dips about 7deg. to the North. It is almost black in colour and again contains small Osteolepid scales.
A massive sandstone band separates this fish bed from the next one.
Fish Bed No 3E dips about 10deg. to the North. It is almost black in colour and contains small Osteolepid scales.
Fish Bed No 4E dips about 10deg. to the North. It is blue in colour and contains small Osteolepid scales and some coprolites.
Fish Beds Nos 5E and 6E dip about 10deg. to the North. They are blue in colour and contain small Osteolepid scales.
Fish Bed No 7E proved to be a great surprise; it is blue in colour and contained a median dorsal plate of a large Coccostied (Dickosteus?), Gyroptychius milleri and, loose, the "new" Dipnoan. It also yielded several delicate Dipnoan tooth plates. Since these are unlike those of Dipterus, it must be assumed that they belong to the "new" Dipnoan. These first two fishes are typical of the Spittal Beds and therefore are in the Upper Caithness Flags. It also dips about 10deg. to the North.
Fish Bed No 8E dips about 10deg. to the North and contains larger fragments of small Osteolepids. It is blue in colour.
Fish Bed No 9E dips about 10deg. to the North; it is almost black in colour and contains complete specimens of Thursius macrolepidotus.
Fish Bed No 10E dips about 10deg. to the North; it is dark in colour and contains complete specimens of Thursius macrolepidotus and a single small Acanthodian.
Fish Bed No 11E dips about 10deg. to the North and contains small fragments of Osteolepids. It is a cliff face exposure and no bedding plane can be seen.
There are further fish beds in the direction of Dounreay, but these fall outside the East Sandside region. There they contain the typical fossil fauna of the Upper Caithness Flags. In the normal stratigraphic column we would expect to find the Achanarras Horizon between the Lower and Upper Caithness Flags. This important horizon appears to be completely absent. On reflection, Achanarras represents the deep water part of the lake whereas Sandside is clearly a lake margin deposit and may represent oxbow lakes in a mature river valley. It is probable that there is no "normal" Dipterus in this section and that the scattered Dipnoan scales belong to the "new" Dipnoan.
The "new" Dipnoan is the subject of a further scientific paper which is in preparation.
Mem. Geol. Surv. Gt. Bt.
Donovan, R.M., Foster, R.J. and Westoll, T.S. 1974. A Stratigraphical revision of the Old Red Sandstone of North-Eastern Caithness. Trans. R. Soc. Edin. 69, pp176-201.
Donovan, R.M. 1975, Devonian lacustrine limestones at the margin of the Orcadian Basin. Jl. Geol. Soc. Lond. 131, pp489-510.