THE ORCADIAN MIDDLE OLD RED SANDSTONE
Jack Saxon

The Middle Old Red Sandstone of the Orcadian cuvette consists of both lacustrine and lake margin sediments. The lacustrine sediments are dominated by a monotonous rhythm, or cyclic sedimentary sequence, resulting from long-term cycles of transgression and regression in the level of the lake or lakes. These rhythms vary in thickness from 3 to 20m (occasionally even more) with an average thickness of 8 to 9m. The rhythms are attributable to a climatic cycle.

During periods of regression the rivers eroded and reworked their flood plain deposits and the older lake sediments. As the lake level rose during periods of transgression, the in-flowing rivers silted up and the transfer of sediment into the lake slowed down. During the early stages of transgression lake shoreline processes sorted the sediments under the action of wind resulting in symmetrical ripple marks. Minor fluctuations in water level resulted in sub-aerial desiccation of the mud flats. "Bird's eye" structures probably resulted from gas bubbles being released from decaying organic matter. Ripple marks and desiccation polygons are well developed at Thurso East.

As water cover became more permanent sub-aquaeous shrinkage (syneresis) cracks formed owing to increases in salinity caused by dry season evaporation. These were infilled with coarse silt during the succeeding wet season. In the older literature these strange features were referred to as "repousse ornament".

During deep water stages some degree of stratification of the lake waters developed (thermoclines and chemoclines). Carbonate deposition in the surface waters was induced by photo-synthesising algae. The sediments produced at this time were non-glacial varves (or laminates) resulting from an annual rhythm. Seasonal algal bloom led to increased photosynthesis, an increase in pH, and carbonate precipitation. Dead algae accumulated as organic matter on the lake floor and elastic debris accumulated continuously or periodically as micro-turbidity flows. These sediments may contain as much as 4% organic carbon; gas chromatography of the hydrocarbons indicate that most of the material is of algal origin.

As the lake regressed, rivers in the surrounding alluvial plains eroded their channels and transported an increased amount of sediment into the lake. Alluvial sediments in the basin include scree breccia, alluvial fan conglomerates and braided river sandstones, siltstones and mudstones.

During Middle Devonian times some 4000m of sediment accumulated in the Orcadian basin owing to these processes.

THE ACHANARRAS HORIZON
The Achanarras Horizon occurred during the maximum period of transgression and the cycle is one of the thickest in the Orcadian cuvette (60m in Orkney). The cycle starts characteristically with the fish

bed lithology of dark laminated flagstone (laminites) deposited in near stagnant anoxic conditions beneath the thermocline in the thermally-stratified lake. Calcareous sandstone beds occur immediately above the fish bed which, at Achanarras, contain the remains of primitive vascular plants. Above these are papery shales. The Achanarras horizon probably represents the greatest extent of the Orcadian lake since it is found as far north as Melby in Shetland and as far south an Gardenstown in Banff(1).

At Achanarras quarry the fish bed comprises about 2m of dark, finely laminated flagstone. By averaging the thickness of the seasonally- deposited laminae Trewin(2) estimated that the lake was in existence for some 4000 years during the deposition of the achanarras fish bed. Some 14 genera of fishes occur at Achanarras(1). These are not randomly distributed but occur in zones e.g. Palaeospondylus and Mesacanthus occur on the same bedding planes but Dipterus and Coccosteus appear to be mutually exclusive whilst Osteolepis appears to occur only in the lower laminae.

It is clear that the fish did not live in the environment represented by the fish bed since the conditions were anoxic. Some of the fish may have lived in the surface water above the thermocline but most would appear to have drifted out as decomposing carcases. It is also possible that local mass extinctions may have taken place due to algal blooms similar to the "red tides" which occur in the Red Sea.

The quarry began life as a farm quarry but when the flags were found to be capable of splitting quite fine the quarry continued as a "slate" quarry. The overburden was removed in trucks on narrow gauge railway lines and tipped to form a bing down the slope. The fissile flags were converted into roofing slates in a low drystone building. As demand fell off the spoil was dumped on the quarry floor. Quarrying finally became uneconomic in the 1960's. The quarry is now derelict and full of water.

The approach to the quarry is by a gated farm track. The last gate is kept locked to deter commercial fossil collectors who did a great deal of damage during the 1970s, so the last few hundred metres is on foot. The quarry is under the jurisdiction of the Nature Conservancy Council and collecting is permitted only from the quarry spoil. After 20 years of being picked over by experienced collectors ther1e is little chance of making a find.

RED POINT
Whereas Achanarras quarry represents the sedimentary deposition in the quiet waters in the middle of the Orcadian cuvette, Red Point, by contrast represents the complex conditions which existed at the lake margin itself(3).

The underlying granodiorite basement displays strong relief. Steep hills up to 50m in height have been exhumed from beneath a cover of screes, sandstones and lacustrine marginal limestones. At the west end of the outcrop the limestones pass rapidly into typical basin centre carbonate laminites. The present author found a single specimen of Dipterus here which confirms its Middle Devonian age.

The approach to Red Point is over heather-covered peat bog and the exposure is on the cliff face. This involves an easy scramble. Waterproof boots are required and they should have soles with plenty of grip for the final scramble.

The walk is worth it for the scenery alone. Botanists will find that the acid peat bog is worth looking over. This gives way to a green belt fringing the cliff edge. Geos, sea stacks and cliffs form a haven for a variety of sea birds.

R E F E R E N C E S