Caithness Field Club

FIELD EXPOSURE ON THE A9.
J. Saxon.

During recent roadworks on the A9 between Knockinnon and Laidhay, two visiting German geologists collected fish fossils from the cutting. It had been found necessary to blast the country rock revealing, on the south west face, about 4m of country rock. To the south there is a gentle anticline, followed by a small syncline to the north and then another gentle anticline, the rocks dipping below the till in the direction of the Latheron fault.

At the lowest part the rocks consist of rough blue-grey limestones with a good proportion of mica. These pass up into a thin, fissile calcareous flagstone and then into a yellowish sandstone. This gives the typical dark- light cyclothem associated with the east coast exposures of the Middle Old Red Sandstone.

The dark rocks were full of fragmentary fish remains but whole fish were recovered at the junction between the dark and light rocks, just in the yellowish sandstones.

From the fossils examined, the writer was able to determine the following: Osteolepis macrolepidotus (?), Gyrorptychius agassizi (?) Dipterus valencienesi, and interminable acanthodians. This is therefore, on faunal grounds, the Achanarras Horizon. The lithology is, however, totally unlike that at Achanarras, being more like the Sandwick fish bed in Orkney which is quarried for road metal.

This exposure raises two interesting points:-

1.  The whole fish were just above the junction of the light and dark parts. No fragments occurred in the bulk of the yellowish sandstone. This must mean that, until the onset of conditions leading to the yellowish sandstones, the fishes which had died had been broken up, presumably by scavengers. On the onset of the yellowish sandstone the scavengers had, presumably, vanished. This fits in with the writer's earlier theory of a climatic cycle in the Devonian, the onset of drought conditions brought fish life to an abrupt end within two or three years. The lower part of the yellow sandstones containing the fish were varved, while the upper part was not.
    

2. The gentle dip towards the Latheron Fault suggests that the throw of the fault is very modest as the rocks to the north of the fault belong to the Spital Beds which overlie the Achanarras Horizon. These pass downward north of the Latheron Fault into the Niandt Limestone, equated by Carruthers (Sum. Prog. Geol. Surv. 1908) with the Achanarras Horizon.