Caithness Field Club 

Caithness Field Club Bulletin 1993

CYCLIC SEDIMENTARY STRUCTURES
IN ROCKS OF PROBABLE MESOZOIC AGE IN NORTHERN SUTHERLAND

 By Jack Saxon

Art and Science have a great deal in common. Both start with the careful observation of Nature, both order the observed facts, both draw conclusions. By art I mean the kind that Ruskin would have been comfortable with. It hardly applies to heaps of bricks or to paint thrown haphazardly onto a canvas.

Careful observation of Nature in my case leads often to a landscape painting or to a geological discovery. Often the two are inseparable. A few weeks ago I noticed, for the first time, a cyclic sedimentary rhythm in the hill called Cnoc an Freiceadain above Coldbackie.  I have climbed the hill more than once, looking in detail at the unusual rocks and I have never noticed the cyclothems. They first became obvious looking back over the Kyle of Tongue from near the head of the Kyle. When you are drawing you see things.

Figure 1 shows the Mesozoic outcrops of Northern Sutherland. There is a further outcrop south of Ben tongue at Meall nan Clach Ruadha. I have not visited this site.

There are a number of these outliers mapped on the Geological Survey maps as outliers of the Old Red Sandstone. The main bulk of these are in the area of Tongue.  There are two others at Poulouriscaig and Kirtomy. It is a six-mile round trip from the nearest metalled road to Poulouriscaig but Kirtomy is easy to get to by car. There may be a third outlier near Farr if the botany of the area is any clue to its presence. Thick coverings of boulder clay and peat make geological mapping a very inexact science. The absence of fossils make geological dating difficult.

According to Johnstone and Mykura(1) these outliers are probably Mesozoic in age; Permo-Trias or perhaps even younger. They bear little or no resemblance to the Old Red Sandstone. The matrix of the Old Red Sandstone breccias is extremely hard; individual pebbles cannot be separated. The matrix of the Permo-Trias breccias and conglomerates is friable and the individual pebbles weather out relatively easily. The Old Red breccias are purple-red in colour whilst the Permo-Trias are red-brown.

The Kirtomy outliers occupy N-S trenches and consist of red-brown pebbly arkose with conglomerate and breccia. They also appear to fill valleys with a northerly paleoslope.  The Poulouriscaig remnant is some 80 m higher than that at Kirtomy.

The outcrop at Coldbackie has a basal breccia overlain by up to 120 m of bright red fluvial sandstone with fining upward cycles and some incipient cornstones in the overbank deposits; these are overlain by up to 300 m of massive conglomerates with syenite clasts suggesting a southerly derivation.

The outliers around Tongue consist of conglomerate and basal breccia with clasts of local Moine racks and syenite from the eastern outcrops of the Ben Loyal intrusion.

By contrast the islands of Eilean nan Ron are made up almost entirely of up to 500m of coarse grey-brown conglomerate with clasts which include abundant Cambrian quartzites and Durness limestone which suggest deposition by braided rivers from a north-westerly source.

The outcrop at Coldbackie which Johnstone and Mykura call Watch Hill and, the OS map calls Cnoc an Freiceadain display a clear set of cyclothems.  These are not obvious on the adjacent Ben Tongue. Cyclothems usually form when fresh water and saline conditions alternate and, since the nature of these Permo-Triassic rocks indicate predominantly arid conditions, the presence of a nearby salina is indicated.

Figure 2 shows the view across the Kyle of Tongue. The hill with the clear step-structure (cyclothems) is Cnoc an Freiceadain and the hill with the mast on the summit is Ben Tongue.

REFERENCE
1   Johnstone, G. S. and Mykura, W. The Northern Highlands of Scotland, 4th Ed. HMSO, 1989.  

GLOSSARY
Arkoses are arenaceous rocks (sandstone) containing more than 25% feldspar and are commonly regarded as indicative of erosion under arid conditions.

Breccias consist of angular fragments implying minimal transport of material. They may also indicate the absence of chemical weathering, ie arid conditions.

Conglomerates consist of rounded or sub-rounded fragments implying more transport than breccias.

Cornstones are concretionary limestones generally formed under arid conditions.

Cyclothems are cyclic sedimentary rhythms usually caused by alternating cycles of fresh water and more saline conditions.

Syenites are coarse-grained intermediate igneous rocks characterised by the presence of alkali feldspars.