Upper Old Red Sandstone of Orkney
|Mykura, W. 1976. British regional geology: Orkney and Shetland. Edinburgh, Her Majesty's Stationery Office.|
Upper Old Red Sandstone
The only Orkney rocks classed as Upper Old Red Sandstone are the thick sandstones and underlying volcanic rocks which occupy the greater part of western Hoy. The latter rest on an irregular eroded surface floored by faulted and gently folded Upper Stromness Flags and Lower Eday Sandstone. The sandstone of Hoy has been equated with the Dunnet Head Sandstone of Caithness, as the two have identical lithological characters. Neither had, at the time of writing, yielded any diagnostic fossils [Scales of Holoptychius, a characteristic Upper Old Red Sandstone fish, have since been found in the Dunnet Head Sandstone, Caithness, by Mr. A. McAlpine (personal communication)] and their suggested Upper Devonian age was based solely on the fact that a phase of tectonic activity, followed by a lengthy period of erosion, intervened between the deposition of the Middle Old Red Sandstone sediments and the eruption of the Hoy volcanics. The results of radiometric age dating of the Hoy lavas are somewhat ambiguous, but it is now thought that the most likely age of these rocks is around 353 million years, which is Upper Devonian.
Hoy volcanic rocks
The first products of the period of Upper Devonian volcanic activity were mils and tuffaceous sandstones which crop out in a number of localities in north-west Hoy. These volcano-detrital sediments originally occupied a somewhat different area from that covered by the later lavas, as in many places the tuffs are now directly overlain by the Hoy Sandstone. Elsewhere in Hoy the lavas rest directly on the underlying flagstones and sandstones without an intervening tuffaceous layer (P915587and P915588).
The tuffaceous sediments (given as tuffaceous sandstone on P915587) were laid down on an undulating surface, and in consequence they vary greatly in thickness. On the north coast of Hoy between the Bay of the Tongue (HY 207 047) (P000596) and the Geo of Hellia (HY 190 042), and on the west coast between Rack Wick (HY 197 990) and Too of the Head HY 192 988) they locally reach a thickness of 15 m. Slightly farther west, however, at the Geo of Hellia itself and at the Old Man of Hoy (HY 176 008) they are completely absent. They consist of brownish red, locally cross-bedded sandstone with angular blocks and pebbles of basalt, as well as finely comminuted detritus of the same material. At Rack Wick, where the tuffaceous sediment rests on a hummocky basement formed by the Lower Eday Sandstone, its bedding planes are parallel to the slopes of the old land surface, following them up and down the hummocks. This suggests that a large proportion of the volcanic material was deposited, not by water, but directly from the air.
The Hoy Lava forms five disconnected outcrops in the north-west of Hoy and one at Melsetter (ND 265 886) on the south coast of the island. Only one flow appears to exist at any one locality, but it is not possible to state definitely whether over the whole of Hoy there was originally but a single lava flow or a number of small flows of similar composition. In some of the outcrops the lava thins and dies out within a very short distance, which suggests that, if all the lava outcrops belonged to a single flow, its shape must have been either very irregular, or else it was affected by subsequent local erosion.
The Hoy Lava is an olivine-basalt which contains porphyritic crystals of olivine and feldspar, set in a groundmass of iron ores, augite and plagioclase. It forms part of the impressive sea cliffs at Hellia on the north coast (P915587) and at the Too of the Head just west of Rack Wick. At Hellia it has a maximum exposed thickness of about 90 m comprising a grey-weathering vesicular lower part, a massive 60 m thick columnar central portion and an upper slaggy zone which is 15 m thick. The Old Man of Hoy sits on a platform of lava, which varies in thickness from over 7 to 3 m and appears to wedge out westward. In this area the lava may have been eroded before deposition of the Hoy Sandstone started. The most interesting exposure of the lava is at Too of the Head, where it thickens from nothing to over 60 m within a distance of about 4OO1n and appears to occupy a pre-existing hollow or valley. At Melsetter, on the south shore of Hoy, only the ropy, highly amygdaloidal top of the lava is exposed.
The Hoy Sandstone comprises a series of red and yellow sandstones with some thin marly partings, which attains a thickness of at least 1000 m. No remains of fish or plants have as yet been found in these beds, though Stephens and Edwards (in Wilson and others 1935, pp. 140-41, fig. 20) have recorded the presence of rather doubtful ‘tracks left by some creature as it crawled about over the mud’ in blocks of sandstone in the Burn of Redglen (HY 219 014) on the west side of Ward Hill.
The Hoy sandstones are medium-grained, red or yellow in colour, and generally trough-cross-bedded, with individual sets ranging to little more than 1-2 m in height. Slumped cross-bedding and convolute-bedding are common at certain horizons. Many sets contain chips of red and purple siltstone, and intraformational conglomerates at the bases of troughs are common. There are also rare small lenses of extraformational conglomerate as well as scattered pebbles of quartz, schist and gneiss. In a limited sector of the south shore of Hoy, where the author has recorded palaeocurrent data, the sandstones appear to have been deposited by currents moving to the east or east-north-east, but these findings do not necessarily apply to the entire group. It is likely that most of these beds were laid down in a fluvial environment, probably by braided rivers.
There appears to be some variation in the lithology and the resistance to weathering of the sandstone in different parts of the sequence. Thus in the hills of north Hoy and on the cliffs of the north shore the lower beds of the Hoy Sandstone are soft and friable and in places weather readily into loose sand. The overlying beds are much harder and form the prominent crags along these hillsides. The presence of the softer layers overlain by harder sediments has thus given rise to the steep-sided outlines of the hills in this part of Hoy. The near-horizontally bedded sandstone with its Vertical joints has been eroded by the sea into the spectacular vertical cliffs and seastacks of western Hoy. The Old Man of Hoy (P000613) is a vertical rectangular pillar of sandstone which towers to a height of 137 m above sea level. Almost equally spectacular is the rectangular pinnacle of rock just south of St John’s Head (HY 187 035), which has been almost separated from the main cliff by movement along the vertical joint planes. Further to the south-east, along the shore south of Little Rack Wick (HY 233 932), the vertical sandstone cliffs are undercut by closely-spaced rectangular sea caves.
- WILSON, G. V., EDWARDS, W., KNOX, J., JONES, R. C. B. and STEPHENS, J. V. 1935. The Geology of the Orkneys. Mem. geol. Surv. Gt Br.