June 1994 Index

Caithness Field Club Bulletin
June 1994

THE GEOLOGY & PALEONTOLOGY OF THE SHORE SECTION
BETWEEN THURSO & SCRABSTER
By Jack Saxon

Abstract
The Caithness memoir of the Geological Survey dismisses the coastal section between Thurso beach and Scrabster Harbour with the statement that it: "...... is largely occupied by bands of rather soft false-bedded sandstone, weathering pink and yellow". It is, however, a very interesting section consisting of a large number of cyclothems each with its own distinctive fish bed, representing hundreds of thousands of years of deposition with climatic conditions ranging from wet to arid in a repetitive series of cycles. My main interest being palaeontological, the section has been mapped in detail showing the fish beds which represent the lacustrine phases of the Old Red Sandstone (the climatic maxima).

The Historical Field Descriptions
In the 1960s I first found fragments of fossil bone in the shore section between Thurso and Scrabster. Unable to identify them, I went to Thurso Library and was permitted to look at the Caithness memoir (1).

The report, by C.B.Crampton and R.G.Carruthers was little help. On page 66 of the Caithness Memoir Crampton reports, "The flagstones in the vicinity of these flagstones at Murkle and in Thurso Bay have yielded Thursius pholidotus and Coccosteus minor."

Coccosteus minor is now known as Millerosteous. On page 67 of the Caithness Memoir Carruthers reports "The shore section immediately west of Thurso is largely occupied by bands of largely soft false-bedded sandstone, weathering pink and yellow." Clearly Messrs Crampton and Carruthers were not going to solve my problem. Either something dramatic had happened to the shore west of Thurso since the Memoir was written or, unthinkably, Carruthers had not taken the time to look at the rocks in detail.

I wrote to the Geological Survey in Edinburgh but received no reply. I later went in person armed with an introduction from the Royal Scottish Museum. Eventually I was shown the manuscript maps of the area. Carruthers had not consulted them before writing his brief, non-committal sentence.

I was lucky to obtain a copy of "Robert Dick, Geologist and Botanist" by Samuel Smiles(2). Here, in chapter X, I had better luck:

"Shouldering an old poker, a four pound hammer, and two chisels in my pockets, I set out for the burn of Scrabster. After a great deal of hammering, I found no end of young Coccosteus. I might have filled a barrel with them, but they were all broken. What hammering! what sweating! Coat off: got my hands cut to bleeding. Found a very hard bituminous bed. It rings like a piece of metal. What pokering! Got three or four fish, not much worth. Don't think them new. Found a plant. Found scales of Holoptychius. Wrought on till the moon shone clear in the water of the burn. Returned home at twenty minutes past ten"

He wandered westward on the sand, and then on the rocks, hammer in hand, ready to strike a blow, or any number of blows, for the honour of science.

"Passing on," he says, "I walk over a bed of loose sand smoothed and levelled by the tide, and after a time I reach the solid rocks, of a bluish-grey cast, and dipping northerly, with a little of west. The first beds I meet are not decidedly fossiliferous, though a few scales and droppings may be found. A little farther on I see some warty bones, and still farther, there is a bed decidedly charged with organic remains. Pieces of fish jaws, bones, and tail-half plates of Coccosteus, are seen in considerable numbers.

"Moving on, I reach an opener space, strewed with fragments of a dark blue flag, charged, more or less, with organisms. Some very fine fossil fish have been found there. I next come in sight of the human burying-ground on the top of the bank, as distinguished from the fish burying-ground on the rocks underneath.

"A little past the burying-ground, and on the beach, I find a change in the dip of the strata. The beds dip east, though almost immediately thereafter they return to their former dip - namely northerly, with a little of west, and continue so until we arrive at the Bishop's Palace, where yellowish, whitish, and striped beds of sandstone prevail. The beds on which the ruined palace stands are reddish and yellowish looking, and dip in the same direction.

"Sixteen years ago, I remember making an attempt to explore the inner recesses of the ruined palace. I entered the inner recesses of the ruined palace. I entered the cave underneath with a lighted candle; but I found it utterly impractical to make my way without pick and spade. There is a low door, which seems to lead to subterraneous chambers; but the passage is choked with rubbish.

"The little burn of Scrabster runs round the rock, entering the sea at its north-west side. The water would be useful to the castle inmates. I have sometimes seen sailors ashore filling their barrels there.

"Close beside the burn, a ridge of clay occurs, and sweeping round Scrabster Bay it rises in some places to about a hundred feet. It is blue and full of stones of various sizes. I have often been astonished at its appearance, and wondered where it could have come from. Some call it boulder clay, and say that it is similar to what skirts the base of some Alpine mountains. It may be a Moraine. It seems to fill an irregular hollow. The bare rocks are through the soil on the hill-top, immediately behind. Can it really be that those hill-tops, now so insignificant, once towered above the clouds, capped in snow, bound up in ice, and that they have gradually mouldered away down to their present elevation of a few hundred feet above the level of the sea?

"Low down, at the Coastguard house, beneath a weight of clay, the strata crop out, and are at first slightly charged with organisms. A little farther on I find beds charged with warty bones; and the strata dip northerly. Then there is a fault, the strata are in confusion, and dip westerly. They then become nearly horizontal, and continue so until the extreme end of Holborn Head, where I find them slaty, and highly calcareous, bituminous, and containing many remains of fish.

"There is a noted fault to be seen almost atop of the point of the promontory. The strata slope in different directions. They are bent, twisted, contorted, and in great confusion. At one place, they are quite on end. What a subterraneous convulsion there must have been here at one time!"

--0--

The fault at Holborn Head is a reversed fault which can only be really appreciated from the sea. I have often wondered about its relationship, if any, with the reversed fault which brings down the red and yellow fluviatile sandstones of the Upper Old Red Sandstone of Dunnet Head against the grey lacustrine Middle Old Red. The junction can be clearly seen at Dunnet by the change of vegetation from the green pasture land to the rough heathland on which the House of the Northern Gate stands.

I searched the literature for a more complete field description but without any success; I next turned my attention to the literature on the fossil fishes.

The Descriptions of the Fossil Fishes

I was fortunate early in my studies to obtain from the Royal Scottish Museum a copy of Jarvik's very thorough descriptions of the Osteolepid Fishes.(3) This enabled me to identify the Osteolepids in the fish beds in the shore section. It also provided a reading list which gave me almost all the literature I could wish for. If only the remainder of the fishes had been so carefully described!

Miles and Westall (4,5) described Coccosteus, Dickosteus and Watsonosteus but Miller's description of Dick's "baby Coccosteus" as Coccosteus minor is no description at all and we had to wait for Adrian Desmond(6) to produce a really good description. Forster-Cooper(7) gave a good description of Dipterous but Miller's description of Homosteous(8) which he mistakenly called Asterolepis was a real disaster. Not only did he fail to understand the structure of the fish but he married the jaw of Gyroptychius to his Asterolepis and we had to wait for Heintz(9) to produce a text figure. Sadly, an adequate description already existed but it was in German and did not include text figures.

I know of no comprehensive description of Glyptolepis paucidens but it is distinctive enough to be easily recognised from the pattern of the scales, the pectoral fins and the bones of the skull. Watson(l0) described the acanthodian fishes but a good description of Mesacanthus peachi is still wanting.

These represent the fishes likely to be met with on the costal section referred to.

A brief description of the Caithness topography would be useful at this point. A mountain range is found along the length of the County March from the Ord of Caithness on the cast coast to Drum Holistan on the north coast. From this range of hills the Caithness plateau or peneplain slopes gently towards the sea ending, usually, in striking cliff scenery. Here and there, however, the sea has made inroads against the cliffs, along lines of weakness and where softer rocks occur, to produce bays and wave-cut platforms which are largely covered at high tide.

Thurso Bay is one such area; the cliffs are mantled with glacial drift which, to the west of the Coastguard station, forms a considerable cliff. It is probable that a dead sea cliff lies behind the glacial drift. To the east of the section a dead sea cliff can be clearly seen in the gardens of the houses on the seaward side of Durness Street in Thurso. This is again covered by glacial drift. One possible explanation is that the cliff already existed prior to the development of the ground moraine.

The cliff section to the north of Pennyland farm is cut by numerous "geos" or narrow inlets which are, in the main, joint-controlled. The cliff of glacial drift lies largely below Scrabster Mains Farm.

Miles acid Westall,(4) P199 suggest that:

"The costal exposures just east of the sands off Murrkle Bay clearly resemble the Mey Beds, and the presence of yellow and pink sandstones in some abundance suggests a position fairly high in the Mey Beds, as sugggested by the survey (CM.,p.65) The same is true of the exposures in Thurso Bay under Pennyland; both yield M. minor, Th. pholidotus and other fossils. The exposures east of Thurso, between the Thurso River and Murkle Bay (i.e. what is often called on old labels "Thurso East Shore" and "Clairdon Shore" are highly characteristic of the less sandy Mey Beds lithology (C.M., p.66) and yield the Mey Beds fauna except for M. minor, which has not yet been recorded. Thus the Survey (in our opinion properly) recognised both the Thurso Bay (Pennyland) and "Thurso East" exposures as pertaining to the Mey beds."

The Kirk Ebb is separated from Thurso East by the Thurso River. The Caithness Memoir suggested that the estuary lies on a fault. However, during dry weather and with exceptionally low tides, the rocky bed of the river can be clearly seen and it shows no sign of faulting. Furthermore the fossils found in the Kirk Ebb are the same as those at Thurso East being mainly Homosteus, Gyroptychius milleri and Dipterous. The western section under review here contains mainly Millerosteous minor and Thursius pholidotus which appear to be completely absent from Thurso East.

Finally the Thurso East exposure would appear to be older than that of the west shore but no junction between the two can be seen owing to the sand and shingle covering the shore between them.

The western dip of the strata means that a greater thickness of rock is seen on the western foreshore than on the eastern one. At least twelve sedimentary cycles and, at most, 21 are to be seen, the doubt as to the number of cycles being due to the presence of a number of faults of unknown throw.

The sedimentary cycles or cyclothems are probably due to a climatic cycle of alternating arid and pluvial periods, the fish beds representing permanent water in the Orcadian lake, each period lasting several thousands of years.

The section was mapped during the winter of 1964-5 by myself and my son E.R. Saxon, using a measuring tape, compass and inclinometer. Fossils were collected from each fish bed (numbered 1 to 21 from west to east) and a representative collection was deposited in the British Museum (Natural History) together with the locality (The Pennyland Fish Beds). The dip of each fish bed is indicated by an arrow (the direction of the dip) and a number (the angle of the dip).

It should be noted that our mapping concentrated on the fish beds, largely ignoring, the lithography of the non-lacustrine sediments.

Fish bed no.1 consists of thin laminae which are plane and flat so that, although the rock, is hard, it splits easily. Most of the collecting was done from pieces which had been broken off the main fish bed and were lying nearby. Millerosteous minor was reasonably abundant in this bed, together with (fewer) specimens of Thursius pholidotus. Two small acanthodians were found in this fish bed and were placed in the University College, London. They were believed to be Mesacanthus peachi.

Fish bed no. 2 is separated from fish bed no.1 by a fault. It is possible that they are one and the same bed, both on lithological and faunal grounds.

Fish bed no.3 consists of poorly-fissile flagstones and contained M. Minor and Th. pholidotus. The green barren rocks, so common in the sequence separating fish bed no.3 from fish bed no.4.

Fish bed no. 4 contained osteolepid fragments together with fragments resembling Dipterous. Green barren beds underlie this bed.

Fish bed no. 5 is probably separated by a fault from fish bed no.4. Fossils were rare; the onlydeterminable one was a badly crushed skull of Dipterous valenciennesi, which was later deposited with the Royal Scottish Museum, and a fragment of GIyptolepis paucidens.. The underlying rocks are the curious "nodule beds" characteristic of the Thurso Flagstone Group. A fault probably separates the nodule beds from a section containing a small syncline, a further fault separates it from fish bed no. 6.

Fish bed no. 6 is quite small and contains fragments of osteolepids, probably Thursius. Large loose boulders near this fish bed contain bituminised fragments which might be Homosteus. A mysterious specimen found loose at this locality was sent to the British Museum (Nat. Hist.) It turned out to be an uncrushed skull of Glyptolepis paucidens. Two faults or joints exist between fish beds 6 and 7.

Fish bed no. 7 contains thin cycloid scales which are not yet accurately determined but presumably they belong to Glyptolepis or Dipterous. These beds are fine splitting flags similar to those of fish beds 1 and 2. Scrabster Burn debouches over the shore at this point.

Fish bed no.8 lies in part of the bed of Scrabster Burn and this may be the classic locality which has yielded fine specimens of Th. pholidotus. We found nothing identifiable in this fish bed. The underlying rocks, and those on which the ruins of Scrabster Castle (the Bishop's Palace) stand, are false-bedded and to a large degree arenaceous, consisting in some parts of coarse, sub-angular, gravel. These overlie green barren beds.

Between Scrabster Castle and Thurso, the Old Red Sandstone forms cliffs about fifty feet (16 m) high, covered with glacial drift. The cliff passes inland at the eastern end, the houses on Durness Street being built on top of the cliff. The steep gardens cover this dead sea cliff. The houses of Shore Street may also be built on this dead sea cliff. The cliffs are truncated by Thurso River. The Kirk Ebb is clearly part of the Thurso East succession.

A complex of faults, probably of very small throw, exists in the rocks between Scrabster Castle and the Mausoleum and two of them truncate fish bed no.9, where we found M. minor in some abundance, together with osteolepid remains, probably Th. pholidotus. Fragments of indeterminate woody plants, having the consistency of jet, occurred here. The top of the fish bed consists of fissile flags like those of fish bed no. 1 and these pass downward into rough-splitting grey flags.

Fish bed no.10 is also truncated by a fault and contains osteolepid fragments, and the same fault produces fish bed no. 11 which also contains osteolepid fragments.

Fish bed no. 11 is truncated by a fault and another truncated an adjacent fish bed labelled 11b because it is almost certainly the same bed. In fish bed 11b we found M. minor, Th. pholidotus, and fragments of woody plants. The sea-stack adjacent to 11b consists, in part, of false-bedded sandstone, similar to those beneath the Bishop's Palace.

Fish bed no. 13 consists of coarse grey flags containing M. minor and Th. Pholidotus. False-bedded rocks, underlaid by green barren beds, underlie fish bed no. 13.

Fish bed no. 14 consists of coarse, grey flags and contains many plates of large osteolepid fishes, probably Th. Pholidotus, fish bed no. 15 being almost identical. Green barren beds occur between fish beds nos. 15 and 16.

Fish bed no. 16 contains plates of Th. Pholidotus and barren green rocks occur between fish beds nos. 16 and 17.

Fish bed no. 17 contains plates of osteolepids and is curious in that the colour shades from yellow in the upper region to the usual grey in the lower. A joint runs through the middle of the bed.

A fault occurs between fish beds 17 and 18 and the lithology of both beds is identical. Osteolepid plates are plentiful in fish bed no. 18.

Fish bed no. 19 contains plates of osteolepids and is truncated on the landward side by the same fault which truncates fish bed no. 20; this also contains osteolepid plates.

Fish bed. no.21 lies almost vertically below fish bed no.20.

A great swarm of faults and joints ends the succession, before it passes under Durness Street.

Discussion
Millerosteous minor and Thursius Pholidotus are common in these beds; Dickosteus threiplandii is very rare. This strongly suggests that they are the equivalent of the East Murkle development of the Mey Beds where M. minor and Th. pholidotus are characteristic fossils (these genera are also characteristic of the Rousay Beds in Orkney). Millerosteus is otherwise only infrequently present in the fossiliferous bands in the Mey Beds. The yellow friable sandstones found in the Pennyland succession are similar to those on the S.E. side of Murkle Bay.

The Thurso East development of the Mey Beds contain Dickosteus but not M. minor or Th. Pholidotus: these beds are believed to occupv a lower position in the sequence. Fishes typical of Thurso East, in addition to D. thriejplandii, are Homosteous milleri, Gyroptychiu milleri, Dipterous valenciennesi and Glyptolepis paucjdens.

The beds exposed at the Kirk Ebb, to the west of the Thurso river, have yielded H. milleri, G. milleri, G. paucidens and D. valenciennesi; so far they have failed to yield D. thrieplandii. It is, however, likely that they belong to the Thurso East succession, because the lithology is very similar to that at Thurso East, the fauna are present in similar numbers, and are similarly preserved.

The rocks exposed to the west of the Pennyland Beds (Holborn Head) are separated from them by a fault scarp and fall within the Spittal Beds. They are lithogically similar and, at Taldale, have now yielded D. thrieplandii, G. milleri, and D. valenciennesi; neither M. minor nor Th. Pholidotus have ever been recorded from this locality, though rare specimens have been found on Holborn Head.

The frequent occurrence of barren green beds between the fish beds in the Pennyland succession has its counterpart at East Murkle.

Conclusions
The shore section to the west of Thurso consists of a wave-cut platform flanked on the landward side by cliffs. Those east of the Bishop's Palace are of the Old Red Sandstone whilst to the west they are of glacial boulder clay. Twenty one fish beds occur in the platform containing mainly Millerosteous minor and Thursius pholidotus. These represent the deep, permanent water deposits in the cyclothems of the Orcadian basin. Shallow water conditions can be seen in ripple-marked pavements and arid conditions are represented by desiccation polygons, what may possibly be "desert roses" and by false-bedded sandstones, some of which may be fluviatile.

It is one of the most interesting sections in the Orcadian Middle Old red Sandstone.