THE JURASSIC BOULDER BEDS OF
The Jurassic boulder beds of east Sutherland appear to be of shallow water origin. The beds are cyclic in nature consisting of dark shales alternating with shelly limestones in which angular boulders have been deposited. The Jurassic beds appear to have been thrown down along the Brora-Helmsdale fault to form a narrow strip of rocks extending from Golspie in Sutherland to Dun Glas on the Ord of Caithness. The boulder beds themselves are of Kimmeridgian age and are exposed on the coast from Kintradwell to Dun Glas.
The deposit can be interpreted as a warm temperate or sub-tropical, shallow water coastal deposit with alternating clear water conditions and stagnant lagoons. The cycle could have been caused either by periodic subsidence or periodic destruction of a bioherm or, possibly, a combination of both mechanisms.
The history of the research into the nature and formation of the boulder beds is long (since 1827) and some very ingenious theories have been advanced for the mechanism of their formation, some of them involving land ice, glaciers. catastrophic floods, landslides, in situ Old Red Sandstone breccias, fault scarps, submarine fault scarps with shallow water at one side and deep water at the other, tidal waves, earth quakes. In short, there are as many theories as there are papers on this remarkable formation. Whatever the mechanism the facts are there to be examined by any field worker, however inexperienced. It can be shown that some of the theories put forward are inconsistent with the facts ascertainable by relatively simple field work, provided we are prepared to accept that plants and animals which today live in certain climatic conditions also lived in those conditions in Jurassic times e.g. that corals grew in warm oceans and that cycads grew on equally warm continents, though probably with a wider distribution in earlier times.
THE FIELD OCCURRENCES
The dark shales contain vast quantities of the broken up remains of land plants, together with occasional marine animals such as brachiopods and belemnites. It seems clear that these shales were deposited in shallow water, under stagnant conditions and not far from land. Since the fossil fauna are marine the only environment which fits is that of a land- locked lagoon with some access to the open sea. A land with a fringing coral reef or a 'bioherm' fits this environment to a T.
The shelly matrix in which the boulders are found contains only shallow water fossil fauna. It also contains occasional well preserved remains of land plants. This suggests that it was deposited on an open beach in clear water and, since the fossil shells are frequently broken, there was probably plenty of tidal action, i.e. there was no barrier reef or bioherm to shelter this shell beach from the open sea.
The boulders in the matrix are all massive and angular and have distorted the strata in the shelly matrix. This means that they have probably not travelled far, otherwise they would show evidence of rolling and the effects of tidal action. They appear to have dropped into the shelly matrix from a height, possibly from a cliff. It must be further noted that none of the boulders in the boulder beds match any of the country rocks in the vicinity. Most of them are flaggy Old Red Sandstone and the nearest present-day outcrops are at Berriedale. There is no doubt that the boulders are of Caithness/Orkney type and the possibility that they were derived from much further north is quite high. This could have occurred if we were to postulate movement along the Brora-Helmsdale fault in this direction, i.e. the boulder beds have moved laterally in a south- westerly direction relative to the mainland mass of Caithness and adjacent Sutherland. If this was the case the minimum movement must be of the order of the distance from Kintradwell to Berriedale, the movement probably taking place in small stages over many millions of years. The enormous time-scale involved makes this amount of movement seem extremely slow, and therefore feasible.
THE FOSSIL EVIDENCE
The fossil are all of Jurassic age (excepting the derived fossils in the boulders) and zone fossils show the deposit to be Kimmeridgian, which is late Jurassic. The marine fossils consist of ammonites, belemnites, molluscs and brachiopods, together with masses of colonial corals. This suggests a warm, shallow water environment. The considerable fossil flora is typically Wealden and consists of cycads, araucarias, giant horse-tails and similar tropical or sub-tropical plants. A checklist is appended, though it is probably by no means exhaustive.
The presence of the 'fallen sea-stack' at Portgower suggests the presence of sheer cliffs of Old Red Sandstone, similar to those of modern Caithness, which beetled over a quiet lagoon fringed with a great barrier reef, beyond which the plesiosaur hunted. The cliffs were mantled with tree ferns and ginkgoes and the estuaries with great horse-tails. Small dinosaurs probably roamed through the forests, and pterosaurs and primitive birds soared on the thermals.
It can thus be seen that any mechanism which involves glaciation and ice action can be safely discounted. At the boulder beds we are dealing with the tropical shore of a late Jurassic age.
|This article was first published in the Caithness Field Club Bulletin April 1983|