Mark Wilkinson (UK)
Scotland has a number of sites of historical interest to geologists. I described one of these, Hutton’s Unconformity at Siccar Point near Edinburgh (see Hutton’s unconformity and the birth of ‘Deep Time’). James Hutton described several Scottish unconformities in his book of 1795 and, while the one at Siccar Point is easily the most dramatic and most easily accessible, there is another unconformity on the Isle of Arran that is well worth a visit if you are on the island. There is a third unconformity in the Scottish Borders that is sufficiently well known to be actually called ‘Hutton’s Unconformity’, but is on private land and is thought to be presently inaccessible. There are also a number of other locations that Hutton described, but which have sunk in the mists of time back into obscurity. It would make an interesting project to resurrect these.
It was on the Isle of Arran that Hutton first observed an actual unconformity surface, in 1787. Arran is the seventh largest Scottish Island at around 32km long, lying in the Firth of Clyde some 64km to the southwest of Glasgow. Sometimes described as ‘Scotland in miniature’ due to the range of scenery, Arran has both highland and lowland landscapes.
This is because the varied scenery reflects the underlying geology, with rocks typical of the Highlands of Scotland, and the lowlands. There is a good range of sedimentary, igneous and metamorphic rocks, many of which have well-exposed field relationships, as well as areas where students can practise geological mapping. The island is a popular location for tourists, as well as geology field parties at all levels of education.
The Arran unconformity is at the north end of the island, close to the village of Lochranza. For those who don’t fancy the geology, there is a ruined castle and the Arran Distillery with tearoom, gift shop and (unsurprisingly) whisky. For those who are so inclined, either walk or drive to the end of Newton Road, which runs along the south and then east side of Loch Ranza, where there is parking for a few cars. The road soon turns into a dirt track and then a path. It is worth looking at the rocks along the shore here (Fig. 1) – they are turbidites that were deposited in the long-since closed Iapetus Ocean, which once separated what we now call England from Scotland.
They were deformed in the Caledonian orogeny, as the intervening ocean closed and the continents carrying England and Scotland collided. Look out for graded bedding, ripples and erosive structures on the base of the turbidites to see if you can work out which way is up – have the beds been overturned by folding? Quite large parts of the Scottish Highlands are inverted, as part of Alpine-scale folds that can only be picked out by careful mapping.
Continuing along the coast, the path more or less dies out as you walk along a classic wave-cut platform backed by fossil cliffs (Fig. 2). This is a relic of higher relative sea levels when the west coast of Scotland was still rebounding from the effects of a kilometre or so of ice cover in the Pleistocene glaciation.
Arran has outstanding glacial topography and geomorphologists find it just as interesting as do geologists. Around a 15 minute walk from the end of the road brings you to the vicinity of the unconformity. The first thing most people spot is an abrupt change in rock type – from grey, steeply dipping turbidites to pale brown or buff conglomerates and sandstones that dip rather more gently towards the sea (Fig. 2). The actual contact between the turbidites and the overlying conglomerates and sandstones is not exposed where these are first seen, and the exposed unconformity can take a few minutes to locate if you’ve not been there before – look for a small cliff (about one metre high) that faces the land (Fig. 3).
Once located, the unconformity surface is fairly clear, although, as is usual with such features, deciding where exactly to draw the line between the turbidites and the overlying sediments can be tricky. Is a partly detached block of turbidite a part of the bed rock, or has it moved sufficiently to be thought of as a clast in the overlying breccias and conglomerates? Just to make things trickier, recent research has suggested that there are two unconformities here, one above the other with only a metre or so between them.
The sediments directly above the turbidites are Devonian in age (these are light grey or buff coloured, as they were cemented by soil-forming processes in a semi-arid climate), while on top of these lie brown sandstones of Carboniferous age. So you get two unconformities for the price of one, although only the lower one, which is the more obvious, was described by Hutton. The upper one has virtually no angular discordance, so is probably technically termed a paraconformity, which is one of those terms that appears in geology textbooks but that nobody ever seems to use in practise.
As the Carboniferous sediments dip seawards, the whole area must have been tilted in that direction some time since the Carboniferous. The most likely time for this was in the Paleocene, as the granite that forms much of the north of Arran was intruded at this time. A good look at a geological map shows that the overlying sediments form a dome around the granite and it seems likely that the granite was half solid as it pushed upwards.
This means that the turbidites were rotated too – and from a vantage point where the different dips of the two (three?) sets of sediment can be seen (Fig 4), it is obvious that, although the turbidites currently dip inland, they must have been pretty much vertical before the granite was intruded. The granite, incidentally, is associated with the opening of the North Atlantic Ocean in the Paleocene. The granite forms dramatic mountain scenery, the outline of which is said to resemble a sleeping warrior. Some of the local people seem to be less than impressed by the resemblance and I’ve heard it referred to as the “sleeping jelly baby”.
Hutton’s account of the unconformity, in his almost unreadable book Theory of the Earth, includes the following description of the relationship between the turbidites and the overlying sediments:
From this situation of those two different masses of strata, it is evidently impossible that either of them could have been formed originally in that position; therefore, I could not here learn in what state the schistus strata had been in when those of the sand-stone, &c, had been superinduced”.
For schistus read ‘turbidites’ and for superinduced read ‘deposited’. I’m not sure what aspect of the ‘state’ of the turbidites Hutton could not deduce, as the dip at the time of formation of the unconformity is easy enough to work out – simply imagine the overlying sediments rotated back to horizontal. But then, not being sure what Hutton was trying to say in his writing is (and was at the time of publication) a common problem.
In his excellent popular history of science A Short History of Nearly Everything, Bill Bryson includes a quote from Hutton’s book which is, frankly, incomprehensible. When John Playfair published a much more readable version of Hutton’s work (Illustrations of the Huttonian theory of the Earth, 1802), making it fit for popular consumption and hence massively promoting Hutton’s theories, he described rather more unconformities than Hutton’s original three. The text is fairly brief, with references to Hutton’s book for more information. However, together with Lord Webb John Seymour, Playfair had discovered his own unconformity below the Carboniferous Limestone in northern England, and understandably this receives a rather more detailed description.
The Playfair-Seymour unconformity, as it is sometimes known, is well exposed in one of the most iconic geological sites in Northern England and well worth a visit. The site is in the Yorkshire Dales National Park, a scenic area of moorlands and sheltered valleys (the ‘Dales’) of outstanding natural beauty. It is also an area famous for its limestone caves, of which two are developed as show caves for tourists. Tony Waltham’s book The Yorkshire Dales, landscape and geology provides an excellent introduction.
The starting point to visit the unconformity is the attractive village of Ingleton, which sits on the east bank of the River Twiss. There is a scenic circular walking route, known as the Ingleton Waterfalls Trail (www.ingletonwaterfallstrail.co.uk) of 7km length that will appeal to most people. There is a modest charge for use. The main attraction for most people is Thornton Force, a waterfall where the Twiss drops some 6m from horizontally bedded Carboniferous Limestone onto rather steeper bedded turbidites of Silurian age – right over the unconformity surface.
The surface itself can be examined by anyone agile enough to walk out along the ledge below the undercut limestone. From close up, it becomes apparent that the basal limestone includes well-rounded boulders of schist, which presumably represent the remains of a beach as the limestone seas transgressed over the eroded Silurian landscape. My attempts to photograph this interesting feature have all failed, a combination of poor light below the over-hanging limestone and the lichen cover of the rocks has resulted in a series of quite unpublishable photos. Perhaps someone else can do better?
The unconformity is also exposed at White Scar Cave around 2km northeast of Ingleton on the road to Chapel-le-Dale. The cave is now developed as a show cave, so you can pay to look around, and very impressive it is too – two friends of mine, both keen cavers, were married here in the huge underground cavern known as the Battlefield. In Playfair’s days, there was no show cave, only a tiny cave with a stream flowing from it, which was subsequently known as Playfair’s cave. The entrance section of the cave required extensive engineering to allow visitors to walk through it in comfort – it is now possible to negotiate the cave in a white wedding dress as my friend demonstrated.
Playfair and Seymour did recognise the significance of the cave, small though it was – rain water draining through the Carboniferous Limestone above the cave encounters the impermeable turbidite sequence and flows out along the unconformity. At this point, there is no conglomerate at the base of the limestones, which rest directly on the folded and eroded turbidites. Playfair describes testing the turbidites for reaction to ‘acids’ (hydrochloric acid?) and found no evidence that the limestone had altered or penetrated the turbidites.
An interesting feature of all the Hutton and Playfair-Seymour unconformities is that they are all, fundamentally, related. They all record the onset of sedimentation onto the eroded remains of the Caledonian mountain chain, after the collision of modern England and Scotland in the Devonian period. The Hutton unconformities are all on the Scottish side of the chain, while the Playfair-Seymour unconformity is on the English side. The time when sedimentation restarted was variable, from the Devonian to the Carboniferous and the depositional environment varied too, from an arid inland area to shallow tropical seas. Whatever their exact age, these unconformities have played a major role in our understanding of both deep time and the nature of the geological record, and are well worth a visit.
For anyone who wishes to read for themselves two of the most historically important books in geology, facsimiles of both Hutton’s and Playfair’s works are on the web and can be easily located with a web search. Hutton’s work is easier to find than to read.
About the author
Mark in a Senior Lecturer in Geological Carbon Storage in the School of Geosciences at the University of Edinburgh. He has lead field excursions to many parts of Scotland.