Hutton’s unconformity and the birth of ‘Deep Time’

I sometimes ask a question to students in an introductory class about geology: “What is the most famous geological site in the world?” For students from the western hemisphere, the Grand Canyon in the USA is a popular choice. However, if you were to ask the same question to a group of geologists, you might get a different answer, and one option is Siccar Point on the coast some 65km southeast of Edinburgh in Scotland. Although the site itself is relatively modest, a gently sloping platform of rock partly washed by the sea at high tide, and it lacks the spectacular grandeur of the Grand Canyon, the historical significance easily outweighs the lack of scenic drama. I’ve taken several groups of visiting geologists to the site, and so far only one of them has knelt and kissed the ground, but the site could be considered to be one of the ‘holy’ sites of our science.

Fig1 — Fig. 1. The steep slope that leads to the exposure seen from below, with a large group of students from the University of Edinburgh ascending. It doesn’t look too bad from this angle.

It is difficult for most modern geologists to imagine the world when any interpretation of the geological record had to be constrained by the literal interpretation of the Bible. A particular problem is the short timescale of the account of the creation of the Earth in Genesis, and the age of the Earth as calculated by Bishop Ussher, who allowed only some 6,000 years for the whole of geological time. The person who is frequently credited with expanding geological time to the ‘deep time’ we know of today is James Hutton. Hutton lived in or close to Edinburgh during the late 1700s, and he had realised that the geological record required vastly longer time periods than allowed for in the Bible. Convincing yourself is one thing, convincing other people is often trickier. Hutton wrote a book describing his ideas. Unfortunately, although he was apparently a good speaker, he was not a good writer and his book was pretty much universally ignored.

A boat trip in 1788 with two friends secured Hutton’s place in scientific history. Accompanied by James Hall, an Earl who carried out melting experiments on rocks, and John Playfair, who might today be described as an ‘opinion former’, a man that learned people listened to, they sailed along the coast deliberately looking for a particular geological feature, which we now term an unconformity. Hutton had previously visited the coast and knew that there were two main rock types – what we now call turbidites, which are deformed into sometimes spectacular folds, and a reddish sandstone or breccia, which is unfolded, although it is sometimes slightly tilted from the horizontal. What Hutton was searching for was the contact between the two – where the younger, undeformed sandstone or breccia rests on the eroded turbidites. Hutton explained this as evidence for cycles of sedimentation, mountain building and erosion in the evolution of the Earth’s crust – the folded turbidites are sediments from an earlier cycle caught up in a mountain building episode, with the sandstones and conglomerates being later sediments that were deposited on top of the roots of the eroded mountain belt. Playfair was so convinced by Hutton’s arguments that he later described how “the mind seemed to grow giddy by looking so far into the abyss of time”. Playfair later popularised Hutton’s theory by writing Illustrations of the Huttonian Theory of the Earth in 1802, which, as a well-written text, sold many copies and effectively promoted Hutton’s theories.

A visit to Siccar Point is aided by a convenient car park with an interpretive board sited on the minor dead-end road leading to Old Cambus West Mains (NT 804 706). Beware of HGVs on the narrow road as there is a vegetable packing plant further on. The dry valley in which you park is a glacial outflow channel, cut when the nearby sea was occupied by glaciers. A ten-minute walk across the edge of fields reaches another interpretive board (safer from vandals here) and the edge of the grassy slope that leads to the rocks below. From here, it is apparent that the famous exposure is, unfortunately, at the base of the slope and you are at the top (Fig. 1). Every year, numerous students descend the unmade path, which in dry weather is steep, and in wet weather can be horribly muddy (and still steep). I’ve seen plenty of trousers coated in mud by small slips, but mercifully no real falls – we never make a trip to the bottom of the slope compulsory even for geology students, as you can see the big picture from a grassy ledge some way down or even from the very top – but binoculars are useful.

Assuming that you make the trip down to the exposure (Fig. 2), there are several things to look at. The turbidites below the unconformity, now known to be of Silurian age, were deposited in deep water (at least tens of metres deep) in the Iapetus Ocean that once separated what we now call England and Scotland. They have been folded and rotated to more-or-less vertical during the mountain building event that welded Scotland and England together – the Caledonian orogeny. It is difficult to get a sense of the scale of the folds here, as the overlying Devonian sediments really get in the way. The turbidites consist of alternating sandstones and shales – the sandstones resist erosion compared to the shales and stand out while the shales weather back (Fig. 3). If you have a hand lens, you can try to see the grains in the sandstones, but this is made difficult by a clay matrix between the grains that gives the rock the overall grey colour. The poor sorting (sand grains plus clay) is a result of the rapid deposition of each sandstone bed in a turbidity current – a body of sediment-charged water flowing over the old seabed at tens of kilometres an hour, triggered by a storm in adjacent shallower water or perhaps an earthquake.

Fig3cropped — Fig. 3. Looking along the vertical bedding of the grey Silurian turbidites (middle distance), with the Devonian sediments in the foreground. Note how the turbidites weather: the sandstones stand up, with gaps where the shales have weathered away.

The shales are even more tricky to see anything in, as they are weaker than the sandstones and tend to get quite badly fractured and deformed during folding. There are a few graptolites in the shales that have been used to date the rocks. However, do not hammer the rocks to try to find one, as the chances of success are remote: the preservation is poor and the exposure is too important to be damaged. An interesting exercise is to try to work out which way is ‘up’ in the beds – as they are vertical, this is not immediately obvious. Clues might come from sedimentary structures such as ripples, which are concave up, or erosion at the base of the turbidite beds, compared to the tops which are gradational to the overlying shale. Incidentally, the shale represents far more time than the sandstone beds – one sandstone bed represents only as few minutes or hours, whereas the intervening shale may have formed over hundreds of years, by the gradual settling of mud on the sea floor.

While the upturned turbidites are interesting in themselves, it is their relationship with the overlying sediments (reddish sands and breccias, that is, rocks made of angular clasts) that is the main attraction. The surface separating the two rock types is irregular, with a few metres of relief visible across the exposure. And there are small blocks of the turbidites included in the basal sediments, that appear to be almost in situ, but not quite. This is a fossil land surface, with some loose weathered material still in place – which we now recognise as an unconformity (Fig. 4). And like most modern land surfaces, the unconformity isn’t flat. Overall, the surface appears to form a shallow valley, but there are numerous lumps and bumps of the turbidites sticking into the Devonian sediments.

The sands and breccias entirely lack visible fossils or even traces such as burrows. This makes it unlikely that they were formed in a marine environment, and the study of these and other sediments of this age nearby suggests they were laid down in a wadi that housed a seasonal stream when the climate was rather warmer and drier. Note that some published accounts of the formation of the Hutton’s unconformity describe the sediments above the surface as marine and describe sea level changing to flood the area – this is incorrect. Where did the sediment in the stream come from? Cross-bedding is frequently used to decide which way a stream or river flowed, but is unhelpful here and largely absent from the breccias anyway. However, where the clasts in the breccias are elongate, they are sometimes imbricated – that is, arranged like books on a bookshelf that have slid over. These show that the stream flowed from the present day sea towards the present day land – evidently the geography of the area has changed beyond recognition since the Devonian.

Fig4 — Fig. 4. A close-up of the unconformity surface – the long edge of the compass is roughly parallel to the vertical bedding in the turbidites, while the more horizontal bedding of the Devonian runs across the top of the picture.

So how did James Hutton view this unconformity? He correctly identified that the turbidites were part of an old mountain belt, which had been folded and uplifted. We now know that the deformation occurred during the long, slow closure of the Iapetus Ocean, which separated Scotland and England before the Devonian. Obviously, Hutton had no concept of what drove these changes – it would be a good 150 years before plate tectonics was identified as the driving force for shaping the Earth’s surface. Hutton also realised that the mountain belt must have been eroded, so that new sediment (mostly derived by eroding the mountains themselves) could be laid on top of them. This surely demanded time, and lots of it. Hutton also realised that the process of erosion was ongoing – he knew that the precious soil from his farm was being washed from this fields into the sea whenever it rained heavily and that this must surely accumulate on the seabed as sediment. This sediment would one day itself be uplifted and the cycle would continue. Hutton had no idea exactly how long all this would take – we now know that the time gap between the deposition of the turbidites and the deposition of the overlying sediments was some 50 to 70myrs. But he knew that it was a good deal more than 6,000 years, a very great deal more. Exactly how much more would have to await the discovery of radioactivity in minerals and the absolute dating of the geological timescale.

Gallery

Translate:

Media RSS

Hutton’s unconformity and the birth of ‘Deep Time’

Recommended reading

Leave a Reply Cancel reply