Another delightful posting from my friend Marli Miller.Thanks, Marli. I’ve blogged here earlier myself about the famous unconformity at Siccar Point, and the depositional contact at the Giants Causeway between a later lava flow, and the paleosol formed by weathering of the one before it.
Geologic contacts are the surfaces where two different rocks touch each other –where they make contact. And there are only three types: depositional, intrusive, or fault. Contacts are one of the basic concerns in field geology and in creating geologic maps –and geologic maps are critical to comprehending the geology of a given area. For those of you out there who already know this stuff, I’ll do my best to spice it up with some nice photos. For those of you who don’t? This post is for you!
Depositional contacts are those where a sedimentary or volcanic rock was deposited on an older rock (of any type). Intrusive contacts are those where igneous rocks intrude older rock (of any type). Fault contacts are… faults! –surfaces where two rocks of any type have moved into their current positions next to each other along a fault.
In a cross-sectional sketch they may…
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I have recently visited two very different sites where time is turned to stone, where just looking at the rocks shows the passage of enormous lengths of time, dwarfing all of recorded human history. At the first site, the rocks I was looking at were ancient sediments, with the clearest possible evidence of prolonged interruption. In the second, they were comparatively (!) recent volcanic outpourings showing the traces of slow continuous change. In the first case I was looking at an 80,000,000 year gap in the record, in the second, at the signs of a hundred thousand years of continuous weathering. The first site is indicated by nothing more than a small information board behind a farm gate off a minor track, although it occupies a special position in the history of geology as a science. The second is visited by over 750,000 tourists annually, has its own well-appointed visitors centre, and was the occasion of a recent shameful episode of science denial. The first records events connected with the closing of an ancient ocean; the second with its reopening.
The first of these sites was Siccar Point, on the Scottish coast between Edinburgh and Berwick-upon-Tweed. Despite its significance, it has remarkably few visitors; in fact my family and I had it all to ourselves on a lovely summer’s afternoon. It is hidden away of a minor road, and access is on foot, culminating in a steep descent across grassland. When the pioneering geologist James Hutton visited it in 1788, he came by boat, and was delighted (but not surprised) by what he saw – a spectacular example of an unconformity, a mismatch between
one set of rocks, and those above them. The lower rocks are a sediment (greywacke) rather like a very coarse sandstone with lots of embedded small pebbles, of the type formed on continental shelves. As is common with sediments, different strata are clearly visible, but what is much less usual is that the strata are standing almost on edge. Immediately above these are another accumulation of rock, a reddish sandstone, with the strata lying almost horizontally. The boundary between the two sediments is also roughly horizontal, but with minor ups and downs, all filled in by the upper sandstone. Down on the beach to the immediate Southeast, the upper layer has been stripped away, and one can see dark lines corresponding to the upended strata, gently curving parallel to the coastline. (For a brief video tour with commentary, see the Open University’s https://www.youtube.com/watch?v=oQzmfcUKSFM , although I must fault this for repeating the myth that this is where deep time was first discovered.)
As Hutton realised, we are looking at a complex sequence of events, which we would now describe as follows:
- The initial coarse sediments were laid down in moderately turbulent offshore conditions. Turbulent enough to mix up debris of different sizes, but not so turbulent as to erase the boundary between different sedimentary layers.
- Enough time passed for them to form solid rock.
- Then came at least one, and possibly two (remember the curve in the strata exposed on the beach) episodes of mountain building, folding the sediments so dramatically that here they are standing on end.
- Next a lengthy interval, how lengthy Hutton had no way of knowing, in which these mountains were worn flat, apart from irregularities caused by local streams,
- The deposition, and eventual consolidation, of the upper sandstones
- And finally, the erosion of later deposits, exposing the sandstone and, down on the beach, the ner-vertical strata of greywacke.
Here and elsewhere, Hutton had discovered confirmation of deep time; conclusive corroboration of his views that the Earth was many millions of years old; indeed, as far as he was concerned, indefinitely old.
For over 100 years, we have been able to do something that Hutton could only dream of, and determine the actual age of the rocks using radiometric dating. This is not quite so straightforward an exercise as the textbooks sometimes pretend. Relative ages were worked out in the nineteenth century, using the principle that generally speaking, younger rocks lie on top of older rocks, and what are known as index fossils to cross-match strata deposited around the same time in different locations. This use of index fossils goes back to the work in the 1810s of William Smith, who produced the world’s first geological map. Finally, the scale is made absolute by the radiometric dating of rocks formed from the melt, such as volcanic ash deposited between strata, or basalts or granites that have forced their way up through them. In this way, we can date the lower coarse rock as some 425 million years old, placing it in the epoch we now call the Ordovician, while the upper rocks are Upper Devonian sandstone, some 80 million years younger.
Hutton lived in Edinburgh, and was a member of the movement known as the Scottish Enlightenment. He would have known and frequently conversed with such other members as the mathematician John Playfair, who accompanied him to Siccar Point, the chemist Joseph Black, the engineer James Watt, and the economist and moral philosopher Adam Smith. None of these would have had any difficulty in discarding Bishop Ussher’s bible-based creation date of 4004 BC. By Hutton’s time, even the churches were moving beyond such a restrictive chronology, and clergymen such as Buckland and Chalmers were to play a major role in the nineteenth century development of modern geology.
Like other members of this movement, Hutton combined intellectual and practical interests. His original training was as a lawyer and physician, but after inheriting a farm from his father he became interested in the land itself and what lay underneath it. He was also a shareholder, director, and what we would now call scientific adviser to the Forth and Clyde Navigation Company, which in 1790 completed the Forth and Clyde Canal. The activity of canal building, like that of railway construction the few decades later, involved cutting through surface soil and the rock directly beneath it, exposing successive layers, and Hutton realised that, generally speaking at least, the younger sediments were younger than the platform on which they rested. Hutton also formulated the principle of uniformitarianism, according to which the processes that we observed that work at present are the key to those that will work in the past. For a while there was a perceived conflict between such uniformitarianism, and the alternative doctrine of catastrophism, according to which the Earth had been shaped by sudden and dramatic events, but by 1865 T.H. Huxley was able to tell the British Association that these views had been fully reconciled. The laws of nature, and the kinds of process responsible for geological change, have indeed been strictly uniform over time. As for specific features, this overall long term uniformity is interrupted by episodes of local catastrophically intense activity. So we have uniformity of process, but uniformity of rate only applies if we paint with a very broad brush indeed.
Siccar Point is sometimes described as the birthplace of the concept of deep time. That is not actually true. Hutton had already been led to his theory of uniformitarianism much earlier, by a mixture of philosophical and
religious considerations. Like many Enlightenment thinkers, he was a Deist. He rejected scriptural religion, but believes that the world had been fashioned in such a way as to make it suitable for long-term human habitation. As he put it in his Theory of the Earth, included in the first (1788) volume of Transactions of the Royal Society of Edinburgh but drawing on materials first pubished in 1785,
THE form and constitution of the mass are not more evidently calculated for the purpose of this earth as a habitable world, than are the various substances of which that complicated body is composed. Soft and hard parts variously combine, to form a medium consistence adapted to the use of plants and animals; wet and dry are properly mixed for nutrition, or the support of those growing bodies; and hot and cold produce a temperature or climate no less required that a soil. Insomuch, that there is not any particular, respecting either the qualities of the materials, or the construction of the machine, more obvious to our perception, than are the presence and efficacy of design and intelligence in the power that conducts the work….
IT is not only by seeing those general operations of the globe which depend upon its peculiar construction as a machine, but also by perceiving how far the particulars, in the construction of that machine, depend upon the general operations of the globe, that we are enabled to understand the constitution of this earth as a thing formed by design. We shall thus also be led to acknowledge an order, not unworthy of Divine wisdom, in a subject which, in another view, has appeared as the work of chance, or as absolute disorder and confusion.
Superficially, this may seem to resemble the viewpoint of the present day Intelligent Design (ID) movement, but the reality could not be more different. ID invokes the operation of a designing intelligence as a sufficient explanation for natural phenomena, passing over the obvious fact that design is nothing without fabrication. This reduces the Owner of the intelligence to a worker of arbitrary miracles. For Hutton, as for his contemporary Paley, Divine wisdom was evident in the fabric of the world, as shown by the perfection of its natural operations. Thus there was no conflict between science and religion, because science was in itselfsomething approaching a religious activity. This belief in design was inseparable from the search for explanations and the enlargement of our understanding. ID, in contrast, seeks to enlarge the domain of the unexplained, so as to have more occasion to invoke the operation of an intelligence, and therefore sets out to denigrate the understanding that we already possess. So it is no accident that a newsletter addressed to the ID community goes by the name of Uncommon Descent, and endlessly rehashes arguments about evolution dating back to the 1860s. If Hutton could have known of the concept of evolution by natural selection, he would have embraced it with enthusiasm.
From Hutton’s starting point, certain things necessarily followed from his premises and daily observations. As a farmer, he knew that soil eroded, and therefore on his theory must be replenished. This could be accomplished by the weathering of rocks, but that also is a process that could not continue indefinitely. And so he was led to the concept of a cyclical Earth, in which the eroded soil gave rise to sediments that were consolidated, in his view by the action of heat, and then experienced uplift, again by the action of heat, thus completing the cycle. Hutton was very taken with the concept of natural cycles, perhaps under the influence of his friend James Watt, who completed the steam engine’s cycle by adding a condenser. Thus he drew analogies between his rock cycle, the Newtonian cycling of the planets, and even the circulation of the blood.
What, then, was the force that folded and buckled the Ordovician strata? To understand that, we had to wait until the twentieth century, and the emergence of what we now call plate tectonics, generally accepted by geologists only in the 1970s, although, as I have already explained here, the prescient Arthur Holmes had correctly described the process decades earlier. Convection currents in the Earth’s mantle drove two great plates together, closing the ancient ocean that we call Iapetus, a precursor to the modern North Atlantic. This collision deformed the rocks that make up the crust, throwing up a range of mountains as high as the Himalayas, whose remnants now stretch from the Appalachians across the Southern Scotland Uplands to Scandinavia. Here at Siccar Point, the folds were then worn down flat over tens of millions of years, and eventually fresh material was deposited to form the upper sandstones. Much, much later, the movement of the plates was to reverse itself as the convection currents shifted, reshaping the globe to its present form, and this will be the subject of my next post.
I thank Keith Montgomery, of the University of Wisconsin Marathon County, and the Rev. Michael Roberts, F.R.Hist.S., for information about Hutton and about geological opinion in the 19th century churches, respectively. Portrait of Hutton by Sir Henry Raeburn, via Wikipedia. Photographs of Siccar Point, and of the Forth and Clyde Canal near Falkirk, by the author. An earlier version of this piece appeared in 3 Quarks Daily.
Subject: 12/00929/FUL Objection Comment. Addressees firstname.lastname@example.org and email@example.com. Cc. Scottish Natural Heritage: firstname.lastname@example.org; Scottish Environmental Protection Agency: Stephanie.Balman@sepa.org.uk; Crown Estates: email@example.com; Dunglass Estate: firstname.lastname@example.org
I write to protest in the strongest possible terms about plans to build a waste pipeline cutting through land immediately adjacent to Siccar Point, which is an integral part of its geological and aesthetic setting.
I had the privilege of visiting Siccar Point this summer. The appearance of the Point itself was familiar to me from illustrations, but the setting, with Old Red Sandstone cliffs exposed to the north, and to the south a beach along which ran the older strata, tilted up on end, was a revelation. Yet it is precisely across this shoreline that there are now plans to build a concrete-encased waste pipe for the disposal of vegetable washings of high organic content.
Siccar Point is the most important single site in the entire history of geology. It is here that James Hutton, prominent member of the Scottish Enlightenment, found the first convincing evidence of deep time, of ongoing cycles of uplift and erosion, the insight central to the newly born science of geology. It features in textbooks the world over. It is a place of pilgrimage not only for professional scientists, but for all who cherish the workings of our planet. It is a unique educational resource, attracting students, scholars, and members of the general public from far and wide, and I must say that for me the experience of visiting it surpassed all expectation.
The entire significance of the location is, that it is the meeting point of the Old Red Sandstone with the much older underlying greywacke, with the latter folded and tilted so dramatically that the originally horizontal strata now lie vertical. These older strata are actually seen to better advantage on the shoreline directly south of the Point, where they present a spectacle of unrivalled majesty and beauty. It is precisely across this shoreline that it is now proposed to build a waste pipeline. I find it incredible that anyone could wish to do such a thing.
There are other, secondary, considerations which in themselves should give pause for thought. How wise is it to run a pipeline of this kind down a steep and unstable cliff? What will the consequences be of erosion and possible exposure, or even fracture, of the pipe? Is it sensible to be discarding such a volume of rotting vegetable matter along this historic coast line, also home to busy holiday resorts, and a playground for young children? Have not serious doubts been expressed about how close the end of the proposed pipeline is to the low water mark? What will the impact be on the marine environment and local fishing of all this added organic matter? Should we, indeed, be disposing of this material as waste, or converting it into a resource by composting? But all of these are, as I said, secondary to the primary concern for the site itself.
My own credentials: I am a former chemistry professor, now honorary senior research associate at Glasgow University. I have been involved in research topics related to geology for over 30 years. My work in this area has been funded by NASA, I have been an associate of NASA’s Astrobiology Institute, and I have served on the editorial board of the journal Origins of Life and the Evolution of Biospheres. Hutton’s work features in my most recent book, From Stars to Stalagmites, as it must in any book that describes the relevant period in the history of science, the fundamentals of geology, or (coming closer to home) the achievements of the Scottish Enlightenment.
Paul S. Braterman, MA, DPhil., DSc.