by Paul Braterman
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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.
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.
