One of the key differences between geology and most other science is that later workers can see the exact same “experiment” as the original workers. In other fields, replication is the standard; while you cannot see the original experiment, you should be able to reproduce it. For us in geology, though, we can usually see the exact features interpreted before.
And so in visiting Siccar Point, GG could look at the same rocks that James Hutton saw and used in arguing for the great depth of time needed for geological relationships. Although Hutton found many unconformities in Scotland, this was the one that has impressed geology more than the others. The neat thing in visiting is appreciating some of the less obvious characteristics of the place. Here is the classic view found in most geology textbooks:
Texts often trim away that righthand side and sometimes zoom in even closer to the unconformity. But if you come to visit, that righthand side stands out: those Silurian graywackes are rising up above the unconformity. Keep that in mind; we’ll come back to it.
Hutton came to this place by boat, but visitors today are usually arriving by land. And when you get out on the top of the hill above the seascape, you see something like the view above. It seems there really isn’t much of that Old Red Sandstone left. Things are really not obvious here, but if you match up points with the photo taken down on that rocky platform, you can separate the Old Red Sandstone from the underlying rocks. If Hutton had been hiking the coastline, this might not have attracted much attention. But go down to the point (not easy, as it is really steep and was wet and rainy when these were taken) and look back and you see how Hutton got excited here.
The bulk of the platform at right is the Old Red Sandstone. In the bottom center you can see an older resistant sandstone bed poking up. To the left, the big unconformity is quite clear (though the vertical bedding of the lower Silurian rocks isn’t so clear). It is also a lot higher than down by the ocean. And it is in that subtlety that the significance of this site becomes clearer, for this isn’t just and angular unconformity, it is a buttress unconformity.
OK, what does that mean? It means that rather than sedimentary rocks continuing laterally until they get thinner and thinner, these run up against a boundary, in this case the upturned edge of the Silurian sandstones. The significance of this is that the Silurian rocks had to be rocks when the later sandstone was deposited; they were not just sand. (You can find “angular unconformities” in big sand sheets like the Navajo Sandstone that were created by cross bedding and syn-depositional slumps that do not reflect much elapsed time). Also, in the basal parts of the Old Red Sandstone are chunks of that Silurian sandstone (a few are visible in the top photo here); again, this wouldn’t happen unless the Silurian sands were already a sandstone. And so the total time represented here was time enough to deposit the Silurian sands, then time enough to turn them to rock, followed by time to erode away a lot of that rock, time to deposit the Old Red Sandstone, time to turn that into stone, tilt it and finally time enough to erode it away as well. While you might be able to catastrophically deposit a lot of sand in a short time and make something like an unconformity, adding in the “make it into rock” step demanded by the buttress unconformity and inclusion of clasts of the older sandstone greatly increases the time needed. And so from this site came the recognition that geological relationships demand a lot of time–tens to hundreds of millions of years at least.
There are some handy webpages with more thorough descriptions of the site and its history, as well as a page describing how to get to the point. (Of all the places visited in Scotland, this was the only one lacking other tourists). 2018 update: That might change after this BBC piece on Siccar Point.