If we had to choose one iconic location for the birth of modern geology, it would probably be here:
This is a photograph of Siccar Point, on the Scottish coast. It shows the interface between steeply tilted (essentially vertical) layers of “greywacke” rock, topped by nearly horizontal layers of conglomerate and the distinctive Old Red Sandstone.
This is an example of an “angular unconformity”, where one set of rock layers was deposited at a different angle from the underlying layers. A generic sketch of an angular unconformity is shown below:
Scottish geologist James Hutton first observed this greywacke/sandstone formation exposed in a river valley in Jedburgh in 1787. This interface has become known as “Hutton’s Unconformity”. He had found similar formations earlier, and spent years working out the implications. He realized that this sort of formation required the following sequence of events:
(1) Thousands of feet of sediment accumulated underwater to form what we now see as the greywacke. Even more sediment was laid down on top of that, to compress and cook it to form solid rock layers. Sediments typically derive from the weathering of pre-existing rocks on land, so it would take many years to produce all the sediments we see in the rock layers around us.
(2) This whole assembly was tilted and raised above sea level.
(3) Hundreds of feet (meters) of solid rock were eroded away to form the top surface of the greywacke that we now see as the unconformity.
(4) The greywacke then sank below the waters, and many more feet of sediment were deposited, starting with the conglomerate and the Old Red Sandstone, then more layers on top to lithify them.
(5) Finally, this formation was again lifted above sea level, and many feet of the overlying rock were eroded away to form the present land surface in that area of Scotland.
All of these events cannot be fitted into a one-year Flood, or any reasonable reconstruction of a 6000-year-old earth.
In 1788, Hutton took mathematician John Playfair and geologist James Hall on a boat trip along the Scottish coast to look for rock formations that might illuminate the geologic past. At Siccar Point they found “a beautiful picture of this junction washed bare by the sea”, which helped convince his companions that Hutton’s theories were correct. These men who had grown up thinking that the earth was only a few thousand years old was overwhelmed by the implications of what they were seeing. Playfair would later write:
On us who saw these phenomenon for the first time the impression will not easily be forgotten…We felt necessarily carried back to a time when the schistus on which we stood was yet at the bottom of the sea, and when the sandstone before us was only beginning to be deposited, in the shape of sand or mud, from the waters of the supercontinent ocean… The mind seemed to grow giddy by looking so far back into the abyss of time; and whilst we listened with earnestness and admiration to the philosopher who was now unfolding to us the order and series of these wonderful events, we became sensible how much further reason may sometimes go than imagination may venture to follow.
Another type of unconformity occurs where there is an erosional surface between two substantially horizontal layers of rock. This is called a disconformity:
Sometimes pebbles or chunks of the lower layer (here, Layer C) are found embedded in the subsequent deposit (e.g. Layer D). This demonstrates that the lower layer had hardened into solid rock and been eroded prior to deposition of the upper layer. Thus, such a location must have gone through the same cycles of deposition/ lithification/ uplift/ erosion/ subsidence/ deposition/ lithification/ uplift/erosion as with an angular conformity.
In places, two or more unconformities are found among successive layers, making it extra clear that these layers were not laid down in a single, brief Flood. For instance, in the Grand Canyon, at least three distinct disconformities are found among the main visible horizontal layers, with a dramatic angular unconformity at the base of these layers. Davis Young  noted that by the mid-1830’s, on the basis of observations like these, geologists had realized that the features of the earth were incompatible with Noah’s global Flood:
As long ago as 1834 the great Christian geologist and ordained minister Adam Sedgwick charged the authors of the “Mosaic Geology” of this day with having committed “the folly and the sin of dogmatizing on matters they have not personally examined, and, at the utmost, know only second-hand – of pretending to teach mankind on points where they themselves are uninstructed.” And a year later, Christian geologist and theologian Edward Hitchcock wrote that diluvianism “has been abandoned by all practical geologists.”
(By “practical geologists” Hitchcock meant those who actually go out in the field and observe). Young earth creationists sometimes portray the modern scientific consensus for an old earth as a conspiracy to prop up godless Darwinism. However, these quotes show that, well before Darwin, geologists (many of them Christians) had concluded that the earth must be far older than allowed for by a literal reading of Genesis, and that its features have been shaped by millions of years of erosion and deposition, uplift and subsidence.
Believers who feel threatened by this might reflect on what is the purpose of the Scriptures, as taught by Paul in II Tim 3:15-17. The Scriptures were not given to teach us geology or astronomy, but to make us wise for salvation and equipped for good works.
 Davis A. Young, “The Discovery of Terrestrial History”, in Portraits of Creation, by H.J. Van Till, R.E. Snow, J. H. Stek, and D.A. Young, Wm. B. Eerdman’s, Grand Rapids (1990).
Appendix: More Details on Hutton’s Unconformity
The lower, now-vertical formation is a mixture of greywacke and shales. Greywacke is typically formed on the edges of continental shelves by periodic sloughing off of accumulated deposits, which slide along the sloping sea floor in “turbidity currents.” Many, many episodes of such slides are represented here, with intervening periods of non-deposition. Shales are laid down slowly, in relatively calm water, from the settling of fine-grained sediments. Thus, this formation was not deposited in a 1-year catastrophic Flood. This whole underlying formation has been metamorphosed through long exposure to heat and pressure.
After the greywacke layer was tilted and raised up and eroded and then re-covered with water, some chunks of it got commingled as a conglomerate with the succeeding layers, as shown here. Also, eroded fingers of greywhacke in spots protrude up into the overlaying sandstone. This positively demonstrates that the lower, tilted layers had lithified before the deposition of the upper, more horizontal layers. Again, this is incompatible with a young earth. Stephen Moreton has described in more detail the features of Siccar Point geology which show its great antiquity.