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 at lower right, topped by nearly horizontal layers of conglomerate and the distinctive Old Red Sandstone (upper left in the picture).
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 were 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 often said to ourselves, ‘What clearer evidence could we have had of the different formation of these rocks, and of the long interval which separated their formation, had we actually seen them emerging from the bosom the deep?’ 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 superincumbent 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, along with a dramatic angular unconformity at the base of these layers. The picture below is of one of these disconformities:
Young Earth creationists will admit that there are erosional gullies to be found in what are otherwise flat-appearing interfaces between certain Grand Canyon layers, but claim that these gullies were cut into soft, unconsolidated sediments as the Flood waters ebbed and flowed. However, Davis Young shows that these channels (e.g. below and also above the Redwall Limestone) must have been cut into fully lithified rocks, not into soft deposits:
The Muav is separated from the Redwall by a distinct surface of erosion. This erosional surface includes small channels that were scoured into the upper surface of the Muav and filled with Redwall sediments containing small pebbles of eroded Muav. These features indicate that before deposition of the Redwall, the Muav sediments had hardened into rock, risen above sea level, and been weathered to form pebbles and boulders that could be incorporated into the overlying sediments once the sea returned to the area. A global flood would have provided neither time for the sediments to be consolidated nor opportunity for the materials to be weathered by exposure to air.
Another important example of an unconformity is the contact between the Redwall Limestone and the overlying Supai Group. Observations by professional geologists indicate the upper surface of the Redwall Limestone, though generally horizontal and conformable with the base of the overlying Supai, has many deep channels scoured into its upper surface – some as much as 400 feet deep. The channels are filled with layered mudstones, sandstones, and limestones and commonly contain pebbles derived from the Redwall. These features indicate that the Redwall lime deposits were hardened into solid rock, lifted up from the seafloor to at least 400 feet above sea level, and there cut by flowing streams that dislodged pebbles from the exposed Redwall land surface and redeposited them in the channels.
The time gaps indicated by the erosional surfaces are consistent with changes in the fossils between the lower and upper layers at these interfaces. Comparisons with similar fossils in other rocks in dated layers elsewhere in the world indicate that the lower and upper layers were deposited many millions of years apart. For instance, the types of trilobites and brachiopods in the Muav (below the Redwall) are similar to those in other rocks which have been dated to around 505 million years ago, in the Cambrian period. The fossils in the Redwall Limestone correspond to those typically found in rocks around 335 million years old, in the “Mississippian” period. This indicates a gap of over 200 million years between the deposition of the Muav and of the Redwall. The fossils in the Supai (above the Redwall) are like those usually seen in rocks dated about 285 million years old, indicating that erosional surface at the top of the Redwall represents a gap of nearly 50 million years.
YE creationists complain that sometimes the surface between an old (lower) and young (upper) layer is so flat and level that it is hard to discern that it is really there ; they take this as justification to reject an age gap across the interface. This objection does not hold water. First, as just noted, if you look carefully enough, you can often find the evidence of erosion. Second, it is clear that some land areas can get eroded down to be very, very flat so any new deposition onto such a surface would form a flat, level interface.
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). Deborah and Loren Haarsma describe eight other lines of observational evidence that led “virtually all practicing geologists, including Christian geologists” by 1840 to believe “that the earth must be at least millions of years old.”
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), pp.68-69.
 Ibid., pp.45-46
Appendix: More Details on Hutton’s Unconformity
The lower, now-vertical formation consists of alternating layers 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 in this photo by Clifford Ford:
Also, eroded fingers of greywacke in spots protrude up into the overlaying sandstone, as noted by Playfair during his 1788 visit to the site:
The rugged tops of the schistus are seen penetrating into the horizontal beds of sandstone, and the lowest of these last form a breccia containing fragments of schistus, some round and others angular, united by an arenaceous cement.
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.