Sedimentary rock forms when layers of deposits (often in water, but also on land) gradually build up and are compressed into rock. Organisms become fossilized in some of these layers, and we can trace the gradual changes in the organisms on Earth by looking at those fossils, since newer ones are on top of older ones (unless there has been some geological upheaval). These patterns have allowed palaeontologists to construct family trees of life on Earth that are independently corroborated by the DNA analyses I discussed in my “language analogy” post.
Each type of fossil is found only in a restricted range of rock layers (strata) – sometimes a large range, sometimes a small range. For instance, you just don’t find the familiar mammals anywhere except the top layers. These regularities are so good that oil companies employ experts in the subject in order to identify layers, the better to find oil deposits.
All this suggested to early geologists that the Earth is very old indeed, and that there was a succession of different creatures throughout this extremely long history. But many geologists still wanted to hold on to the Biblically derived model that the Earth was only thousands of years old – they clung to the idea that all those layers were deposited by Noah’s Flood. That hope finally died with the advent of modern dating techniques, which demonstrate that these rocks go all the way back to roughly 4 billion years ago. Here is a partial list of those dating methods:
Amino acid racemization (L-to-D) (AAR), Argon40 to argon-39 chronometric, Astronomical polarity time scale (APTS), 10Be/9Be isotopic analysis, chronostratigraphy (requires chronometric methods as well), Coral reef annual layering, Dendochronology (tree-ring), Deuterium-hydrogen isotope analysis, Electron spin resonance (ESR), Fission track dating, Fluorine-uranium-nitrogen analysis (FUN), Geomagnetic (archaeomagnetic/paleomagnetic) reversal time scale (GTRS), Geomagnetic secular variation (around magnetic pole), Helium dating, Infrared-stimulated luminescence, Lutetium-176 to hafnium-176 geochronology, Meteorite cosmic-ray exposure, Milankovitch cycle astrochronology, Neon-21 to helium-3 dating, Obsidian hydration analysis (OHA), Ocean sediment core analysis, Optically stimulated luminescence (OSL), Oxygen-16/Oxygen18 stable isotope analysis, Polar ice-sheet core, Potassium-40 to argon-40 chronometric, radiocarbon dating, Radioluminescence (RL), Radon-222-lead-210-lead-206 chronometric, Rhenium-187-osmium-187 chronometric, Rubidium-87-strontium-87 chronometric, Samarium-147-neodymium-143 chronometric, Strontium87 to strontium-86 chronometric, Tephrochronology, Terrestrial rock cosmic-ray exposure, Thermoluminescence (TL), Thorium-232-lead-208 chronometric, Uranium-235-lead-207 chronometric, Uranium-238-uranium-234-thorium-230-radium-226-lead-206 chronometric (U-series) and Varve analysis.
Each method covers only a certain time range, from a few thousand years to billions of years. In each case, we take some natural process that changes with time, we measure its pattern of change today, and use that information to infer the dates of old things the process has affected. The marvellous thing is that we have so many completely different independent dating techniques that all agree where they overlap that we can be certain that we’re dating the rock layers correctly. Anyone who is not desperately hanging on to the inerrancy of Genesis will see that this is proof of an old Earth, and proof of the succession of organisms.
What can young earth creationists do with this evidence? They’re forced into contortions you wouldn’t believe:
1) They say that the strata were laid down from “settling” after Noah’s flood (except for the “creation week” layer at the very bottom). If you go to the McAbee fossil beds near Cache Creek (highly recommended!), you’ll find several hundred thousand micro-thin layers chock full of fossils (which geology dates to around 50 million years ago). After the rain stopped, the flood waters must have been as calm as a perfectly still aquarium for months or years to achieve that kind of settling! (But why the exactly similar layers?? I’ll tell you why: it’s a seasonal accumulation of debris, roughly one per year. But John can’t say that. And why are they now rock? Eons of compression, but John can’t say that either.)
2) They say that the settling occurred in such a way that each creature is found only within a certain range of layers across the globe. For example, hippopotami are always inexplicably found far above cockroaches with external ovipositors – I guess they were really, really heavy cockroaches! In the Grand Canyon, there are 2000 feet of sediment (containing marine organisms) before the first reptiles and plants appear. Were the reptiles treading water for months on end? (And the plants too?) Not only that, but the reptiles left tracks in the sediment (e.g. in the Hermit Shale layer). How did they manage to walk around on the bottom of the ocean, after having all been killed in the first 40 days of a violent global deluge, I wonder? And then other animals left more tracks on top of the layer above? and then the flood deposited thousands more feet of sediment? You can see why geologists laugh at so-called “flood geology.”
3) Moreover, they have to say that the many independent dating methods that make it *look* as though each layer is independently confirmed to be a certain age, and where these layers *appear* to follow a beautiful succession of dates, millions to billions of years old, are all wrong, in completely different ways for completely different reasons. That, I hope you can see, is utterly preposterous! All to save a literal interpretation of Genesis – there’s no independent confirmation of any of these crazy hypotheses.