Added by edit:
I started this post this morning, before JonF added his explanation to his previously written post, hence my comment that I didn't notice a clear answer to AF's question. I checked for additional messages, but I didn't reread the thread when I returned and resumed writing so I missed it. JonF does give an explanation for geologic dating, so my comment isn't a slight against him.
End of addition.
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First of all, how can this geologic column be of any use today when it was created using none of the dating methods that are wholly, if not unfortunately, accepted today?
I didn't notice a clear answer to this in the previous posts, so I'll venture to offer a very simplified explanation of the history of geologic dating. It'll be very brief and very simple, so it'll leave out important details. If I get anything wrong, or if essential details are missing, experts are welcome to correct me. Also, I'm sure that there will be questions; since I'm not a geologist, and since my knowledge of geology comes from the first year college sequence and whatever subsequent reading I have done, I'm sure that there will be questions I won't be able to answer.
It was noticed that fossil species always occur in the same order in the geologic column. (Well, there are some exceptions, but these exceptions are very rare, and when they do occur, there are some things that are very "wrong" with the units in which they occur. I'll still use the word "always" with this caveat in mind.) Dinosaurs always occur below mammmoths. Trilobites always occur below dinosaurs. You never see trilobites above dinosaurs or mixed with dinosaurs. And for individual species, trilobite species
Brachyaspidion microps is always found lower down that the species
Paciphacops birdsongensis. There is a definite, consistent order to the fossils in the geologic strata, an order that cannot be explained by any flood model.
And that is how the geologic column was originally constructed. As one moves down into the geologic column, there is a definite order to when one first observes occurrences of the various fossil species and when one observes that the various fossil species no longer occur. Any given species will occur in the same rock unit with only certain other species. And so the labels for the geologic units, like "Paleozoic, Mesozoic, Cenozoic" and "Triassic, Jurassic, Cretaceous" were defined, and are still defined, based on which species occurs in those units.
So, when a geologist in Britain observes a certain fossil species in the rock unit she is studying, she knows that that particular unit is "Devonian". What that really means is that there are only a certain numbers of particular fossil species that will appear in that unit (one of which is the particular species she is observing), and that these species, generally, don't appear in any other rock unit. So a geologist in Russia, say, reading her description already knows which known fossil species will be found in that unit, and which ones will not.
So this is the basis of the geologic column. It is noticed that rock units occur in horizontal layers, that in each layer only certain fossils species occur, and, in fact, only certain combinations of species will occur in the same rock units, and that there is a definite order in which the fossil species appear as one "reads" the column from top down.
I will repeat to make sure that this is clear: this is what the geologic column means. Rock units generally occur in horizontal layers, only certain combinations of species will occur in the same layers, and there is a certain pattern to what species occur as one reads from top to bottom.
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What can be the interpretation of these facts? Why does this pattern exist? It is a simple observation that when sediment layers form, the layers below are older than the layers above. It also makes sense logically: how can sedimentary layers be formed
beneath already existing rock? There are exceptions to this general rule, but for brevity in this initial post I'll just say that these are usually pretty easy to identify.
So this was another indication that the flood does not explain geology. When sediments settle out of water, we almost always see the courser materials settle first, then the finer materials on top. If the environment was chaotic, then maybe they won't sort very well. But one never sees finer material settle out first, then the courser materials over that. Yet this is exacly what we see in the geologic column. We often see mudstone and siltstone and sandstone alternate in various orders as one goes down the geologic column: if all these strata were formed from the same event we would either not see much differentiation, or we would always see large rocky material on the very bottom, above that sandstone which would grade into siltstone above, which, in turn, would grade into musdtone above that. Instead, we see mudstone, sandstone, and siltstone in all sorts of various orders, sometimes with quite abrubt boundaries between them. A single flood event cannot explain this; it is evident that these layers were formed at different times in different environments.
So we have these layers that were obviously formed at different times. And we can pretty much conclude that the lower layers were formed before the upper layers. And, by observing processes today, geologists were able to come up with some estimates as to how long it took each layer to form. That is how they originally came up with the estimates of the lower levels of the geologic column being tens of millions or even hundreds of millions of years old. Of course, they were aware that these were very rough estimates, but what else could they do? They didn't yet know that some rocks have a date stamp included in them.
So it was proposed that each layer in the geologic column formed long ago, and the species that occurs in that layer were species that lived at the same time on the earth. So the reason that trilobites are always found below dinosaurs is because they lived earlier in the history of the earth, and then disappeared before the dinosaurs appeared. Then dinosaurs disappeared before mammoths appeared, and so dinosaurs are always found in lower strata than mammoths. And so forth.
So, if it is known that species A and species B can occur in the same rock layer, then if a geologist in Britain observes species A in the unit she is studying and a geologist in Russia observes species B in his rock unit, then, by the above hypothesis, the two rock units are roughly the same age. If a geologist in North America observes species C in the rock units she is studying, and it is known that species C always occurs in layers higher in the geologic column than either species A or B, then one draws the conclusion that her rock units are significantly younger than the other two.
So, based on the observation that the only known processes that could have formed all the sedimentary rocks would have taken a long amount of time, and based on the observation that the fossil species always occur in a definite order in the geologic column, we now have the basis for geologic dating. At this point we don't have any way of putting absolute numbers on these dates. We know that there was a period of time when the rocks labeled "Triassic" were formed, and they include the fossils of species that lived at that time. There was a period of time when the rocks labeled "Jurassic" were formed, and they include fossils of species that lived at that time.
Except for a very few that seem to have lived on the boundary, the species found in the Triassic rocks are different from the species found in the Jurassic rocks. They lived at different times. And we don't (at this point in the discussion) really know how long ago these species lived, all we know is that the Triassic species lived before the Jurassic species because the Triassic species are always found below the Jurassic species.
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So we now have geologic dating, but we can only give
relative dates, that is, we only know that some species and some events occurred before others, but we cannot pin any dates on them. It's sort of like knowing that the American Civil War occurred at roughly the same time as the Franco-Prussian War in Europe, and that both occurred before the World War I, but not knowing the exact dates at which they occurred.
Then radiometric dating was discovered. Physics, an entirely different discipline from traditional geology, potentially allows us to put actual dates on the geologic rock layers. Now this may not work: it might be possible that our interpretation of the rock layers as individual times in a long history may be wrong; it may be that in practice the radiometric dating may not work reliably. In either case, we would not expect to get consistent dates for the various rock layers.
But, as it turns out, radiometric dating does work reliably. When the rock units that contain species labeled as Jurassic are dated, the answers are always between 199 million years ago and 145 years ago. When the rock layers containing Triassic fossils, assumed under the above hypothesis to have occurred in a distinct period in time that occurred before the Jurassic, the dates are always between 251 million years ago and 199 million years ago. And similarly for other strata.
So it is important to note that radiometric dating provides additional information: we now have absolute dates when we previously had only relative dates. Also, the absolute dates observed from radiometric dating provides an independent check on the relative dating assumptions, and, in fact, is consistent with them and so provides amazing confirmation of the previous relative dating.
So, to sum up, geologic dating was already well developed long ago, long before radiometric dating, based on the simple observation that the fossil species always occur in the same order in the geologic column. This dating was only relative -- it only allowed us to tell which periods of time occurred before or after which other periods.
Radiometric dating finally allowed us to put precise ages on the rock units. But what is most important, radiometric dating gives an important confirmation that the previous geological dating system was correct. Traditional geologic dating was based simply on noting which fossil species were in the rock units, and in what order the fossil species appear. This has nothing whatsoever to do with chemistry or atomic physics. Radiometric dating is based on simply counting the amounts of radioactive isotopes and/or their daughters; it has nothing to do with geology or fossil species. Yet the two methods agree: rock units in different parts of the world that contain the same fossil species have the same radiometric dates. Rock units always date younger than the rock units that occur below them. If whenever species A and species B are found in the same locality it is observed that species A always occurs in layers below that of species B, then any rock units containing A will always have an older radiometric date than those containing species B, even when they are found in different parts of the world, even when they are collected in localities where A and B are not both found.
This is the kicker. Radiometric dating did not have to agree with the conclusions drawn from traditional geology. But they do, and consistently.
Edited by Chiroptera, : No reason given.
Computers have cut-and-paste functions. So does right-wing historical memory. --
Rick Perlstein
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