In the early part of the 19th century the science of geology underwent a major period of development. The primary result of this renaissance was the geological chart, which laid out the history of this planet as read from the very solid evidence provided by the rocks of the Earth itself.
The geological chart was the product of the discipline of stratigraphy. "Strata" are layers; "graph" means pictures or writing. So stratigraphy was the reading of the history of the planet in layers of rock. The rock in question is sedimentary rock rock formed by the usually slow deposition of particulate matter (usually under water), which gradually packs and condenses until it forms rock.
This discipline was based upon a basic law: the Law of Superposition. This law stated that, if a sequence of layers of rock was undisturbed, then the lower a layer was, the older it was. In other words, the bottom layer had to be there before the next layer could be deposited on top of it. And anything in the bottom layer had to be older than anything in the layer above it.
This is all pretty much straightforward, but the development of these concepts by the early 19th century geologists qualifies as a feat of true collective genius. What they noted was that, while no place on earth has a complete sedimentary record of the past, the layers present in different places could be correlated to each other. In other words, they could create a very, very extensive continuous record of strata by comparing overlapping series of layers from many different places. Thus they built up a long history of depositional activity.
Of course, for most of us one rock looks pretty much like another. Geologists know a lot more than most of us about the composition of different rock types, etc., and they have systematic ways to categorize and classify different rock layers. But a very significant tool that these early geologists used to identify specific rock layers was the inclusions found in those layers. These could be a number of different kinds of things like certain grades of gravel or certain kinds of minerals. However, the most significant inclusions for stratigraphy are index fossils.
Not all fossils are useful as index fossils. They must be inclusions which are found over a widespread area (to allow comparisons between different geographical locations) but which are not found in many layers of rock (so they are diagnostic of rock from a particular time period).
Note that the identification of fossils as the remains of organisms is completely irrelevant to their use in stratigraphy. They could be frozen thunderbolts or simply funny shaped rocks. All that matters is that they are widespread, but found in only one or a few strata, and that they be distinctive.
There is an interesting footnote to this discussion of geographic achievement. One criticism that is sometimes aimed at the theory of evolution goes something like this: "Evolution depends upon the geological chart, and the geological chart was constructed following evolutionary principles, therefore they represent circular reasoning."
Unfortunately for those who wish to undermine evolution, this is an empty criticism. The geological chart was well developed by about 1815 by old-earth creationist geologists. Darwin's theory wasn't published until 1859.
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