A geologist may observe a sequence of rocks in one road cut: mudstones, sandstones, limestones, etc., and another sequence in the next road cut a mile up the road. If the sequences are identical, the geologist might begin to form the hypothesis that they are the same rock beds, and so of the same age, and this might allow him or her to infer that the rock beds were continuous between the two roadcuts, where they run under the ground/grass/poison oak and so cannot be seen. The whole science of studying rock layers is stratigraphy, and the process of matching identifying rocks laid down at the same time is correlation.
Unfortunately, sequences of common kinds of sedimentary rock frequently look alike. If there is a special, rather distinctive or rare sediment in the sequence, that would act as a "marker bed" to allow the geologist to be more confident in identifying two sequences as identical. Examples might be beds laid down by a tsunami (a wave kicked up in the ocean by an earthquake), or a volcanic ash bed. These marker beds are not only distinctive, but are formed practically instantaneously, so that they are precise time markers. We do not know how old they are in thousands or millions of years, but we do know that they formed on the same day!
So simply matching rock types (lithostratigraphy) can be a powerful method of correlation. However, there is a more powerful way, based on the existence of specific time markers in rock sequences.
If Klingons had visited the Earth every zillion years, and had sprinkled colored BBs over the entire surface of the planet in some sort of religious ceremony (different color for each visit), we could explore the planet, seeing always that certain colors were in older layers than others. We could then draw time lines, guaranteed accurate in different continents, using the BB layers. Every time we found blue BBs, we would know, perhaps, that we were in older rocks than the layers that had red BBs. We might not know the actual dates to within 100 m.y., but we could work out the relative sequence of colors.
That did not happen, but we have a perfectly acceptable substitute, in the form of fossils. Since life formed on this planet, it has evolved. The organisms alive at any time are different from their ancestors, and different from their descendants. What we have to do is to examine the sequence of fossils that occur in sedimentary rocks, and (laboriously) to collect up and down the layers, noting the changes, and getting to know which fossils are the best ones for telling us the age at which they were alive. This process is biostratigraphy.
(Just as an aside, you don't *have* to believe in evolution to make this work. All you have to recognize is that fossils have changed through time. You can believe that fossils were planted by Klingons if you like, and you can still be an effective biostratigrapher. In fact, biostratigraphy began, and was very successful, long before Darwin explained the mechanism for evolution.)
Early geologists in Western Europe collected fossils in the extensive chalk deposits that extend from England and France all the way to Russia. The fossils (called Cretaceous after the Latin for "chalk") were generally alike, and they were different from those collected in the rocks sequences just below (named the Jurassic after the Jura Mountains of eastern France). So rather quickly, geologists began to set up a sequence of time periods, during which certain sedimentary sequences of rocks were laid down, each period with a recognizable set of fossils that reliably occurred in the rocks and defined the period. Geologists competed to discover and name these sets of rocks-with-their-own-fossils, and they were often named after the areas in which they were defined. So Devonian rocks were named after the county of Devon, in southern England; Permian rocks were named after the city of Perm, in Russia. Overall, the geological time scale has now been stabilized, codified, and refined to the point where a good fossil collection from any particular rock bed can usually be identified as belonging to a particular time. Often we can do much better than "period": periods are typically divided into stages, and each stage is dividied into fossil zones.
Some fossils are more useful than others for telling time. Animals that evolve slowly are obviously not as useful for telling time as those that evolve quickly, and organisms that are widespread in geography and in habitat are more useful than those that are restricted in their range. Those that are abundant are more useful than those that are rare. It helps if the evolutionary changes that occur are easily recognised. So certain fossils (called zone fossils) have come to be used more than others. I showed pictures of some fossil cephalopods that were big, beautiful, and had complex markings on the shell that allowed them to be identified not only as certain species, but also as belonging to specific time periods.
Oil companies may spend $100,000 a day to drill an oil well, so it is important to know precisely what age of rock the drill bit has reached. If it has gone beyond (below) the level at which the company expected to find oil, the hole is a "dry" one and it is time to stop. A trained paleontologist will be able to tell the age of the rock at the base of the hole within a matter of hours, so may be able to save the company more than her annual salary, perhaps several times a year!
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