Let’s know about what are the geologic column and the fossil record? As you know the Earth is about 4500 million years old. Clearly, this is an immensely long period of time, difficult for us with our life span of about 70 years to imagine. To make consideration of the history of the Earth a little easier, this huge time scale has been divided up into smaller units, and then further divided into still smaller units.
The largest unit of time we use is called an era. There are six of these eras, but they do not by any means represent equal lengths of time. They are the Eozoic, and Cenozoic. Paleozoic, for example, means ancient life, and the rest have equivalent meanings.
What are the geologic column and the fossil record
During any given period of time, however, beds of rocks will have been laid down somewhere around the globe and these must be given names too. The strata or beds of rock deposited during an era are referred to as a Group. Eras are divided into Periods and the groups into Systems and it is the Periods and Systems which make up the geologic column shown in the picture.
Sometimes confusion arises between time and rock units. If you wish to divide say, the Devonian period you might refer to the divisions as Early, Middle, or Late Devonian time, whereas the equivalent rock units would be the Lower, Middle and Upper Devonian. It is important that these terms should not be mixed up, although they often are, even by experienced geologists.
As you can see from the picture, the last 600 million years only comprise three Eras. This is only a tiny period compared with the 4000 million years that went before, which are usually referred to loosely as the Precambrian, that is, the period of time before the Cambrian, the lowest Period in the Phanerozoic time scale.
At first, it was thought that the first life appeared on Earth at the beginning of Cambrian time because it was in rocks described as Cambrian in Wales that found. Later, however, it was discovered that the first life forms may have occurred at least 3500 million years ago, but still the most detailed fossil record exists for the last 600 million years.
The boundaries between the time Periods or rock systems are often established on the appearance of some new form of life. It may be a completely new type of life form or just a distinctive species of an animal or plant form that has existed earlier. In fact, the availability of the fossil plays an important part too.
For example, although graptolites had existed in Cambrian times, the Ordovician period is usually sub-divided on the basis of the species of these animals.
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