Almost from the start, geologists have argued, sometimes passionately, about exactly where the divisions of geologic time should begin and end. In the early days, when all ages were relative, meaning this is older than that, but we didn’t know absolute age dates closer than tens of millions of years or more, it was challenging to assign a particular time to the start of a period like the Cambrian. When I was in college, the start of the Cambrian was put at 600 million years ago, and honestly, it was a fairly arbitrary number even though it was generally accepted. My historical geology textbook, published in 1960, has a question mark after 600 million years.
The Cambrian Period today is considered to have lasted until about 488 million years ago, for a total length of 54 million years, just 1% of Earth’s history. Yet we’re spending all of February on the Cambrian. Stuff happened!
Like the months of the year, the periods of earth history are subdivided and given names to make it easier to refer to them. So we have Early, Middle, and Late Cambrian as general names, and in parts of North America those times have been related to packages of rocks called the Waucoban Series, Albertan Series, and Croixan Series. You can imagine that everything geologic didn’t happen simultaneously, or in the same way, all around the world, so there are dozens of regional variations in the nomenclature. The subdivisions are not arbitrary, but are often based on the fossils that can be found in each subset of the rocks. Fossils can be related to age, and can help correlate between rocks of different type that may be of the same age.
There are ongoing attempts to create internationally acceptable time scales. So there is actually an International Subcommission on Cambrian Stratigraphy that works on this kind of thing, and at present the Cambrian is divided into four major subdivisions, based in part on the timing of major extinction events. I think we’ll settle for early, middle, and late.
International Commission on Stratigraphy