How do geologists talk about geological time?
Geologists have divided the 4.5 billion years of geological time into smaller parts using the geological time scale (table 1). As we have discussed, geologists have determined the age of the Earth by studying the concentrations of radioactive isotopes in certain rocks. Only certain types of rocks, however, can be dated using radioactive isotopes. In many cases, the best way to determine a rock’s age is to look at the fossils contained within that rock. Many of the rocks in Kansas, like those found along many of our state highways and in stream banks, have abundant fossil remains. Geological time is divided into a number of eras and periods, each characterized by a specific set of fossils, the remains of organisms that lived in those remote geological times. The oldest rocks, which were formed before large animals had evolved, are termed Precambrian rocks. Precambrian rocks do not contain many fossils, and most of the fossils that occur in them are rather small–in fact, most are micros