why didn't the early geologic time scale include the number of years ago that events happened?
The early geologic time scale didn't include the number of years ago that events happened primarily because it was developed before the advent of radiometric dating techniques, which are used to determine the absolute age of rocks and fossils based on the decay of radioactive isotopes.
During the 18th and early 19th centuries, geologists were primarily focused on identifying and classifying different rock layers and the fossils found within them, rather than determining their exact ages. They recognized that certain organisms were restricted to specific layers of rocks, allowing them to establish a relative sequence of events.
To build the early geologic time scale, geologists relied on the principle of superposition, which states that in an undisturbed sequence of rocks, the youngest rocks are on top and the oldest rocks are at the bottom. By studying the order of different rock layers and the fossils they contained, they could establish a relative chronology, with older events occurring earlier in time.
It wasn't until the early 20th century that radiometric dating techniques were developed, allowing scientists to accurately measure the absolute ages of rocks and fossils. Radiometric dating uses the decay of radioactive isotopes, such as carbon-14 or uranium-238, to determine the time that has elapsed since the formation of a rock or the death of an organism.
With the advent of radiometric dating, geologists were able to assign numerical ages to the events in the geologic time scale. This led to the development of the modern geologic time scale, which includes not only the relative order of events but also the actual number of years ago that those events occurred.
In summary, the early geologic time scale didn't include the number of years ago that events happened because radiometric dating techniques were not yet available. It was only with the development of these dating methods that scientists were able to determine the absolute ages of rocks and fossils, leading to the inclusion of numerical ages in the modern geologic time scale.