What is the Rock Record?

Geologists and other earth scientists often refer to the rock record. The rock record is nothing more than the rocks that currently exist. The rock record does not show a tidy, orderly progression of geologic events.  Rock formations are eroded, buried, torn apart, melted, squashed together, even turned upside down. The only parts of the Earth history "recorded" are "leftovers" that haven't yet been recycled. That is, when an area undergoes change due to a geologic process, the original rocks are often changed or destroyed, making the investigation of the events that created the rock quite difficult. Nevertheless, every thing we know about the history of the Earth has been learned from studying the rock formed by geologic processes. 

 Geologic time is measured in billions of years.  Most of Earth's history is very sketchily recorded, and the rock record for most of Earth's history is composed of rocks that have been changed physically and chemically many times since it was first laid down.  The appearance of fossils in the rock record has made geologic investigation easier, because the organisms that the fossils came from give us markers in the rock record.  Fossils also tell us many things about the environment present when the organisms were alive.