Geologic Time
Putting it all together . . .
Geologist use the Principles of Relative Dating together with unconformities and fossils to help order events. The following animation illustrates this:
Index fossils
Geologists also use index fossils, fossils of organism that existed over a wide geographic area but for a short geologic time span. When found in a rock these fossils can help constrain the age. For example, if a bone from a T. Rex is found in a rock, a geologist knows that the rock can only be between 83.6 - 65 million years old.
Stratigraphic Correlation
Geologists create stratigraphic columns of an area to give a brief glimspe of the rocks exposed there. The rock units are drawn to scale to portray the original thickness, the rock types are depicted by graphical fill patterns, and specific units/formations are identified. Then, when working in a nearby area, the geologist can use that scale and compare it to the rocks exposed there, and compare them. Comparing stratigraphy in this manner is called stratigraphic correlation. This has been done all over the world. One of the more famous locations is what is termed the "Grand Staircase" in the Colorado Plateau region of Arizona and Utah.
