He worked in Southern England, and he got to see all kinds of different rock strata that were exposed in outcrops and canals.
William Smith collected fossils from his work sites and, over time, he learned to recognize which fossils tended to show up in which rock strata.
We want fossils of plants and animals that lived for a relatively short amount of time, like a few hundred thousand years or so.
I know that doesn't seem like a very short time span, but it is when we're talking about geologic time.
They follow an ordered progression that is very clear and predictable.
We wouldn't want to use a horseshoe crab fossil, because horseshoe crabs have existed for over 400 million years and are still alive today!
Watch this video to find out how we use index fossils to establish the relative ages of rocks.
In previous lessons, we talked about the Geologic Time Scale and how scientists use it to piece together the history of the earth.
The upper layer had scallop fossils, and the lower layer had trilobites.
Smith would have brought these two arrangements together, overlapping the common scallop layer, to produce a larger succession of three rock strata!