Horizons are specific layers of rock representing a particular point in geologic time.  When you make a sandwich, the first thing you do is put a piece of bread down, which becomes the bottom layer of the sandwich.  In nature the first layer of rock that gets laid down is also at the bottom.  That is why on this picture the horizon numbered one is at the bottom, because it was put down first.  The term that geologists use for this idea that layers are added one top of the next is “superposition.”

At the Fossil Finders site, some samples received decimal numbers, such as “2.2,” which means the horizon is above horizon 2 and below horizon 3. The top (youngest) horizon is numbered 4.2. This image does not show all of the horizons that have been studied with Fossil Finders.

Data for Each Horizon

        

Biodiversity for Each Horizon