Guide students in reasoning: Viewing layers of sedimentary rock, such as we see in the Grand Canyon, we can logically assume that layers at the bottom are the oldest followed by successively younger layers.
As fossil hunters excavated these layers of sedimentary rock, something profound was discovered. The fossils found differed from one layer one layer to the next. This led geologists to name rock layers according to the fossil plants and animals found in them and create charts accordingly.
Type into your browser: fossils of geological ages chart
For images of addition fossils found in any given sedimentary rock layer (geological period) type into your browser:
fossils of the _______ period images
Put the desired geological period (above chart) in the blank
Note that some kinds of fossils (organisms) persist over several ages while new kinds of plants and animals come into the picture and others pass out (become extinct).