The Cambrian explosion
About 500 million years ago a vast multitude of new life forms appear on Planet Earth. These are recorded in the Cambrian rocks. At the Grand Canyon, an ocean moved into the area forming sandstone cliffs at the water's edge (Tapeats Sandstone). As the ocean deepened, it formed a muddy shale (Bright Angel Shale). As the water got deeper still, it formed the cliffs of the Muav.
When there is a natural progression of rock in this manner, geologist call that a conformity. This conformity is called the Tonto Group.
In the Tonto we find fossil worm tracks, trilobite tracks, and if one is lucky, real trilobites.