The missing links
Below the Redwall, we find the Muav and in some areas, the Temple Butte.
Temple Butte is of Devonian Age (360 MYA), and not often found in the western Grand Canyon. It is more prevalent in the Eastern Grand Canyon where the ocean which formed this limestone was deeper. Along the South Kaibab Trail one can see a lens of Temple butte near the bottom of the Redwall switchbacks.
The Muav(480 MYA) tends to mix in with the Redwall, and in some places it is hard to tell them apart. From a distance, the Muav looks a little more broken than the Redwall.
Somewhere in there we are missing rocks from the Silurian (416 MYA) and the Ordovician (444 mya). These were eroded completely away in the distant past. When sections of rock are missing, geologists call this a nonconformity.
We can tell what was probably happening during the Silurian and Ordovician by studying the rock record elsewhere on earth, but in the Grand Canyon there is no record of these rocks.