The ages of the rocks exposed on the surface are a major source of information when it comes to interpreting the subsurface cross sections from geologic maps. Additionally, these ages are another way to recognize disconformities. In the Grand Canyon for example, the Redwall Limestone sits upon the Muav Limestone in many places. In some cases, the contact is difficult to see-- so inclusions may not be visible to help us determine that erosion must have occurred between the two. But, the Redwall Limestone contains fossils indicating that it is Mississippian in age. The Muav Limestone's fossils suggest that it is Cambrian in age. The entire Ordovician, Silurian, and Devonian fossil assemblages are missing! Therefore, rocks of these ages were either never deposited or eroded off prior to the deposition of the Redwall. This is the evidence of a disconformity.