Grassy Mountain nonconformity outcrop

The upper layer here is the Bonneterre Formation which is 500 million years old.  These sedimentary layers overlie the 1.48 billion year old rocks of the Grassy Mountain ignimbrite which is called a nonconformity (a missing part of the geologic record).  You can also see a dike within the Grassy Mountain ignimbrite which has been eroded relative to the ignimbrite.  The material is brecciated material  representing eroded rock from both the dike and ignimbrite.  The younger sedimentary layers rest on top of this material.