The Great Unconformity

There is a gap in the rock record between Cambrian times and pre-Cambrian, 550 million years ago. An unconformity is a surface in the rock record in a stratigraphic column representing a time from which no rocks were preserved or formed, or when rocks were formed buy eroded away. 

Small unconformities are ubiquitous in the rock record. But the Great Unconformity is important because it represents a long period of time (250-1200 million years in the Grand Canyon), and it is found nearly everywhere in the world. 

​And it divides rocks with familiar fossils from those with no fossils or only fossil bacteria. 

​Below, you will find the Great Unconformity at the bottom of the blue layer.