GRAND CANYON - THE GREAT UNCONFORMITY It is fitting that the Grand Canyon should contain some of the best exposures of The Great Unconformity -- the gap in the rock record between Cambrian times (~550 m.y. ago) and the pre-Cambrian (anything earlier). An unconformity is a surface in the rock record, in the stratigraphic column, representing a time from which no rocks are preserved. It could represent a time when no rocks were formed, or a time when rocks were formed but then eroded away. Small unconformities are ubiquitous in the rock record -- for instance, in the Coconino sandstone, every cross-bed surface represents a small unconformity. The Great Unconformity is important for three reasons: it represents a long span of time -- 250 to 1200 million years in the Grand Canyon;

it is found nearly everywhere across the globe; and

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

