Interesting article, news in the media, too: 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.“