Grand Canyon National Park, Arizona. Section in the north wall of the Grand Canyon illustrating the unconformity between (A) lower Precambrian schists and (B) steeply dipping upper Precambrian beds and the unconformity between (B) upper Precambrian and (C) Paleozoic strata. Precambrian pertains to the earliest era of earth history, ending 570 million years ago, during which the earth's crust formed and life first appeared in the seas. An unconformity is a buried erosional or non-depositional surface separating two rock masses or strata of different ages, indicating that sediment deposition was not continuous. In general, the older layer was exposed to erosion for an interval of time before deposition of the younger, but the term is used to describe any break in the sedimentary geologic record. This image has been color enhanced.