sandstone is younger than shale because the pieces of shale eroded as the sandstone was deposited

Steno realized earth’s history contains a chronological history of geologic events

nonconformity: a boundary between non- sedimentary rocks (below) and sedimentary rocks (above)

angular unconformity: a boundary between two sequences of sedimentary rocks where the underlying ones have been tilted (or folded) and eroded prior to the deposition of the younger ones

disconformity: a boundary between two sequences of sedimentary rocks where the underlying ones have been eroded (but not tilted) prior to the deposition of the younger ones