什么是 Unconformity, angular unconformity, disconformity？

An unconformity is a surface of erosion or non-deposition that separates younger rocks from older ones, and so marks a gap in the record. In an angular unconformity the older beds were tilted or folded before the younger ones were laid across their eroded edges, so the two sets of beds meet at an angle. In a disconformity the beds above and below are parallel, and the gap shows only as an erosion surface or as missing fossils or ages.