Rocks with the laminated structure in the field are all called rock formation. However, when it is concerned with the geological time, the sequence of old and young rocks, this formation is then called rock. Therefore, the stratum may be defined as "rock stratum with time sequence”. The rock formations on the earth’s surface are deposited successively with the old rocks superimposed by the younger ones. Rocks deposited in such a way will form a sequence stratigraphy. Older rocks always underlie the younger ones unless they are changed or reversed by the intense tectonics. Hence, this is the “stratigraphic superimposition law”. When the early sedimentary strata have tectonic changed to form sedimentary breaks, dipping reverses and folding, a clear break will appear between the later deposits and the original rocks. This break surface is called the “rock strata contact”.