The law of superposition is based on the common sense argument that the bottom layer had to laid down first. The bottom layer because it logically had to be laid down first must be older. The layers on top could only be laid down on top of the bottom layer so must be younger.
However the relative ages of rocks is more commonly determined by the presumed ages of the fossils found in the sedimentary layers. The sedimentary layers with the simplest fossils are assumed to be older even if the sedimentary layer is found on top of a sedimentary layer that has fossils that are more complex and therefore assumed to be younger.
Fossils that are in violation of the law of superposition where the older fossil occurs above a younger fossil are said to be stratigraphically disordered. " virtually all sedimentary systems have stratigraphic disorder at come scale is probably a common feature of the fossil record " Fossils out of Sequences Cutler Palaios June 1990 on
The conclusion of some scientists is that the Law of Superposition just doesn't work Shindewolf Comments on Some Stratigraphic Terms American Journal of Science June 1957 " Historical geology relies chiefly on paleontology the study of fossil organisms. Von Englen Geology McGraw Hlll 1952 page 346.
The Law of Superposition makes logical sense but in practice it is the nature of the fossils found in the sedimentary layers that determine the relative ages of the rocks. The theory of descent with modification trumps the empirical evidence of superposition.