It is generally not used alone, because there are some exceptions. And you are right, it is a statement of relative time, not absolute time. "The law of superposition (or the principle of superposition) is a key axiom based on observations of natural history that is a foundational principle of sedimentary stratigraphy and so of other geology dependent natural sciences: “Sedimentary layers are deposited in a time sequence, with the oldest on the bottom and the youngest on the top. ” The principle was first proposed in the 11th century by the Persian geologist, Avicenna (Ibn Sina), and the law was later formulated more clearly in the 17th century by the Danish scientist Nicolas Steno." "From Steno's observation that rock strata form when particles fall out of suspension in a fluid, it then follows that the youngest stratum is on the top of a sequence. However, this principle also applies to other types of rocks that do not form with water, such as volcanic rocks which spread on older flows, by flow banding. Steno realized that other geological processes could create apparent exceptions to his laws of superposition and horizontality . He reasoned that the formation of caves might remove part of a lower layer, and that the collapse of a cave might transport large pieces of an upper layer downwards. He recognized that rocks might be uplifted by subterranean forces. Geologists now recognize that tilting, folding, and faulting may also complicate the analysis of a stratigraphic sequence. Molten rock may force its way through surrounding rocks and may sometimes squeeze between older rock layers, also forming an exception to Steno's law. However, such anomalies leave physical evidence in the disturbed rocks; for example, faulted rock layers may be cracked, broken, or metamorphosed along the fault lines. Steno's law is a statement of relative time, not absolute time: two rock layers, in principle, could form millions of years apart, or days apart." Source and further information: http://en.wikipedia.org/wiki/Law_of_superposition