In a sequence of sedimentary rocks, if there is a bed of gravel, the clasts (or inclusions) of gravel will be older than the bed in which they are contained.
In many instances, the gravel will directly overlie an irregular erosion surface. And sometimes, it will be obvious from the lithology that the clasts in the gravel bed are derived from the underlying partially eroded layer. If this is the situation, it is possible to place several layers and events in their proper relative order: (1) deposition of sedimentary rock A, (2) erosion of sedimentary rock A, producing an irregular erosional surface and "rip-up" clasts, (3) deposition of "rip-up" clasts of sedimentary rock A on top of the irregular erosional surface, producing a gravel bed. This gravel bed is sometimes called a "basal conglomerate".
A similar line of reasoning may be applied to igneous rocks if xenoliths are present. You should remember from Physical Geology that a xenolith (which literally means "foreign rock") is a piece of surrounding rock (or "country rock") which becomes caught up in an intrusion. As magma moves upward, forcing itself through cracks in the surrounding rock, sometimes pieces of these surrounding rocks break off or become dislodged and incorporated into the magma without melting. These pieces of rock are called xenoliths, and they move along with the magma. According to the Principle of Components or Inclusions, xenoliths are older than the igneous rock which contains them.
Intrusion containing xenoliths.
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