Which principle or law of geology states that in a sequence of undeformed sedimentary rocks, each bed is older than the one above?

The law of superposition related to the strata in geology (also called Steno's Law) states that "sediment layers are deposited in a temporal sequence, in which the oldest are in a position inferior to the most recent"

This principle was initially proposed in the 11th century by the Persian geologist Ibn Sina, and was first proposed as a law in the 17th century by the Danish scientist Nicolás Steno.