Superposition of rock units is a very simple and straightforward method of relative age determination. The principle states that in a sequence of undeformed sedimentary rocks the oldest beds are at the bottom and the youngest ones are at the top. Underlying assumptions are 1) that layers were originally deposited horizontally, 2) and that beds are not overturned (sedimentary structures can be used to dermine whether a sedimentary succession is overturned or not).