The best indication of the relative age of a rock layer is the position of the layer compared to other layers, according to the law of superposition. This principle states that in an undisturbed sequence of rock layers, the oldest layers are at the bottom and the younger layers are deposited on top. By examining the order of the layers, one can determine their relative ages without knowing their exact numerical age. Other factors such as thickness or chemical makeup do not reliably indicate relative age.