How would a geologist use the principle of superposition to determine the relative age of the rocks in the grand canyon

A geologist could use the principle of superposition to determine the relative ages of the rocks in the Grand Canyon because this canyon exhibits what geologists call 'layer cake' geology with mostly horizontal beds and formations going from the oldest near the bottom of the canyon and the youngest at the top and if the actual age of an underlying formation is known say Cambrian then it can be correctly surmised that the formation lying on top of it is at least younger than Cambrian, for example. 

Answer barren with few trees, in the northern hemisphere;