What is the principle of cross-cutting relations and why is it important for relative dating?
A cross cutting relationship tells us that in order for say rock 2 to cut across another rock 1, rock 1 had to be there originally for the rock 2 to cut through it. Its a logic thing
Cross cutting relationships in rock give us some relative idea of which rock came first, second and so on. See pic for a simple example. Rock 1 came first, then rock 2, then rock 3, then the orange volcanic dyke rock 4 cut across rocks 1 to 3 and then rock 5 was deported on top of them all.
Here is another picture I took of three rocks showing cross cutting relationships. the light brown rock cut through the darker brown speckled rock and then the pink dyke cut across the two brown rocks. So, the speckled rock came first (oldest), the light brown rock next, and the pink rock last (youngest)