Answer:
4.index fossils, intrusions, and cross-cutting relationships
Explanation:
There are various geologic features or phenomenon that determines the relative ages of rocks . This geologic features or events includes index fossils , intrusions, cross cutting relationship etc. Note relative ages of rock does not give the actual ages oft the rocks but instead compares the ages of these rocks. Knowing the relative ages of rocks, one can decipher which rock is older or which one is younger. The geologic features give details on the sequence of event that happens through the formation.
Like the principle of cross cutting relationship states that the geologic feature which cut across or intrudes another geologic feature is younger than it surrounding rock.
Index fossils are remains of organisms that are widely spread. The deposition of this remains can give the sequence for deposition of the surrounding rocks which these fossils are found.
Intrusions occurs when geologic features like plutons invades another rock body. Geologically, the plutons will be younger than the surrounding or host rock.
