Strata originally extended in all directions until they thinned to zero or terminated against the edge of their basin. surfac. Intrusion must be younger than the host rock. Igneous rocks find the weak spots in sedimentary rocks and fill those in. Xenoliths are older; basaltic rocks are younger. The fault must be younger than the surrounding rocks. Rocks above follow erosion of rocks below. There"s a length of time that is missing in the middle of rocks (most probaly because for a time, there was no sedminetary rocks during that length) Nonconformity : a surface of erosion that separates younger sedimentary strata above from older igneous or metamorphic rocks. Angular unconformity : this is the easiest to determine. It is a surface of erosion between two groups of sedimentary rocks in which the orientation of the older strata, below, are at an angle to younger strata, above.