![]() ![]() Second, we observe that rock layer H (which is an igneous intrusion) cuts into rock layers B-F. Let's work through the imaginary example above.įirst, we know from the principle of superposition that rock layer F is older than E, E is older than D, D is older than C, and C is older than B. The fault cut the curb and is thus younger than the curb itself. We know that the curb was originally straight when it was first constructed. As it turns out, the famous San Andreas fault runs below the curb at this location, which has caused the curb to be broken and displaced. You can see that the curb is offset: the bottom half does not line up with the top half. Have a look at the photographs below, which show the curb of a road in a neighborhood in Hollister, California. Because these features are the ones doing the cutting, we know that they are younger than the rocks that they cut into. ![]() When investigating rocks in the field, geologists commonly observe features such as igneous intrusions or faults that cut through other rocks. Because of cross-cutting relationships, the cut that divides the slice from the rest of the loaf is younger than the loaf itself (the loaf had to exist before it could be cut). Imagine cutting a slice of bread from a whole loaf. The principle of cross-cutting relationships states that a rock unit (or other geological feature, such as a fault) that is cut by another rock unit (or feature) must be older than the rock unit (or feature) that does the cutting. A third key principle- faunal succession-is reviewed in Section 3. Geologists employ a handful of simple principles in relative age dating two of the most important of these are are the principles of superposition and cross-cutting relationships. Relative age dating has to do with determining the temporal ordering of events in Earth's past.
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