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Different species of ammonites lived at different times within the Mesozoic, so identifying a fossil species can help narrow down when a rock was formed.
Correlation can involve matching an undated rock with a dated one at another location.
Recognizing unconformities is important for understanding time relationships in sedimentary sequences.
There are a few simple rules for doing this, some of which we’ve already looked at in Chapter 6.
[SE] The principle of cross-cutting relationships states that any geological feature that cuts across, or disrupts another feature must be younger than the feature that is disrupted.
An example of this is given in Figure 8.7, which shows three different sedimentary layers.
Correlation with them has helped geologists date many New Zealand rocks, including those containing dinosaurs.
Bring relative dating principles to life with the activity Rock layers and relative dating.
For example, a xenolith in an igneous rock or a clast in sedimentary rock must be older than the rock that includes it (Figure 8.6).