Laws of relative dating geology
The boundary between the two represents a time gap of nearly 300 million years.[SE ]Figure 8.9 The four types of unconformities: (a) a nonconformity between non-sedimentary rock and sedimentary rock, (b) an angular unconformity, (c) a disconformity between layers of sedimentary rock, where the older rock has been eroded but not tilted, and (d) a paraconformity where there is a long period (millions of years) of non-deposition between two parallel layers.(The near-vertical stripes are blasting drill holes.The image is about 7 m across.) [SE photo] An unconformity represents an interruption in the process of deposition of sedimentary rocks.[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.
The flat-lying rocks at the top are Paleozoic (540 to 250 Ma).The simplest and most intuitive way of dating geological features is to look at the relationships between them.There are a few simple rules for doing this, some of which we’ve already looked at in Chapter 6.Geologists have studied the order in which fossils appeared and disappeared through time and rocks. Fossils can help to match rocks of the same age, even when you find those rocks a long way apart.
This matching process is called correlation, which has been an important process in constructing geological timescales.
Relative dating does not provide actual numerical dates for the rocks.