Laws of relative dating geology Chatphonsex
Tilting and erosion of the older rocks took place during this time, and if there was any deposition going on in this area, the evidence of it is now gone.
Figure 8.8 The great angular unconformity in the Grand Canyon, Arizona.
(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.
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.
For example, the principle of superposition states that sedimentary layers are deposited in sequence, and, unless the entire sequence has been turned over by tectonic processes or disrupted by faulting, the layers at the bottom are older than those at the top.
The method of reading the order is called stratigraphy (layers of rock are called strata).
The tilted rocks at the bottom are part of the Proterozoic Grand Canyon Group (aged 825 to 1,250 Ma).
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.