Rules of relative dating
Exercise 8.1 Cross-Cutting Relationships The outcrop shown here (at Horseshoe Bay, B. Buff/pink felsic intrusive igneous rock present as somewhat irregular masses trending from lower right to upper left 2. A 50 cm wide light-grey felsic intrusive igneous dyke extending from the lower left to the middle right – offset in several places Using the principle of cross-cutting relationships outlined above, determine the relative ages of these three rock types.(The near-vertical stripes are blasting drill holes.Not only did the rock layers indicate changing environments they also revealed that different life forms have existed in different times.The simplest and most intuitive way of dating geological features is to look at the relationships between them.An example of an unconformity is shown in Figure 8.8.The Proterozoic rocks of the Grand Canyon Group have been tilted and then eroded to a flat surface prior to deposition of the younger Paleozoic rocks.This law was independently discovered by William Smith (1769-1839), a British engineer, while working on excavations for canals in England (Winchester, 2002 p.
Put another way, the natural laws that we know about in the present have been constant over the geologic past.
The image is about 7 m across.) [SE photo] An unconformity represents an interruption in the process of deposition of sedimentary rocks.
Recognizing unconformities is important for understanding time relationships in sedimentary sequences.
The principle of inclusions states that any rock fragments that are included in rock must be older than the rock in which they are included.
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).