The topics covered in this chapter can be summarized as follows:

Section Summary
7.1 The Geological Time Scale The work of William Smith was critical to the establishment of the first geological time scale early in the 19th century, but it wasn’t until the 20th century that geologists were able to assign reliable dates to the various time periods. Geological time is divided into eons, eras, periods, and epochs and the geological time scale is maintained and updated by the International Commission on Stratigraphy.
7.2 Relative Dating Methods We can determine the relative ages of different rocks by observing and interpreting relationships among them, and applying the principles of stratigraphy. Gaps in the geological record are represented by various types of unconformities.
Lab 7 Exercises The principles of stratigraphy can be applied to determine the geologic history of an area. Road cuts, cliff faces, and cross-sections give us an excellent insight into the geology of the subsurface, and the sequence of geologic events responsible for the strata observed can be solved like a puzzle. Keep in mind, that these stratigraphic principles can also be applied to relationships displayed in any outcrop, hand sample, and even in thin section! For example, using the principle of inclusions: a microscopic fluid inclusion within a crystal of quartz must be older than the quartz itself.


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A Practical Guide to Introductory Geology Copyright © 2020 by Siobhan McGoldrick is licensed under a Creative Commons Attribution 4.0 International License, except where otherwise noted.

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