Geologic Time: Concepts and Principles - The Changing Earth: Exploring Geology and Evolution - page 434

OUTLINE
Introduction
How Is Geologic Time Measured?
Early Concepts of Geologic Time and the Age of Earth
James Hutton and the Recognition of Geologic Time
Relative Dating Methods
Correlating Rock Units
Absolute Dating Methods
GEO-INSIGHT: Uluru and Kata Tjuta
Development of the Geologic Time Scale
Stratigraphy and Stratigraphic Terminology
GEO-FOCUS: Denver's Weather—280 Million Years Ago!
Geologic Time and Climate Change
Geo-Recap

OBJECTIVES
At the end of this chapter, you will have learned that
The concept of geologic time and its measurements have
changed throughout human history.
The principle of uniformitarianism is fundamental to geology.
Relative dating—placing geologic events in a sequential
order—provides a means to interpret geologic history.
The three types of unconformities—disconformities, angular
unconformities, and nonconformities—are erosional surfaces
separating younger from older rocks and represent signifi cant
intervals of geologic time for which we have no record at a
particular location.
Time equivalency of rock units can be demonstrated by vari-
ous correlation techniques.
Absolute dating methods are used to date geologic events in
terms of years before present.

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The Grand Canyon, Arizona. Major John Wesley Powell led two expeditions
down the Colorado River and through the canyon in 1869 and 1871. He was
struck by the seemingly limitless time represented by the rocks exposed in the
canyon walls and by the recognition that these rock layers, like the pages in a
book, contain the geologic history of this region.

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