Chapter 2: Section 6 - Interpreting the Geologic History of Your Community

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In this section you will find materials that support the implementation of EarthComm, Section 6: Interpreting the Geologic History of Your Community.

Learning Outcomes

  • Use models and cross sections that explain the basic geologic principles for relative dating of rock layers and structures.
  • Analyze and interpret data on a geologic cross section of an area to describe its geologic history.

Inquiring Further

  1. To learn more about radiometric dating, visit the following web site:

    Radiometric Time Scale, USGS
    Describes the use of radioactivity to determine the geologic ages of rocks.

Radioactive Decay and Geologic Time: Simple Models, Carlton College
How can popcorn be used to model parent and daughter atoms? Find out how geoscientists decide which material to date.


To learn more about this topic, visit the following web sites:

Interpreting Geologic History

Relative Time Scale, USGS
Describes the development of the basic geologic principles and how they are used to determine the ages of rock layers.

How Do Geologists Know How Old a Rock Is?, Utah Geological Survey
Descriptions of the basic geologic principles and how they can be used to determine the age of a group of rocks.

Laws of Superposition and Crosscutting Relationships, USGS
How do we know if a sill is older or younger than the surrounding layers? Find out here.

The Grand Age of Rocks: The Numeric Ages for Rocks Exposed within Grand Canyon, NPS
Detailed description of the geologic history of the Grand Canyon.  Includes pictures of several unconformities that are seen in the Grand Canyon.

The Relative Ages of Rocks, Berkeley
Unravel the relative ages of the rock units in another challenging cross section.

Web Geologic Time Scale, University of California Berkeley
Use the links on the timescale to learn about different periods in Earth’s history from 4.5 billion years ago to the present.