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Houston Activity: Texas Rocks


Geologic maps can tell you a lot about the rocks beneath your feet. You can use the legend with the map to figure out what rock types are in various geographic areas. The legend can also tell you in what geologic period those rocks formed. Geologists use such maps to help identify where natural resources are and where natural hazards are likely to occur. They are also critically useful in other ways, such as in making wise land use decisions.

While you may be familiar with paper geologic maps, you may not have worked with interactive online geologic maps. For this investigation, you can start with exploring the U.S. Geological Survey’s (USGS) online version of the geologic map of Texas. The geologic time scale shown here will help you to figure out how old the rocks are in various parts of Texas. As you get familiar with how to use the USGS interactive maps, you can widen your exploration to your own geographic area.


  • Computer with Internet access
  • Color copier (optional)
  • Pen


  1. Go online to http://mrdata.usgs.gov/sgmc/tx.html. What you’ll see is an interactive geologic map of Texas.
  2. Click on any part of the map. A new window will open on your computer screen that will show this information:
    • Name of the rock unit
    • Age of the rock unit (geologic period)
    • Primary rock type (such as limestone)
    • Secondary rock type (such as dolostone)
  3. If you can make a color copy of the map on this page, label the rock types for various parts of the map.
  4. Use the geologic time scale to find out which are the oldest rock types in the state. Which are the youngest?
  5. If you do not live in Texas, go to http:// mrdata.usgs.gov/geology/state/ and explore the rocks that make up your own state. How do the rocks in your state compare with those of Texas? How might you explain any differences?

To dowload and print the USGS timescale, click here.
To download and print as a PDF, click here (To change orientation of page, select "landscape" under Print Properties).

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