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Central Texas

Geodetective

Finding faults in west Austin

The Balcones Fault Zone is composed of numerous faults. Take a look at the outcrops near MoPac, all the way from Loop 360 to the south, just north of Braker Lane. The original rocks were lying flat before faulting. The rock affected by the faults shows offset, tilting, and/or folding. See if you can spot any of these features. Also, take a look at the geologic map of Austin. This map shows the location of faults in west Austin.

Finding evidence of prehistoric life in west Austin

Many of the rocks in west Austin on both sides of the fault contain abundant evidence of prehistoric life in the form of fossils. Great fossils can be collected in the road and stream cuts of the west Austin hill country. Many fossils can be seen with the naked eye, although for others you will need a magnifying glass. Here’s a book that will help you identify many of the fossils:

Matthews, W. H., 1960, Texas Fossils: an amateur collector’s handbook: University of Texas, Austin, Bureau of Economic Geology, Guidebook 2. Click here to order this online.

Good hunting!

Limestone from the Lower Cretaceous Walnut Formation.
Limestone from the Lower Cretaceous Walnut Formation showing shell fragments and imprints of bivalves (clams) and gastropods (snails).
Limestone showing ancient burrowing
Limestone showing intense burrowing by an ancient shrimp-like creature called Thalassinoides. These kinds of fossils are called trace fossils.
Austin cross section through the Balcones Fault Zone
A cross section of Austin through the Balcones Fault Zone that shows where major faults are located.