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Guidebook to the Geology of Travis County

Field Trip No.2 : Balcones Fault Zone

Mount Bonnell, Zilker Park, Highland Park Elementary School

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I.   To the Teacher and/or Field Trip Leader  

II.  Students' Introduction  

III. The Tour


TO THE TEACHER AND/OR FIELD TRIP LEADER

The Balcones Field Trip consists of three stops: Zilker Park, Mt. Bonnell, and Highland Park Elementary School. None of these three gives a clear, concise view of the Balcones Escarpment alone, but each presents one or more aspects of this huge fault. The tilted and folded outcrops exposed at Highland Park School are an old rock quarry now filled in for a new use as an athletic field in the school recreational-park complex. Mt. Bonnell is a very scenic, dramatic bastion created by the Colorado River carving a deep valley through the Balcones. Zilker Park is the site of natural springs. Many of these springs are located along the Balcones Escarpment.

All three stops have adequate parking for a school bus, and there is no preferred sequence of stops. Your sequence depends upon your initial location; however, Zilker might be the best choice for the third stop since it offers the best facilities for a luncheon break and recreation.

There are a few competencies that your students will need to have to maximize their learning:

1. Bedding - originally deposited horizontally.

2. Law of Superposition - youngest layers on top, etc.

3. Accretion - how our coastal regions and the continental shelves are thought to have been formed.

4. Graben - what it is, and how it is formed.

5. How the forces of stress, tension, and shear act upon rock.

6. The definitions of aquifer and water table so that students will know the mechanics of a spring.

The actual length of time for this field trip obviously will vary from one schoolls location to the other. Also, it is dependent upon how long you wish to dwell in any one location. To give you an idea of timing, thirty minutes at the Highland Park School is ample to view the outcrops; longer stays might interfere with the schoolls playground activities. At Mt. Bonnell, the time could range from 30 to 45 minutes. If your group is 8th grade Earth Sciences, then 30 minutes is more than enough time; but an 11th or 12th grade Geology class might want to look in more detail at the topography and the stratigraphy of the rocks. Zilker is a 30 minute tour (without lunch or recreation). The entire trip, from departure to return to school could be accomplished from most Austin locations in a little over three hours.


STUDENTS' INTRODUCTION

You are fortunate to live in Austin, which has many varied and dramatic geological points of interest within its city limits. Our Balcones field trip will concentrate on the few, but very good, exposures we have of the ancient fault we call the Balcones Escarpment. We will have only three stops: Zilker Park, Mt. Bonnell, and Highland Park Elementary School.

The purpose of this tour is to give you an opportunity to learn how a fault occurs, what a fault looks like, and where you can find one. When you complete your tour, you should be able to answer the following list of study exercises:

1. Why is the hill country, just west of Austin, so different from Austin and eastward?

2. List at least three prominent land features that identify the boundary of the Balcones Escarpment.

3. Draw one or more possible results of a rock mass failing under (1) stress by the force of Tension, and under (2) stress by force of Compression.

4. Why are so many Central Texas cities located along the fault zone?

5. Explain the nature of the force that most likely caused the Balcones Escarpment.


A VERY BRIEF HISTORY OF THE BALCONES ESCARPMENT

Not very long ago in geologic time (only a mere 300 million years ago, more or less) our Texas coast did not quite make it to Beaumont, Galveston, Corpus Christi, Brownsville, etc. Take a look at your Texas map (Fig. 54). You will note that both the present coast and the fault zone are heavily outlined. This inland fault approximates very closely the existing coastline during the Permian period. All of the good Texas land from that line to our present coastline (well over 100 miles Gulfward from the ancient coastline) has been added from rocks, sand, and muds washed down from the hill country and carried by the many rivers like our Colorado River. You can easily imagine that it has required an immense amount of silt and all of those millions of years for nature to pile up enough material to make Texas the huge state that it is today. So when you consider Texas' size, think about all the erosion and piling of materials that make it!

When material is piled on other material, the result is that the whole pile tends to sink, as great weights drive out water and causes the material to compact (Fig. 55a). As this great mass slowly sank, it caused tension, or pulling effect, whose force was strongest along the ancient continental border or coastline. One day (still millions of years ago), the stress became great enough to overcome the strength of the rocks, and a gigantic shear occurred along the original coast. A huge graben was formed in an area located against the Permian coastal region and our Balcones Escarpment was born (Fig. 55b).

The original relief of this graben was much greater than it is today. Erosion has worn down the high part, or the hill country, and the deep trench has been filled in considerably with sediments from the hill country (Fig. 55c). When you climb Mt. Bonnell and look at the countryside, you will find that there is still plenty of relief remaining.

There is no large scale faulting occurring in Texas today like that which resulted in the Balcones Escarpment. However, along the entire Louisiana and Texas Gulf coast, a whole series of small slumps and earth movements occur daily somewhere! These movements are of particular concern to city engineers in Houston, and the annual damage to buildings, streets, and underground utilities is very costly.


THE TOUR

Zilker Park

There are more than four distinct points of interest at Zilker Park (excluding the refreshment stand). As you enter the park, you will notice two stone columns at the entrance of the parking lot. These "gates" are made of a sample of every rock-type native to Texas. How many of these rocks can you identify? You should be able to name at least eight.

Zilker Park contains some of the many natural springs which are located along the Balcones Escarpment. Natural artesian waters rise to the surface along a fault or break in the earth's crust and are supplied by underground water that is stored in an aquifer. These springs supply water to the swimming area, and also add to the water flow of the Colorado River. San Marcos, which is only 30 miles south of here, also has many large natural springs. In fact, many of the early towns in central Texas were located where natural springs existed. The nearby historical marker will provide you with some data on the amount of water produced by Barton Springs. Figure 56 shows the relationship between the aquifer and the artesian waters at Barton Springs.

As you are looking down into the springs, you are standing on some rather unusual flagstones. Look at them and see -Lf you can think of anything else that might look like this. Any ideas? These flagstones are like fossils, but instead of being remains of once-living creatures, they are a fixed record of an event. Have you ever noticed how an old mud puddle tends to crack into a definite pattern as it dries out? Now look again at these flagstones. Do you see the similarity? Now turn back to the mud hole with cracks -- what happens to the cracks when it rains again? These ancient mud cracks are not swollen-out and smoothedover again. if you were a geologist studying these rocks, could you assume that the story locked up in these cracks is that there was a very long dry spell and a large pond. or lake, or even a sea, dried up? You can now see how geologists reconstruct events which happened very long ago in time.

The great forces which produced the Balcones Escarpment broke solid rock,shoved some land up, and caused other land parts to fall. If you will now look at the pool toward the upstream or dam portion, you will notice several very prominent and long cracks or faults. You probably have seen faults before on highway banks where the roadway cut into solid rock. Although the Balcones Fault is thought to be one very long fracture, the fact is that nature doesn't do things so neatly. Almost every exposed rock formation in our area shows these kinds of cracks, faults, and fissures to tell us that almost all rock formations near the Balcones Escarpment were under the same great force.

Mt. Bonnell

This point is at a divide. Upstream of the Colorado is the hill country which is the higher, or uplifted, section from the Balcones. Downstream, you will also notice that the land soon flattens out. Also, note the change in the tree cover and the species of trees. You are standing in hill country, and it is full of cedar, youpon, and flowering bushes. Look now at the flatlands and you will see fewer trees, almost none of which are cedar. This point is only a little over 750 feet above sea level, but the soil and the climate are sufficiently different from the flats to make a difference in rainfall and in the types of vegetation.

The rock layers all along Mr. Bonnell and vicinity are horizontal (coming up to Mt. Bonnell it seemed that the layers were slanted, but this is an illusion caused by the incline of the road). This means that this part of the Balcones is the uplifted part. The portion of the graben which dropped shows tilted bedding and lots of fractures.

How long do you think it took for the Colorado River to carve its gentle slope through this hilly area? Do you think there might have been a time when this meandering river might have formed a falls like Niagara Falls at this point? Later, might there have been foaming rapids as the Colorado went over the Balcones?

Highland Park Elementary School

This park was a rock quarry, and the lower field next to all of the exposed rock is a fill. This is proof that an old quarry need not be an ugly, water-filled hazard to the neighborhood.

Look first at the solitary rock exposure nearest the tennis courts. If you look closely at the angle, you will see that it slants. Look closely at the rock surface facing the school. You can see long striations caused by rock scraping against rock as this piece probably moved upward at a slant.

Looking downhill at the field and the rock wall beyond, you can see the slanting layers. This is the downdropped portion, or the graben, since the layers usually tilt at the edges of the graben.

Walk around to get a closer view of the outcrops; you will notice many fractures and faults along the field.

The actual location of the Balcones fault plain is not easy to spot since it is well covered, but it is approximately along a line parallel with Balcones Drive and the first row of houses across the street. Balcones Drive does follow the fault very closely in this area.

Go Back to the Guidebook or Go Back to Field Trip #1