Overview
Watch the virtual field trip then answer the questions
Stop 1: Observations for the Edwards Formation
Head to Stop 1 of the virtual field trip, watch the video at Bee Cave Road, and view the GigaPan image.
Video Questions for Stop 1
1. Describe the Edwards limestone. Be sure to include information such as minerals present, grain size, types of fossils, color, weathering, etc.
2. Based on your description, what was the depositional environment for the Edwards limestone? (see Figure 7.25, p. 222-223). Include evidence to support your interpretation.
3. There is a small normal fault at this outcrop that is part of the Balcones fault system. What is the dip (angle relative to the ground) of this fault?
4. The Edwards aquifer is an underground layer of porous rock with water and supplies water to over 2 million people. This stop is in the recharge zone of the Edwards aquifer.
a. In your own words, describe what a recharge zone is.
b. What are the two main ways that water gets into the aquifer?
c. Do you see any evidence in this outcrop for how water gets into the recharge zone?
GigaPan Questions for Stop 1
5. Zoom in to look at the sedimentary beds.
a. Describe the texture of the rocks.
b. Include a screenshot that highlights your description. Click the box to attach your image.
c. On your screenshot from part b, trace three contacts between sedimentary beds.
d. Using your observations of these contacts, how thick are the individual limestone beds? (Hint: trees/shrubs are 5-10 ft tall).
e. Does the thickness of individual beds remain constant across the outcrop? You may want to zoom out and relook at the entire GigaPan image.
6. Search the GigaPan image for any evidence that chemical weathering affected this outcrop.
a. Has chemical weathering affected this outcrop?
b. Include a screenshot with annotations that supports your answer.
Stop 2: Observations for Slaughter Gap
This field trip stop has two different rock types: an igneous rock that students are standing on and a sedimentary rock behind them.
Video Questions for Stop 2
7. Listen closely to the discussion of the igneous rock on which the students are standing.
a. Using full sentences, describe the igneous rock. Include information such as color,
minerals present, texture, grain size, and the name of the rock.
b. Is this rock ultramafic, mafic, intermediate, felsic? Select one.
c. Interpret whether this igneous rock is intrusive or extrusive and its cooling rate.
8. Listen closely to the discussion of the sedimentary rocks behind the students.
a. Using full sentences, describe the sedimentary rock. Include information such as the type
of sedimentary rock, grain size, grain shape, minerals present, color, bedding thickness, and any other features.
b. There are no fossils at this location, but elsewhere there are reports of trilobite trackways for this rock. What do you think was the depositional environment for this sedimentary rock?
9. What type of unconformity exists between the rock students are standing on and the rock behind them? Explain your answer.
GigaPan Questions for Stop 2
The first image was taken at the location where the students were standing, shown at the red arrow's tip on the topographic map of Slaughter Gap (Figure 1). The second image is the view across the gap from the first location, as shown by the red line on the topographic map. The base of the second GigaPan image is parallel to the red line shown below.
Figure 1 - Topographic map of the Slaughter Gap region provided by Dr. Leon Long, University of Texas at Austin
10. On the first GigaPan image, locate the contact between the igneous rock and the sedimentary rock. You may have to zoom in and move around the image to look at the rock textures to help.
a. Is the contact flat or steep? Select one.
b. Take a snapshot and draw a line that shows the contact between these two rocks.
11. Locate this same contact on the second GigaPan image from this stop. You may want to zoom in to see the rock types within the trees and look for layered rock above the break in slope. Be careful because some of these are large boulders that have fallen down the cliff. Take a snapshot and draw a line that shows the contact between the two rock types.
12. Figure 2 is a topographic profile from one side of Slaughter Gap to the other. Students were standing on the right side of this image. Use your interpretation of the two GigaPan photos to draw the unconformity on the topographic profile in Figure 2. You can make this annotation directly in this PDF by going to tools > comments and then select the drawing tools to create a line or shape.
Figure 2 - Topographic profile of Slaughter Gap from Google Earth.
Stop 3: Observations for Inks Lake State Park
This stop has two different rock types as well, metamorphic and igneous. The students are standing on one type, and the other is across the stream in the GigaPan images.
Video Questions for Stop 3
13. Describe the metamorphic rock that students are standing on. Include observations on its color, grain size, minerals, and texture (terms such as granoblastic, foliated, or layered).
14. Based on the minerals present, what is the metamorphic grade?
15. Deformation has affected this outcrop. Was the deformation brittle or ductile, and did deformation occur before, during, or after the metamorphism?
16. Describe the cross-cutting relationships between the igneous and metamorphic rocks and which rock came first.
GigaPan Questions for Stop 3
17. In the first image, locate a dike, take a snapshot, and then draw a polygon around it. Attach your snapshot with the annotation.
18. In the first image, locate a sill, take a snapshot, and then draw a polygon around it. Attach your snapshot with the annotation.
19. On your snapshots from 17 and 18, use the pen tool to highlight the foliation in the metamorphic rock.
20. In the second GigaPan image, locate a fold, take a snapshot, and draw a box around it. Attach your snapshot with the annotation.
21. When describing metamorphic rocks where geologists cannot tell which layer is older or younger, they use the terms antiform and synform instead of anticline and syncline.
a. Is your snapshot from Question 20 an antiform or synform?
b. Is your fold symmetric, asymmetric, or overturned?
Stop 4: Observations for Enchanted Rock
Enchanted Rock State Natural Area has two exfoliation domes formed by erosion of the Grenville age Town Mountain granite (about 1.1 billion years old), similar to what you heard about at Slaughter Gap. See this video for an explanation of exfoliation domes and watch exfoliation in action.
The Town Mountain Granite intruded into the Packsaddle Schist (the schist is not visible in the GigaPan image). The two granite domes are part of the second-largest batholith in the Llano Uplift. The tallest dome is 425 ft above ground level.
There are no video questions from our Enchanted Rock stop, only observations made from images.
GigaPan Questions for Stop 4
22. Locate the first GigaPan image for this stop.
a. Draw a polygon around the granite block on the dome to show the exfoliation process.
Attach your snapshot with the annotation.
b. Give an estimate of their size.
c. Do you think these blocks are hazardous to anyone who walks near them?
d. Which of the following do you think is most likely to happen? Will the granite blocks 1) undergo further mechanical weathering, break into smaller pieces, slide down the side of the dome, and be added to the granite rubble pile at the bottom or 2) will the granite blocks be chemically eroded by rain? Be sure to explain your answer.
23. Locate the second GigaPan image for this stop.
a. Use a pen tool to trace the path of a black stripe running down the slope of the dome.
Attach your snapshot with the annotation.
b. What type of weathering causes these stripes?
24. The Grenville orogeny represents an ancient convergent boundary. From the videos, you should decide if this collision involved ocean-ocean, ocean-continent, or continent-continent
convergence. Sketch a cross-section for this convergent tectonic setting that represents the Inks Lake and Enchanted Rock stops. You can use these figures as a reference. Your sketch must be hand-drawn and labeled, showing where the Enchanted Rock batholith and Valley Springs gneiss would have been located during convergence. Label geologic features such as oceanic and/or continental crust, lithosphere-asthenosphere boundary, volcanoes, mountains, and trench. Take a picture of your sketch and attach it below.
Geochronology: Putting all the stops together
25. The ages for all of the geologic features are in the videos. Fill out this summary table with the ages of the geologic features.
Feature
Edwards Formation – Stop 1
Balcones Fault – Stop 1
Town Mountain Granite – Stops 2 and 4 Hickory member – Stop 4
Valley Springs Gneiss – Stop 3
Igneous intrusions – Stop 3
Age (millions of years)
26. How much time passed from the oldest rocks seen in this field trip to the youngest fault activity seen in the first outcrop?
27. How much time elapsed between the deposition of the Edwards Formation and movement on the Balcones Fault?
28. How much time elapsed between the two rock types at Inks Lake (Stop 3)? Was convergence active for the entire time?