CLASSROOM EARTH Ripple marks in pool at northeastern end of Canyon Lake Gorge (January 2019)
Field Trips to Canyon Lake Gorge, Texas Heidi Harwick, SA-9084
Going on field trips is one of the things I truly enjoy about studying geology. Studying geologic features hands-on in the field, compared to looking at pictures of them or reading about them, provides a much deeper learning experience. Field trips are often a required part of geology courses: compared to some requirements, they are something I look forward to. Seeing something in person also sometimes results in that “aha!” moment that helps to cement a concept.
Canyon Lake is on the Guadalupe River, northeast of San Antonio in Comal County, Texas. Canyon Lake Gorge is downstream from the spillway on the eastern side of the lake, adjacent to and south of the dam. In 2002, there was a large flood event, and water flowed over the Canyon Lake Spillway. The water carved out the existing gorge and revealed exciting information about its stratigraphy, formations, and fossils.
I first went to Canyon Lake Gorge for a structural geology field trip in April 2016. We mapped and created a cross-section of the fault zone and examined structural and fault-related features. We mapped an area about 130 feet wide and 1,800 feet long, noting features and faults based on aerial photog- raphy and field observations. The gorge has many pools and depressions, with waterfalls and high, vertical banks, with approximately horizontal bedding. It exposes the Hidden Valley Fault Zone, which consists of several normal faults
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that strike to the northeast with a dip to the south between 55 and 80 degrees. We examined slickensides and mineral traces, and measured the trend and plunge of slickenlines to plot on stereonets. Bedding in the hanging wall of fault strands has been strongly rotated to the north, sometimes past vertical. Most exposed fault surfaces were relatively small, except for the fault scarp near the spillway that was taller than a person (8-12 feet high). Based on fault plane and lineation measure- ments, we calculated the principal stresses.
I had the opportunity to return to Canyon Lake Gorge during an AIPG field trip in January 2019. I looked forward to going again “just for fun” to see the gorge without having to concern myself with school obligations or taking measurements. This was part of a larger trip led by AIPG and Dr. Tom Ewing to learn more about the area’s stratigraphy (which includes the Glen Rose Formation and the Edwards Group), as well as springs and recharge features of the Edwards aquifer. We began our tour at Overlook Park, which is between the dam and the spillway and provides a spectacular view of the lake and surrounding area. We walked down to the spillway and saw the dinosaur tracks, then continued down through the gorge. I enjoyed seeing the dinosaur tracks, which I had not had the opportunity to do on my first visit, and learning more about the gorge’s lithology, fossils, and structures. The area is
www.aipg.org
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