The Mountain City Quadrangle is a U. S. Geological Survey topographic map whose boundaries are defined by lines of latitude and longitude. In the geologic profession it is customary to use topographic maps for adding geologic mapping. For this website the MCQ topographic map serves as a base map for adding geologic information in this way. The map information is accompanied by text and photos to describe the features on the map. This website is provided by Thomas Grimshaw, Ph.D., as a public service. It is the result of a hobby (no compensation) and is offered freely. At the same time, Dr. Grimshaw cannot be held liable for any actions that rely on the information provided.
PLEASE NOTE: This website is on hold because of conflicting priorities. Sorry for the inconvenience! Thomas Grimshaw.
The MCQ is in central Texas southwest of Austin. Like other USGS topographic maps, it is named for a geographic feature, in this case the community of Mountain City. The quad is west of Interstate 35. Buda is a short distance to the east, and Kyle is is near the southeast corner. The map area measures about 17.9″ wide by 22.8″ high. Its scale is 1:24,000 (1 inch equals 2000 feet), so it encompasses about 58 square miles. This MCQ geology website is accompanied by a companion website for the San Marcos North Quadrangle (SMNQ), which adjoins the MCQ to the south.
For more complete coverage of the geology of the area, the northern boundary of the MCQ is extended northward into the lower 1/3 of the quadrangle to the north, which is the Signal Hill Quadrangle (SHQ). Similarly, the SMNQ is extended southward to include the northern 1/3 of the San Marcos South Quadrangle (SMSQ).
(Image from GoogleEarth)
The topics covered on this website are indicated in the two rows below the title. They are introduced below, and links are included to their respective webpages.
Geologic Description
The geology of the MCQ encompasses many components and levels. At the highest level is the regional geologic setting, which includes the basement rocks deep in the earth — the Texas Craton and the Ouachita Foldbelt. At the next level are the local components that extend elsewhere in central Texas, such as the Balcones Fault Zone and the Cretaceous limestones, clays and related rocks that comprise the bedrock above the basement. Finally, there are the geologic features that are found specifically in the MCQ. Many of these are described in the “Salient Geologic Features’ webpage, shown below.
Geologic Map
The geology of the MCQ includes the description of the geologic characteristics and how they are distributed over the area encompassed. The geologic map complements the description – stratigraphy, structural geology, etc. – by depicting how these characteristics are expressed within the quadrangle. As noted above, the Mountain City topographic quadrangle serves as the base map on which the geological information is superimposed.
Salient Geologic Features
Several aspects of the geology of the MCQ warrant specific coverage in order to fully characterize the quadrangle. These topics include not only to unique geological characteristics of the area, but also features that contribute to understanding of the broader geologic picture of Central Texas. They include nearly all facets of the quadrangle‘s geology, particularly structural geology, geomorphology and hydrology.
Hydrology
The relationship between geology and water is extremely close in the MCQ, both for surface water and groundwater. For example, a large portion of the area is a recharge zone for the Edwards aquifer. Most of the recharge water comes out at Barton Springs in Austin. Both of the major streams in the MCQ have had major floods historically and more recently: Onion Creek In 2013 and 2015 and the Blanco River in 2015. The Balcones Escarpment as a whole is one of the most floodprone areas of the U.S.
Environmental Geology
Geology and human use of the land are closely related in other ways besides hydrology in the MCQ, both for the impact of the geology on people and vice versa. One example is the potential impact of urban runoff and septic tanks – resulting from rapid urbanization – on the quality of groundwater and surface water. Another example is the geologic hazard of unstable clay units, particularly the Del Rio clay, for foundations of homes in certain areas. The goal of environmental geology is to optimize land uses with the capability of its geologic characteristics to sustain those uses.
Methods
The primary methods used for geologic mapping of the MCQ were to utilize aerial photographs and intensive field checks to verify map units. The air photos were used as stereo pairs to enhance their utility for identifying stratigraphic units and faults of the Balcones Fault Zone. The photos were taken in 1958 and are at a scale of about [TBD]. The photo images were downloaded from the USGS Earth Explorer website, where they are free to the public.
Selected References
The geology of Central Texas has been extensively studied in the past. This previous work serves as a foundation for the more detailed and, hopefully, more precise geologic characterization of the MCQ presented in this website. The objective here is to acknowledge past studies with a list and brief summary and, where possible, a link to the original source.
Future Opportunities
Most of the geologic mapping for this study has been accomplished on air photos and has been transferred to the topographic base map. The main emphasis now is to process and interpret the geologic information as it is added to this website. This work will continue indefinitely. The primary future opportunities will likely be the result of observations and insights that emerge as the analysis and interpretation are accomplished. It is also possible that more quadrangles – in addition to the companion San Marcos North companion website –may be added in the Austin–to–San-Antonio growth corridor.
Dissertation: Environmental Geology of Urban and Urbanizing Areas: A Case Study from the San Marcos Area, Texas
Dr. Grimshaw’s interest in the MCQ and SMNQ goes back to the early 1970’s, when he studied the area for his Ph.D. dissertation. When he returned to UT after completing his M.A. in Geology and a tour of duty in the U.S. Army, the environmental movement was rapidly accelerating tin the U.S. and worldwide. The 1970s are often referred to as the “decade of the environment” because many of the major new laws for environmental protection were enacted, including the National Environmental Policy Act (enacted in 1969 and implemented on January 1, 1970) and the clean air act, clean water act, solid waste act, and hazardous waste act.
Dr. Grimshaw chose a career in environmental geology, which he began with Dr. Keith Young as his supervisor. During this timeframe Dr. Young prepared his book “Geology: The Paradox of Earth and Man”. Dr. Grimshaw’s dissertation utilized the San Marcos area as a case study in the rapidly-growing Austin-to-San-Antonio growth corridor. The dissertation has two major components – detailed geologic mapping followed by an environmental analysis of land capability to support human uses of the land.
About
As noted above, Dr. Grimshaw received his M.A. and Ph.D. degrees in Geology at The University of Texas at Austin. For both degrees, he did geologic mapping in the Texas Hill Country south and west of Austin. After graduation, while engaged in his career in environmental geology and energy policy, he undertook as a hobby more detailed geologic mapping in the Mountain City and San Marcos North quadrangles. His main emphasis is on improving the quality of previous mapping using better air photographs and deeper understanding of the structural style of the Balcones Fault Zone. He also has a career-long interest in applying geologic knowledge to optimizing the relationship between humans and their earth environment.
Progress Documentation Memos
Progress has been made on characterizing the geology of the MCQ as time permits in the context of many other projects and activities. As incremental progress has been made, especially for field visits, memos were prepared for documentation. The memos have been classified by the nature of their content, including general geology, stratigraphy, structural geology, geomorphology and related categories.
Tracking Progress
As noted above, this website will be a work in progress for the foreseeable future. As additions are made, they will be noted, usually with a link to the webpage locations where they occur.
Acknowledgments
Brian Hunt, formerly a geologist at the Barton Springs Edwards Aquifer Conservation District, has been literally a partner in the geologic understanding and mapping of the MCQ. I have greatly enjoyed both his geologic and hydrologic insights and the pleasure of many field trips with him.
Dr. Mark Helper, faculty member of the Jackson School of Geosciences at The University of Texas at Austin, was also very helpful in the early stages, when he led an effort to digitize my previous geologic mapping in the area. He also provided leadership in an attempt to utilize LIDAR in the geologic mapping.