By Mark Wilson
Did you know that Blair Woods, our Travis Audubon urban nature preserve, is the site of an important historical spring? Coleman Springs, cited in Gunnar Brune’s 1981 Springs of Texas, served Frank and Fern Blair as a source of water. And like most habitats and natural features, the spring is heavily dependent on the geology of the area.
Dr. Pete Rose (Ph.D. in Geology, University of Texas), professional geologist for over 58 years (and father of our own Travis Audubon Board Member, Virginia Rose) visited Blair Woods last fall to further our understanding of the natural history of the preserve. Dr. Rose found that the northern part of the preserve has an underlying outcrop of Upper Colorado River terrace deposits, sand and gravel in nature, generally 10-15 ft thick. This terrace is roughly 1 million years old, remnants of river gravels deposited by the ancient Colorado River when it was at a higher level than today. Underneath the sand/gravel terrace is a clay/mudstone layer, 75 million years old, that does not allow water to move through easily. Importantly, the edge of the sand/gravel terrace, sloping to the south, comes out along a small gully in the preserve, the site of Coleman Springs. Unlike the well-known aquifer springs found in western Travis County, Coleman Springs is a Quaternary (gravel) spring. Surface water filters down through the topsoil into the gravel layer and is slowed or stopped by the underlying clay formation. The water then moves slowly down the slope until it finds a natural place to exit–a place like our Coleman Springs.
During an educational walk at the preserve this last February, Dr. Rose pointed out examples of the exposed river terrace deposits and the underlying clay formation. We examined an interesting ledge of a course-grained conglomerate, looking much like concrete to the untrained eye, just north of the old springhouse. We also measured the depth and level of the springhouse well. All those present came away with a deeper understanding of the geological features and workings of Coleman Springs and the nature of the Quaternary aquifer that makes it possible.
Dr. Rose has graciously donated an expert geological report on Blair Woods to Travis Audubon, along with maps and drawings. This report is available in digital format and can be requested from Travis Audubon. Our plans are to use this information in future educational exhibits at the preserve.
We expect to have more information on the historical importance of Coleman Springs later this year. Look for future blog pieces on what part our Blair Woods and Coleman Springs played in the lives of the historical peoples of Texas.