The HAYS HUMM - October 2018 - Online Edition
Tom Jones & Betsy Cross
the devil’s backbone
Article & Photos by Tom Jones
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The Devil’s Backbone is one of the most scenic drives in Comal and Hays Counties. It is a five-mile stretch along Ranch Road 32 just inside the Comal Co. line. The road is on top of a narrow ridge which has an elevation of 1,225 feet and offers unparalleled views of rolling ranch land. I can remember the first time I visited the Devil’s Backbone. It was in my junior year of college at UT working on a Geology degree. One requirement was a multi-week field trip introducing us to the geology of Central Texas. Later, I returned to the Devil’s Backbone to accompany my son to El Rancho Cima Boy Scout camp on the Blanco River. I will never forget the thrill of backpacking with him up to the summit of Sentinel Peak, camping overnight and gazing at the incredible stars.
The Devil's Backbone was created by erosion of the Edwards Plateau as the adjacent Guadalupe and Blanco Rivers became entrenched forming their respective watersheds. One feature of watershed anatomy is known as a watershed divide. However it is more commonly referred to as a ridgeline, which separates neighboring watersheds. On rugged land such as the Devil's Backbone, the divide lies along an elevated ridge. The ridgeline is narrow allowing great views on either side of RR 32. The topographic map graphic illustrates the location of the watershed divide in relation of RR 32 and its impact on drainage patterns near the Devil's Backbone. Rainfall runoff on the Devil’s Backbone feeds into the Blanco or Guadalupe Rivers, depending upon which side of the watershed divide it falls on.
Looking out from the Devil's Backbone you can see deeply carved valleys and rounded hills, which are much-loved features in the Texas Hill Country. What caused this narrow strip of land to stand tall while each side is eroded away? Typically, ridgelines are composed of rock that is harder and better cemented allowing it to withstand the erosive forces of water and wind. Also ridgelines may have fewer fractures or cracks which can create weakness within the rock layers.
Although the Devil's Backbone is in Comal Co., the best views are of the Blanco River valley to the north looking into Hays Co. Illustrated on the geologic map are many black lines marking the surface locations of faults, which are collectively part of the Balcones Fault System. Faults are simply fractures in the rock accompanied by movement of the limestone formations relative each side. The faults were created during the the uplift of the Edwards Plateau.
The deep Blanco River valley is easily seen from the top of the Devil's Backbone, as it is flows southeast in close proximity and parallel to the Devil's Backbone. Take a look at the geologic map and note that the direction of the Blanco River suddenly makes a sharp left turn, changing its course to the northeast away from the ridge toward the City of Wimberley. The abrupt change in the flow direction is caused by the river intersecting a major fault line locally referred to as the Wimberley Fault. The river path then closely follows the fault to downtown Wimberley. Cypress Creek at the Blue Hole is also aligned along this same fault.
Faults have a big regional influence on ground water by creating pathways for rainfall to enter formations at the surface and recharge the deeper Trinity aquifers. A little closer to San Marcos this fault system has the same impact on the Edwards aquifer. In fact, roadside signs along the Devil’s Backbone provide notification when you are entering the Edwards Aquifer recharge zone. When you see one, you can be sure that the Balcones Fault System is nearby. The other local impact is that the faults allow deep underground aquifers a pathway for water to flow to the surface, creating springs. Great examples include Spring Lake, Barton Springs and the springs along the Comal River. I also believe that the springs at Blue Hole are related to groundwater movement up the Wimberley Fault.
The Devils’s Backbone lives up to it name based upon stories and lore reported about this area.
Like the Comanche and Kiowa before us, we also use the view from the Devil's Backbone to spot and monitor the progress of development from an increasing population moving into this beautiful hill country landscape. Living in Wimberley I cross the Devil's Backbone frequently. And every time I catch a glimpse of Sentinel Peak, the wonderful memory of camping with my son at El Rancho Cima returns.