Spanish explorer Bernardo de Miranda y Flores struck out across the Texas Hill Country in 1756, writing of the “hills and rocks…and thickets of cedar and oak” as he crossed what he called “los Balcones.” The name (pronounced băl-COH-nees) was a reference to the dramatic stair-step terrain of the region that resembled a series of rising balconies.

Today, the Balcones Canyonlands remain a unique classroom for the study of the history and structure of the earth known as geology. Geology is essential to learning about the ecosystem of the Bull Creek Nature Preserve. Geological features, both above and below ground, significantly affect the habitat of species. For instance, prime golden-cheeked warbler habitat can often be found in steep-sided canyons dotted with natural springs or seeps. In addition, six of the eight endangered species protected by the conservation plan are small and highly specialized karst invertebrates adapted to living underground in caves.

The term “karst” is of German origin and describes a type of landscape characterized by the dissolution of limestone forming caves, sinkholes, springs, and underground streams.  The limestone in our area, including the rock upon which one hikes in the Bull Creek Nature Preserve, was formed during the Cretaceous Period over millions of years when the area was covered by advancing and retreating shallow seas. The natural steps that can be seen along the trails at Bull Creek are formed by layers of solid limestone and marl. Marl is a softer, clay-containing limestone that crumbles and erodes easily, collapsing underneath the solid layers that lose support and fall, creating a stair. On a larger scale, this process is responsible for the stair-step terrain of the Hill Country. 

Another geological feature to look for on the Bull Creek Nature Preserve is groundwater seeping from the layers of limestone. These seeps can be seen and heard coming from cliff faces as well as along the trail. The ridge on the Preserve is capped with Edwards Limestone—the same type of limestone found within the Edwards Aquifer—which absorbs water during rainstorms. Limestone is naturally full of small pores and conduits and has an amazing storage capacity for water. Once absorbed at the top of the ridge, water moves down through the hillside and is slowly released out of small cracks and fissures. The slow release of water allows the plants and animals of the Preserve to take full advantage of this vital resource. In addition, the gradual infiltration and discharge helps prevent rapid stormwater runoff and potential flooding of surrounding streams and creeks.

For more information on the geology of the region, check out the Guidebook to the Geology of Travis County.