Geological Origins

Charles Woodruff, Jr.

A recounting of the origins of the area would be incomplete without recognizing how the geological substructure has determined land use. Three major geological events shaped the Austin area: the Cretaceous Seas, the formation of the Balcones Fault Zone, and subsequent erosion of the landscape by the Colorado River and its tributaries.

Marine deposits between 65 and 145 million years ago formed the limestone strata, which underlie Austin. At that time, a vast inland sea covered the interior of what is now North America. During the late stages of the Cretaceous epoch, volcanoes formed along a buried hinge (the subsided stumps of the Ouachita Mountains) that would later mark the edge of the Balcones Fault system. Pilot Knob, in southeast Austin, is the remains of one of these volcanoes.

Approximately 20 to 25 million years ago, rocks underlying what is now Central Texas were broken and displaced downward toward the Gulf of Mexico. This resulted in mountain building in Colorado and New Mexico to the west and sediment deposits along the Texas coast to the southeast. This belt of broken rock, which makes up the Balcones Fault Zone, extends from Del Rio, northeastward through San Antonio and Austin, and beyond. It has profound influences on today's landscapes and water resources. The limestone, uplifted to the west, has been sculpted into the Hill Country west of the fault line; to the east, softer chalks and claystones have been eroded to lower levels, forming the Blackland Prairies.

As described by geologist and former President of the University of Texas Peter T. Flawn, "the Balcones fault line is where the West really begins." The cotton economy of the Old South meets the cattle economy of the Old West at the Balcones Escarpment, where the Blackland Prairies abut the hardscrabble hills.

That immense crack in the earth's surface created the anomaly of older Cretaceous limestone hills in west Austin being higher than the younger Blackland Prairie at lower elevations in the east. Parallel fault lines are most evident east of the Mount Bonnell Fault. Both Shoal Creek and Waller Creek follow along the parallel faults. Joseph Jones in his Life on Waller Creek, A Palaver about History as Pure and Applied Education, notes how Waller Creek began as no more than a trickle along a stretch of Cretaceous limestone.

The whole Hancock area is underlain by this Austin Chalk, which is evident by the road-cut along 38th street, west of Red River, on the south end of the golf course. Waller Creek exposes the chalk bedrock in many places, as well as minor fault traces. The creek provides the north-south axis of the Hancock area; it is understandable how it defines so much of the land-use from the beginning of human habitation.