Geologic Fault Zone

The Bryker Woods Fault
by Dr. Leon E. Long

Fault exposed along Shoal Creek
Fault line exposed along Shoal Creek

Did you know that a geologic fault zone passes directly through the Bryker Woods neighborhood? A fault is a fracture marked by displacement of the rock on the two sides. Rock does not break easily, and as the responsible force continues to intensify’, the rock will ‘lock up” for a while, even if a fracture already had been established in that sport. Finally, an overwhelming buildup of force will rupture the fault again, temporarily relieving the stress. Each time this happens the rocks on either side of the fault are offset more, and there is an earthquake.

Fortunately, Austin is no longer Earthquake Country. All that excitement was millions of years ago, and today about all that happens is slow modification of the landscape by erosion.

The Bryker Woods fault is associated with a much larger fault that runs along the base of Mt. Bonnell on its east side, crossing Mt. Bonnell Drive under the bridge near the water treatment plant, and continuing across Lake Austin.

You can see the Bryker Woods fault by visiting the green belt west of St. Andrews School, where Shoal Creek is forced to jog to the east to get past a limestone cliff. As you stand on the deposit of coarse gravel, gaze up the hill past Shoal Creek, aligning your eyes parallel to the trend of the cliff. The cliff marks the plane of the fault, separating the hard rock on the left-hand side from the dark-colored, soft sediment that comprises the gentle slope on the right-hand side. A much thinner cliff made of the same limestone rests upon the soft sediment near the top of the hill. The base of the cliff up the hill on the right-hand side is equivalent to limestone at creek level on the left-hand side. Rock on the right-hand side was displaced upward about forty feet relative to the same stratum on the left-hand side.

Dr. Leon F. Long. Professor of Geological Sciences, UT.