In October of 2015, Becky (my wife) and I took a vacation in the “Hill Country” of Texas.  While there, we explored a spectacular cave called the “Natural Bridge Caverns”. 

     The Natural Bridge Caverns are the largest known commercial caverns in the U.S. state of Texas. The name is derived from the 20 m natural limestone slab bridge that spans the amphitheater setting of the cavern’s entrance. The span was left suspended when a sinkhole collapsed below it.

     While Becky and I walked into the cave we crossed different layers of limestone.  During the Cretaceous period, a warm, shallow sea covered much of Texas. Sediments and dead marine organisms collected on the ocean floor, compacted and formed the these different limestone layers.

     Perhaps around 20 million years ago, a number of faults formed in Texas due to settling of the costal regions. These movements created an extensive series of faults known as the Balcones Fault Zone.

     The eroded face of the Balcones Escarpment marks both the fault zone and the beginning of the Texas Hill Country. In addition to creating the faults, the tectonic stresses also created joints, or cracks in the rock. Underground water moving along the joints eventually carved the passages at Natural Bridge Caverns, creating the most amazing cave formations in the United States.

     The Natural Bridge cavern formed by an underground “river” moving slowly through cracks and pores within the limestone. Rainwater seeping through cracks started dissolving the limestone and redepositing the calcium carbonates in these spectacular formations. In time, the original narrow cracks or joints enlarged to form huge underground conduits or passages within the limestone.

     Perhaps due to changes in climate, vegetation, or other natural forces, the water drained to lower levels within the earth. As the water left the upper passages, it moved deeper and started forming a second and third level.  As the water left these levels, stresses within the rock led to many of the layers collapsing to form break-out domes.  These are the rooms that Becky and I see and walk through as we explore the “Natural Bridge Cavern” within the “Texas Hill Country”, USA.