Geology – NBC

Natural Bridge Caverns is the most extensive cavern within the San Antonio area and one of the largest caverns within the state of Texas, making it one of the top attractions in Texas.

Background Geology

Visitors to the caverns walk through different layers of limestone, a sedimentary rock. Geologists theorize that 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 different limestone layers. Geologists give different names to the various layers, and visitors to Natural Bridge Caverns will find the Glen Rose and the Kainer (Edwards) layers. The Glen Rose, as the oldest rock layer, contains the lowermost chambers, while the Kainer forms the Natural Bridge.

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, one of the main sources of San Antonio fun.

Cavern Formation

The cavern formed by an underground "river" moving slowly through cracks and pores within the limestone. Rainwater seeping through cracks started dissolving the limestone. In time, the original narrow cracks or joints enlarged to form huge underground conduits or passages.

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 level. The water eventually moved to another level even deeper within the earth. As the water left the lower level, stresses within the rock led to many of the layers collapsing to form break-out domes. This final stage of collapse led to the creation of the passages our visitors now see.