Geology
Natural Bridge Caverns is the most extensive cavern within the San Antonio
area and one of the largest caverns within the state of 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.
CAVERN FORMATION
The cavern formed by an underground “river” moving slowly through
cracks and pores within the limestone. Rain Water 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.
