Geological
History
of
the
Region
About 140 million years ago, during the
Cretaceous
Period
most of Texas was covered by a shallow sea.
Calcium-rich
matter from decaying sea life accumulated
in large
quantities on the sea floor and gradually
turned into
limestone. Seashell and other marine life
fossils are very
common in South Texas.
Beds and nodules of sedimentary chert and
flint were formed
when the silica-rich skeletons of
microscopic
radiolarians and
diatoms were brought together and
concentrated
by sea
water. The organic material contained in
flint
is where flint
gets it's dark color from. Flint contains
approximately 99%
silica, while chert has 90% silica. Genuine
flint is not near
as common as chert.
About 65 million years ago, during the
Tertiary
Period,
igneous activity lifted the region out of
the sea, exposing the
limestone, and cracking it along the
Balcones
Fault Zone.
Rainwater absorbed carbon from the soil,
and
became a
weak carbonic acid. The acid in the water
would seep into
the cracks in the limestone, and stand
there
for periods of
time. The acid in the water dissolved the
limestone, and over
time, made the cracks wider and wider,
forming
the caverns
and canyons along the Balcones Escarpment.
Many canyons
were originally caverns, and many of the
large
boulders in
these canyons are fragments of the old
cavern
ceiling.
The Coastal Plains were created by rivers
and dranage
off the Edwards Plateau during the last 60
million years.
These ancient rivers, like present day
rivers,
flowed southeast
to the Gulf of Mexico, and washed and
tumbled
pebbles
eroded from the canyons. Rounded river
pebbles
of dolomite,
quartzite, flint and chert (known as
"Uvalde
gravels") are
found over most of the Coastal Plains.