Page 90 - 609550 Sun City_Text_ _proof_FinalTrim

Geology
ROCKS!
By Paul Swetland
If you love geology, Sun City Texas is
a great place to live. Sun City sits on
the limestones of the Edwards Plateau
which were deposited in a shallow inland
sea during the Cretaceous age about
100 million years ago. Fossil-bearing
limestones that contain marine organisms
such as snails, oysters and clams are
common. The Edwards Plateau area is
marked by aquifers, springs, sinkholes
and caves which developed as rainwater
seeped into the limestone. While most
everyone has heard of Inner Space Cavern
in Georgetown, you may not be aware that
well over 100 caves are found within Sun
City, and 24 of them are named. Some of
their names include Double Dog Hole, You
Dig It, Do Drop In and Venom.
Embedded within the local limestone is
Georgetown flint, a hard microcrystalline
form of quartz. This flint has been prized
by American Indians and earlier cultures
because of the thin, sharp edges that can
be produced from it. Indian artifacts,
including blades and arrowheads made
from Georgetown flint, can be found in
Sun City. The abundance of flint was one
of the reasons the Clovis people settled at
the Gault Site near Florence, Texas 12,000
years ago. Flint blades and spear points
aided in the hunting of mammoths and
other mammals. At the Waco Mammoth
National Monument, you can see the
skeletal remains of a herd of mammoths.
The Cretaceous period was the time of
the largest dinosaurs, as evidenced by
large dinosaur tracks preserved at many
locations in Texas, including Williamson
County. Two and a half hours north of
Georgetown, dinosaur tracks are especially
abundant along the Paluxy River near
Glen Rose at Dinosaur Valley State Park.
The Paluxysaurus, a 60 foot long, 20 ton
herbivore, was named for this area. It
has been designated the state dinosaur of
Texas. In addition to large dinosaurs, there
were also large predatory marine reptiles.
Mosasaurs, which reached lengths greater
88 | SUNRAYS JUNE 2017
Geology SIG members listen to a presentation at their monthly meeting. Occasionally
the group goes on field trips to various geological sites.
than 50 feet, patrolled the Cretaceous
seas of Texas. The vertebrae (segments
of spinal column) of these creatures can
be collected along the North Sulfur River
northeast of Dallas.
Just an hour west of Georgetown is the
edge of Llano Uplift, an area of granite
and metamorphic rocks more than 1.1
billion years old. Enchanted Rock is a pink outside the Texas Drive Fitness Center.
granite dome north of Fredericksburg,
which is the second largest granite dome
in the United States. The beautiful
pink granite of the Llano area has been
quarried for more than 100 years and was
used to build the State Capital in Austin
and a number of county courthouses. The
Llano Uplift is also home to a unique rock
called Llanite. Llanite is an igneous rock
with beautiful blue quartz phenocrysts
only found near Llano. It can be collected
in a road cut just north of town. Central
Texas is also home to a unique gemstone,
blue topaz, found only in Mason County.
Blue Texas topaz has been designated the
Texas state gemstone.
Petrified wood (wood turned into stone)
is probably the most abundant fossil in
Texas, and it can be found throughout
the state. Petrified wood occurs when
PHOTO BY SHERRY MCRAE
the organic materials are replaced with
minerals while retaining the original
structure of the tissue. The Texas state
stone is petrified palmwood. Many cities
in Texas have buildings and homes made
of petrified wood, including Glen Rose,
Fort Worth, Lubbock, Rockdale and
San Angelo. Sun City Texas has a large
petrified tree stump on the grounds just
With all the interesting geology, the
Geology Special Interest Group of the Sun
City Nature Club (founded in 1997) has a
lot to keep it busy. The group studies and
collects rocks, minerals and fossils of the
geologic formations of Central Texas. Field
trips are made to museums, caves, parks
and other areas of geologic interest. The
Geology Group has monthly programs on
the first Tuesday of every month at 11 a.m.
in the Activities Center Atrium. A wide
variety of topics are presented. Recent
talks have covered life in caves, volcanos,
glaciers, mercury mines of Slovenia,
Yellowstone National Park and Loch Ness.
Upcoming talks include Big Bend National
Park, the Colorado Plateau and the silver
mines of Bolivia. All Sun City residents
and guests are welcome to attend.
ONLINE: SCTEXAS.ORG