TOWN MOUNTAIN GRANITE DOME
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FIELD TRIP STOP – QUARRY OF THE TOWN MOUNTAIN GRANITE IN MARBLE FALLS, TEXAS
LOCATION: Enter the town of Marble Falls, TX. Travel west on Route 1431 a short distance to a roadside rest located directly across from the Texas Granite Corporation.
GEOLOGIC FEATURES: Granite Batholith; Igneous; Sheeting; Exfoliation Dome; Phaneritic Texture; Sole markings; Intrusive
DESCRIPTION: Across from the roadside rest is the quarry of the Texas Granite Corporation. Picnic tables at the rest are made of polished slabs of granite quarried across the street. This granite is commercially sold under the names “Texas Pink” or “Sunset Red.” This stone is used worldwide as a building stone in such places as Iceland, Australia, Thailand, New York City, Los Angeles, and Chicago. In Texas it composes the state capitol building on Austin and the jetties along the Gulf Coast. The age of this granite is about 1.1 byo. It is exposed at the surface as a result of uplift and erosion occurring during the Carboniferous and Cretaceous Periods. The rock at Granite Mountain is a thought to be part of the huge batholith that is also responsible for the granite at Enchanted Rock. At the surface it forms a significant Exfoliation Dome exhibiting Sheeting. Crystals in this intrusive igneous rock are large and composed mainly of quartz, orthoclase and plagioclase feldspar.
Also found at this roadside stop is the underside of a slab of the Pennsylvanian Smithwick Sandstone that exhibits sole-markings. These are sediments that once-filled depressions scoured out of stratigraphically lower sediments by the action of currents running down a relatively steep sea-bottom.
STUDENT QUESTIONS:
(1) What is the strict geological definition of a granite?
(2) Why does sheeting occur in many exposures of granite?
(3) How does the word “Exfoliation” relate to the word “Sheeting.”
(4) Why do large crystals suggest cooling deep within the Earth?
(5) What is a Batholith? How does it compare to the term Pluton.
(6) CHALLENGE: How are rough slabs of granite converted to the smooth, polished surfaces of countertops, building stones, flooring, etc.?
(7) CHALLENGE: Commercial stones with interlocking crystals are usually classified as Granite by commercial businesses though these rocks might actually be other types of igneous rock such as Basalt, Gabbro, Diorite, or even metamorphic rocks such as gneiss. Why might this be so?
SELECTED REFERENCES:
-Spearing, D. 1991. Roadside Geology of Texas. Mountain Press Publishing Company, 418 pp.
PHOTOS:
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Figure 1 - 1.1 billion year old Town Mountain Granite exfoliation dome exhibiting sheeting. This quarry has been active since the late 1800s,
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Figure 2 - Just behind the roadside rest are found pieces of the granite that include pegmatitic clusters of large quartz and orthoclase feldspar crystals.
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Figure 3 - Sole markings in a slab of the Pennsylvanian Smithwick Sandstone which was moved to a roadside rest across the street from the granite quarry.