LONGHORN CAVERN, TX

 

FIELD TRIP STOPS – A FLOW CAVE

 

LOCATION: 9 miles southwest of Burnet, TX on Park Road 4 in southwest Burnet Country

 

GEOLOGIC FEATURES: Flow Cave; Stalactites; Stalagmites; Pillars or Columns.

 

DESCRIPTION:. Deposition of Cambrian through Cretaceous sediments occurred on the eroded PreCambrian metamorphic and igneous rock of the Texas Hill Country. Faulting occurred in the area about 280-300 mya during the Llano Uplift. This is thought to have created a fault-block which down-dropped Paleozoic sediments into the PreCambian basement rock preserving them from future erosion. Uplift in the Upper Cretaceous and subsequent erosion removed Mesozoic and Paleozoic sediments covering much of the the PreCambrian crystalline rock except for the Cambrian and Ordovician sediments preserved within the down-dropped fault block. It is in this block that Longhorn Cavern is found within the 500 million year old Ellenburger Limestone in what is now called Backbone Ridge.

      Subsequently, over the past few million years, the cavern’s passages were created by acidic subsurface streams. Longhorn Cavern is unique in that it is a Flow Cave formed by underground flowing rivers of water rather than a cave formed by water simply sinking down through the earth. Besides solution, the underground streams carried abrasive sediment which sculpted the cavern’s many smooth walls. Subsequently, slowly dripping underground waters formed the existing cave deposits such as stalactites and flowstones composed of travertine. Today, water does not run through the cavern except during periods of heavy rains.

      Historically, Longhorn Cavern is of further interest in that it served as an Indian camp (Comanche Indian Council Room), a hiding place for outlaws (Sam Bass), a 1920s Speakeasy Nightclub, and an area for Confederate Troops to manufacture gunpowder,

 

STUDENT QUESTIONS:

(1) Explain two factors directly responsible for the carving of the Cavern walls.

(2) What is the Llano Uplift?

(3) Describe the process by which a Stalactite and Stalagmite can combine to form a Pillar (Column).

(4) CHALLENGE: How do subsurface waters become acidic enough to dissolve limestone?

 

SELECTED  REFERENCES:

-Matthews III, William H. 2010. The Geologic Story of Longhorn Cavern. Bureau of Economic Geology Guidebook 4, The University of Texas at Austin. 46 pp.

-McClelland, Eileen. 2008. Longhorn Caverns. Chron.com. 1/12/2008. Retrieved on May 23, 2020: https://www.chron.com/life/travel/article/Longhorn-Cavern-1760335.php.

-Smyrl, Vivian Elizabeth. Longhorn Cavern. The Handbook of Texas Online/Texas State Historical Association. Retrieved on May 23, 2020: http://tshaonline.org/handbook/online/articles/rql01

 

 

PHOTOS:

Figure 1 - Flowstone hangs from the cavern ceiling. Much of the cave, though, is composed of smooth "onyx-like" walls sculpted bythe flow of underground streams carrying abrasive sediment.

Figure 2 - "Icicles" (Stalactites) hanging from smooth walls of the cavern.  (Photo by Aidan Clark)

Figure 3 - Here, Stalactites forming from above join Stalagmites growing up from below to form a Pillar (Column). (Photo by Aidan Clark)

Figure 4 - In places, large crystals of calcite preciptated from water that completely filled the cave,

Figure 5 - Crystals of calcite, several inches across are found in the cave having precipitated from waters saturated in lime (Calcium Carbonate) filling the cave. (Photo by Grayson McCarley.)

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