INKS LAKE STATE PARK – PreCambrian Granite and Gneiss
FIELD TRIP STOPS – VARIOUS EXPOSURES OF METAMORPHIC and IGNEOUS ROCKS OF THE LLANO UPLIFT SEEN IN INKS LAKE STATE PARK
LOCATION: Just south of Route 29 in Burnet, Texas. Enter Park Road 6. Exposures are found throughout the Park. Devil’s Waterhole found near the beginning of the park entrance is of particular note.
GEOLOGIC FEATURES: Mainly Gneiss and Granite; Foliation; Pegmatite Dikes; Folding; Ductile Flow
DESCRIPTION: Rocks of the park are mainly 1.1 byo pink granites, and 1.3 byo fine to medium grained quartz-feldspar gneiss with some amphibolite and quartzite. They form part of the Llano Uplift of Central Texas and represent the deep core of a mountain whose roots have been exposed by weathering and erosion. The gneiss is named the Valley Springs Gneiss. Granites cut through the gneiss and are part of the Town Mountain Granite. Nearby, lower Paleozoic sedimentary strata unconformably overly the PreCambrian crystalline rocks.
(1) What is metamorphism?
(2) What is a Protolith and how does it relate to metamorphic rocks?
(3) By what specific method might the rocks of Inks Lake Park have been dated? Explain the principle behind this dating method.
(4) Why are the granites younger than the gneisses at Inks Lake?
(5) What is foliation and how does it occur?
(6) What can you interpret if you see foliation that is itself folded?
(7) What are the characteristics of a Pegmatite?
(8) CHALLENGE: Why is the determination of the Protolith of extreme importance to geologist and other students of earth history?
-Spearing, D. 1991. Roadside Geology of Texas. Mountain Press Publishing Company, 418 pp.
Figure 1 - PreCambrian Metamorphic and Igneous rocks of Inks Lake State Park. Note the large intrusion of pink Town Mountain Granite into the somewhat darker metamorphic rocks of the Valley Springs Gneiss.
Figure 2 - Notice the pink granite intrusion into the gneiss. Within the gneiss note the foliation including thin layers of quartz.
Figure 3 - The availability of outcrops as well as the varied rock types and crystal sizes make Inks Lake a great place to explore.
Figure 4 - A high-grade gneiss exhibiting foliation and ductile flow.
Figure 5 - Intense metamorphism showing well-developed foliation and ductile flow leading to folding of layers within the gneiss.