Geology of the Sharp Mountain area, Llano County, Texas



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The area immediately southeast of the town of Llano is underlain by metamorphic rocks, diorite, and several types of granite, all of Precambrian age. Marble is quarried for roofing and flooring chips, and a little graphite schist is worked to produce water filter granules. The Valley Spring Gneiss is exposed on the flanks of the recumbent Llano syncline and in the core of an associated small anticline. The folds plunge gently to the southeast and their limbs dip to the southwest. Overlying the Valley Spring Gneiss are the varied schists and marble layers of the Packsaddle Formation. Foliation in the metamorphic rocks is generally parallel to bedding (compositional layering), and the most prominent lineation is parallel to fold axes. Regional metamorphism attained the almandine amphibolite facies. During the orogenic episode in which the rocks were folded and metamorphosed they were intruded by diorite (Watson Diorite) which underwent considerable metamorphism and later by a series of granites (Town Mountain Granite, Oatman Creek Granite, and Crownover Granite) which are relatively unmetamorphosed. Watson Diorite and Crownover Granite are new informal names proposed here for local use in this area