Detailed field and petrographic study of the Precambrian Rough Ridge Formation of the Packsaddle Schist documents a more complex tectonic history for the Llano Uplift than has previously been reported. The Rough Ridge Formation was affected by two metamorphic events and four phases of deformation. Mineral assemblages in pelitic and mafic lithologies are characteristic of the amphibolite facies, and the presence of cordierite in preference to almandine indicate low pressure during part of the metamorphism. Inclusions of staurolite in a Mn-rich garnet, coupled with the absence of staurolite elsewhere in the rock suggest that pressure may have ranged from medium to low during metamorphism. Textures indicate that post-tectonic recrystallization occurred as a consequence of either slow cooling or of reheating after dynamothermal events. The first phase of deformation (D1) was a complete transposition of original sedimentary layers, with a pervasive foliation (S1) forming parallel to the axial planes of isoclinal folds. D2 is characterized by small folds and a crenulation cleavage S2. D3 formed a crenulation cleavage S3. D4 formed the prominent folds in the area, with a pervasive axial planar cleavage S4. Isolated occurrences of a pre-S1 metamorphic foliation are evidence of still earlier deformation. In one pelitic unit, oriented inclusions show that garnet growth was post-D2, while cordierite growth was post-D4. Growth and recrystallization of micas, quartz and feldspar span all the deformations. The timing of deformation events is bracketed by the pre- to syn-tectonic intrusion of the 1167 +/- 15 m.y. old Red Mountain Gneiss, and the post-tectonic intrusion of a 1080 +/- 15 m.y. old melarhyolite dike. The results of this study demonstrate that rocks of the Llano Uplift have undergone a complex tectonic and metamorphic history similar to that seen in other Grenville Age rocks of North America. / text

The purpose of the author is to present a geological survey of the Backbone Ridge area in Burnet and Llano counties, Texas. During the summer of 1947 while enrolled in a geologic field course in McCullough County, it was noted that a new classification of the Cambrian and Ordovician formations In central Texas had been presented In the literature. It was noted at the same time that the Paleozoic rocks of the Llano uplift are more highly faulted than had been shown on previous geologic maps of the region. The writer concluded from field observations that the Backbone Ridge area was more complexly faulted then had been previously shown, and it was decided to test this conclusion by making a detailed geologic map of the area using the stratigraphic subdivisions recently established by Bridge, Barnes, and Cloud. A detailed study was made of these subdivisions and a large number of the type sections were visited. It was necessary to study the complete geologic history of the region in order that the events which involved the complex structural pattern and the present physiographic forms might be properly understood. The material contained in this report consists of data obtained from the literature and from field observations which were made in the area during the months of June and July of 1948. / text