The Ouachita structural belt in Texas, buried for about 850 miles of its 900-mile course, is composed of: 

1. A frontal zone of essentially unmetamorphosed folded and thrust-faulted rocks which are mostly shale and sandstone of Mississippian-Pennsylvanian age and Ouachita facies (Stanley- Tesnus); pre-Mississippian Ouachita facies rocks are present in the subcrop along the course of the front, and in places Ouachita facies rocks are thrust over foreland facies rocks (Grayson, Collin, Bell, Kendall, and Brewster counties, and probably also in McLennan, Ellis, Bandera, and Terrell counties); a belt of very weakly metamorphosed dark fine-grained clastic rock occurs in the interior part of the frontal zone. 
2. An interior zone of highly sheared phyllite, slate, very fine-grained schist, metaquartzite, and marble.

The tectonic boundary demarking the western and northern margin of the highly sheared rocks is possibly analogous to the Blue Ridge front in the Appalachian belt. In the western part of the belt in Trans- Pecos Texas, the frontal zone is broadly developed in the Marathon salient, but eastward in Val Verde and Kinney counties metamorphic rocks of the interior zone are tectonically juxtaposed with foreland basin sedimentary rocks, and the frontal zone of the belt is missing in the subcrop. Possibly in this area, rocks of the interior zone have overriden frontal zone rocks on the south side of the Devils River uplift. Rocks of the interior zone are exposed in the Sierra del Carmen south of the Rio Grande near Boquillas, Coahuila.