The Beartooth Mountains represent an elongate block of Archean crystalline rocks (metamorphism dated at 2.7 b.y. by Rb-Sr) which was uplifted during the Laramide Revolution. The Lonesome Mountain area in the heart of the range consists mainly of folded tonalitic to granitic gneisses, migmatites, and ortho-amphibolites with minor metasedimentary rocks, small pegmatites, and orbicular granitoid rocks. Mafic intrusions were emplaced during at least four periods from the Archean to the Tertiary; two of these later underwent metamorphism.
Analysis of compositional and mineral foliations, small fold axes, and mineral lineations in gneisses and migmatites defines statistical folds at stations and subdomains. These accurately reflect the geometry of large folds outlined by amphibolite sheets and axial trends constructed from field data. Presence of this pervasive fold system in heterogeneous migmatites and gneisses raises the question of whether the rocks had an intrusive or metasomatic origin.
Independent studies of folds and fabric in the nearby Gardner Lake area by Harris, the Southern Beartooth Mountains area by Wise, the Line Creek area by Casella, and the Quad Creek-Wyoming Creek-Line Creek area by Rowan and Larsen where metasedimentary rocks are common show that mapped large folds and statistical large folds are coincident. Small folds in the Quad Creek-Wyoming Creek-Line Creek area are older than or formed simultaneously with the open, flexural large folds and were formed during metamorphism-metasomatism. Mesoscopic compositional foliations at stations there are not homogeneous with respect to the statistical large fold axis as they are in the Lonesome Mountain area.
The geometry of large folds in the Lonesome Mountain area indicates dominant passive flow rather than flexural-flow and flexural-slip. Homogeneity of gneisses and migmatites on the mesoscopic scale is best explained by this mechanism of folding. Earlier structures would have been destroyed or transposed. Evidences for intrusive structures are measurable in inches or up to a few feet only, but phacolithic intrusion in the catazone cannot be ruled out by present criteria.