ABSTRACT

Compositional layering, lineation measurements and mapping of rock units indicate steeply plunging anticlinal folds in the area of the Madison River canyon, north of Ennis, Montana. The folding maintains an approximately constant NNE. trend but changes in direction of plunge rather abruptly near the center of the area. The southern portion of the area consists of a broad open anticline plunging 70° N. 20° E. However, in the northwestern portion the folds plunge steeply to the southwest. Two theories are advanced to explain the origin of the observed structures: (1) Initial cylindroidal folding about a horizontal axis trending N. 20° E. was followed by a second period of folding with the deforming stresses nearly at right angles to those of the first period; (2) A single stress field with negative tetragonal symmetry (two axes of extension and one of compression) with the positive axis trending essentially NWSE could produce such a fold pattern as was observed. The non-cylindroidal nature of the folding may be attributed to the original attitude of the compositional layering.

A representative section of the Pony gneiss, 6,147 feet thick, consists of interlayered plagioclase-quartz gneisses, microcline-plagioclase quartz gneisses and amphibolites. In addition a thick unit of sericitic quartzite was observed. All the mineral assemblages observed are stable throughout the almandine-amphibolite facies. The Pony series is believed to have been derived from the metamorphism of interstratified limey shales, sandy limestone, and sandstones.