While on a field trip back in 2012, we stopped to view the high-grade gneisses at Powder River Pass as we were traversing over the Bighorn Mountains. These gneisses are part of the Precambrian core of this mountain range, which was uplifted during the early Cenozoic as part of the Rocky Mountains. These gneisses, forming the southern part of the Precambrian core, and on the order of 3.0 Ga (Wyoming Geologic Map). I was hoping these gneisses might contain some good folds, and they did not disappoint.
This first rock was awesome enough to offer two cross-sections of the fold at ~90° to one another. And, one shows and "S" while the other shows a "Z", giving excellent insight into the three-dimensionality of this fold.
This next one is isoclinal, hiding among the layers of the gneiss. Its always important to look very carefully at the layers in high-grade gneisses like these, because they often hide isoclinal folds. Sometimes the hinge area is quite small and you can miss it if you aren't looking carefully.
This next one is also isoclinal and shows significant thickening of the layer in the hinge. The extremely thin limbs suggest it has been stretched considerably. Continued stretching would eventually separate the hinge from the layers entirely.
This final fold is a bit more open. It doesn't show significant thickening in the hinge, but some of the layers are variable thickness and look discontinuous in places. These different kinds of fold patterns suggest they might be different generations of folding, but so far I haven't observed any fold interference patterns here.