How is foliation related to the dominant regional stresses?
Any mineral in a rock subjected to a stress field will start to rotate so that the long axis of the mineral is perpendicular to the stress field.
This tendency of elongated minerals to rotate into this position gives rise to foliation in metamorphic minerals (like in gneisses, schists, slates). This happens in compressional stress fields.
Quick quiz for 10 points - what direction was the stress field in the following gneiss? Parallel to the hammer or perpendicular?
Answer: the compressional stress field must have been perpendicular to the hammer ie. top to bottom squeeze!