In the Rangeley-Phillips area of Maine, a section of lower Paleozoic clastic sedimentary rocks that may be more than 35,000 feet thick has been tightly folded along northeast-trending axes, intruded by large plutons, and variably metamorphosed. Two generations of schistosity are recognized. The older schistosity, S2, pervades pelitic strata in most of the area and grades from slaty through phyllitic cleavage to schistosity, roughly with increasing metamorphic grade. It is typically nearly parallel to axial surfaces of the major folds of the area.
At several localities thinly tabular metasandstone and metashale dikes are about parallel to S2. They are best developed near the axial surfaces of folds, but are present along fold limbs as well, and have been found in all the major pelitic units of the area. Metasandstone dikes, the largest of which are about half an inch thick, at least 6 feet high, and of unknown horizontal extent, characteristically extend downward from graded beds of metasandstone. Some are paired with metashale dikes that extend upward from metashale beds along refracted cleavage in metasandstone. At places folded beds of meta-sandstone are penetrated by many closely spaced paper-thin to half- inch-thick metashale dikes, producing a compositional cleavage lamination.
The discovery of similar clastic dikes approximately parallel to slaty cleavage in the slates of the Delaware Water Gap area of Pennsylvania and New Jersey led J. C. Maxwell to the conclusion that slate exposed there is an end product of compression, folding, compaction, and dewatering of a thick mass of rapidly subsiding, nearly impermeable pelitic sediments. This process, which may be called tectonic compaction, best explains S2 schistosity in the Rangeley-Phillips area. It was followed here, however, by metamorphism, which was accompanied by growth of micas and other minerals parallel to the old S2 fabric, producing phyllites and schists. Later the rocks were sheared and metamorphosed again, which caused younger slip cleavage and schistosity, S3, in the manner proposed by W. S. White in Vermont.