Proterozoic gneiss exposed in readcuts between Fort Ann and Whitehall, New York, is close to the Precambrian/Paleozoic unconformity. It exhibits several features not previously described in Adirondack rocks. These (features include (1) extensive slickensides (azimuth 120°–150°, plunge 0°–30°, southeast-over-northwest sense of movement) on preexisting foliation surfaces; (2) coexistence of calcite marbles that contain granulite facies mineral assemblages with dolomite marbles that contain dolomite, quartz, microcline, and serpentine in unreacted mutual contact; (3) ferroan dolomite that fills veins, faults, and fractures and occurs in thin, foliation-parallel lamellae in gneiss; and (4) minerals of probable low-temperature origin (adularia, celadonite, and cherty silica) that fill veins and interstitial spaces in gneiss and marbles. We suggest that these features can best be explained by minor rotational shear deformation during the Taconic orogeny, accompanied by local metasomatism caused by Mg-rich connate brines expelled from Cambrian–Ordovician sedimentary rocks by overriding Taconic thrust slices.

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