Anisotropy in aquifers has been shown to be common in deformed terrains. When structural fabrics and resulting secondary porosity are a result of regional tectonics, principal transmissivity tensor (Tα) orientation (θ) may be similar over a wide area. Recognition of domains with similar orientation and magnitude of transmissivity tensors is important for effective groundwater management. The Pen Argyl Member of the Martinsburg Formation is a thick sequence of claystone slate with subordinate intercalated quartzose slate and sub-graywacke. Time-drawdown data from four aquifer tests in the Pen Argyl Member exhibited anisotropic behavior at a scale of at least 600 m. The horizontal anisotropic properties were determined using analytical models. Statistical tests for anisotropy were applied, and variance formulas were used for uncertainty estimation and testing for differences in Tα/Tβ and θ from tests at different locations in the aquifer. Principal transmissivity tensor orientation for four pumping tests within the Pen Argyl Member of the Martinsburg Formation were statistically indistinguishable (mean θα = 41.5° ± 8.8°, 2σ) and aligned parallel to fold axes and the strike of joint sets and cleavage. An example of results from a single test shows the aquifer is anisotropic (H0: Tα = Tβ rejected at P < 0.005) with θ = 47.7° ± 19.0° (2σ), Tα/Tβ = 7.9 ± 5.5 (2σ), Tα = 218 ± 441 (2σ) m2/d, and Tβ = 27 ± 53 (2σ) m2/d. This study is a step toward delineation of hydrostratigraphic units within the Martinsburg Formation.