We characterize the crustal and upper mantle structure of the Cascadian fore arc using receiver function analysis of data from long-running observatories located along the North American margin in Oregon and Washington. We identify coherent SV-polarized phases consistent with P-S conversion from the top of the subducting Juan de Fuca plate throughout the area west of the volcanic arc. In many instances we also find evidence for deeper features, likely associated with the subducted oceanic crust. At many locations SH-polarized energy in receiver functions is associated with the surface of the subducting plate. These SH-polarized phases display significant directional variation and show clear polarity reversals, consistent with the presence of seismic anisotropy. Previously, a layer of serpentinized material within the megathrust has been proposed to explain an anisotropic, low-velocity, high Poisson's ratio layer identified by receiver function analysis at Corvallis, Oregon (Global Seismic Network station COR). Our new results suggest that this feature is widespread in Cascadia, but the specific receiver function signature varies. This anisotropic supraslab layer is supported by receiver function estimates beneath seismic stations which, similar to COR, are ∼100 km from the coast and ∼40 km above the slab surface. However, the orientation of rock fabric implied by the anisotropy varies greatly from station to station. Modeling with synthetic seismograms supports the existence of serpentinite beneath station GNW (Green Mountain, Washington). Closer to the coast, where the slab is shallower, an anisotropic layer is likely beneath some stations, but the evidence is less conclusive. We propose that the anisotropic signature beneath the inland stations is related to serpentinization of the mantle wedge at greater depths, best explained by the presence of the polymorph antigorite. Hypothetically suctioned upward by slab rollback, a deformed sliver of serpentinite-rich rock above the slab interface would likely display the observed large spatial variation in anisotropic symmetry axis within the layer, consistent with our observations.