Broad belts of northwest-vergent deformation and southeast-younging Silurian strata constitute a major structural inhomogeneity in the Southern Uplands-Down-Longford lowerPaleozoic terrane. These belts are defined, mapped, and interpreted as obducted slices within an accretionary forearc above an active, northwest-dipping subduction zone. Normal southeast-vergent accretion, involving northwest underthrusting of trench-wedge sediments,was punctuated by intervals of anomalous northwest overthrusting. Overthrusting was temporally associated with abnormally high late Llandoverian sedimentation rates, causing over pressuring and loss of shear strength in the basal hemipelagics. This dynamic relation between the conditions of deposition and the asymmetry of subsequent deformation is analogous to that in the landward-directed overthrusts of the Washington-Oregon margin.