The disclosure features material separators for screening excavated material (e.g., rocks from soil.) In some implementations the materials separators include (a) a supporting frame; (b) a screening surface mounted at an incline on the supporting frame, the screening surface comprising a plurality of fixed bars that are attached to the frame in a manner to resist upward movement, and a plurality of shift bars that are attached to the frame at their upper and lower ends, in a manner to allow upward movement; and (c) a shift bar actuator, positioned between the upper and lower ends of the shift bars, the shift bar actuator being configured to impart a two-stage movement to the shift bars, whereby during a first stage the upper ends are first displaced vertically, and during a second stage the lower ends pivot upward about the upper ends.