A rotary trench cutting or rock milling saw unit (12) is supported on a head (18) for movement about two mutually perpendicular axes (50,62). The head is adapted for pivotal mounting on a backhoe boom (20) or similar positioning structure. Rotary rock cutting wheels are driven by a hydraulic motor and a support vehicle (24) for the saw unit includes a hydrostatic drive system with a variable displacement hydraulic pump driving variable displacement track drive motors which may be set in a slow speed operating mode for trench cutting operations or a high speed mode for tramming between work sites. A constant rate of power input to the saw unit is maintained by sensing saw drive motor supply pressure and adjusting tract drive motor supply pump displace­ment to maintain a constant feed or rock cutting rate. The apparatus includes hydraulically operated actuators for rotating the saw unit and its support boom about a vertical axis (40) with respect to the apparatus undercarriage, elevating the boom with respect to the undercarriage, and moving the saw unit about three mutually perpen­dicular axes (40,50,62) with respect to the support boom. Each of the actuators includes a control valve which may be selectively positioned to operate the actuators in reverse directions, lock the actuators in a predetermined position, or permit the actuators to oscillate or rotate the saw unit with respect to the various positioning axes of rotation when the saw unit is confined in a trench.