Abstract: A rotary trench cutting or rock milling saw unit is supported on a head for movement about two mutually perpendicular axes. The head is adapted for pivotal mounting on a backhoe boom or similar positioning structure. Rotary rock cutting wheels are driven by a hydraulic motor and a support vehicle 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 track drive motor supply pump displacement to maintain a constant feed or rock cutting rate.