Apparatus for controlling the steering mechanism of a mineral mining machine having a nucleonic sensor adapted to sense the cutting horizon of the machine's cutter head relative to a boundary of the mineral seam, the apparatus providing an electronic cavity detector which freezes the machine's steering mechanism according to two operational modes when a cavity or crack/undulation respectively is detected. Thus, the machine is not steered in accordance with an erroneous sensor signal derived when the sensor is adjacent to the cavity or crack/undulation.