the excavator 100 is set to dig on a descending grade making an increasingly deeper cut until the outermost lower probe means 52a register a changing of resistance to a different value from that registered by the upper probe means 54a the grade is then reduced and corrected until the resistance values are stabilized with the lower and upper means penetrating the different strata and picking up distinctly different resistance readings from then on whenever the lower and upper resistance readings become similar the value will indicate whether a positive or negative grade correction is needed the procedure is virtually the same whether mining product or removing parting material except for a reversal of the grade correction response a machine operator will soon become skilled in responding to these indications or if desired a grade control response sequence can be programmed for computerized stratum boundary excavation the depth control technique is much the same regardless of the material property used to distinguish the stratum boundary fig1 shows the split screen measurement and display means 98b lower probe means 52b with microphonic pick up means 86b send noise and vibration signals to be measured and displayed on the left hand side of the screen of measurement and display means 95b as vibratory trace 101 the frequency and intensity of which are characteristic of abrasive sandstone upper probe means 54a send noise and vibration signals to be measured and displayed on the right hand side of the screen of measurement and display means 95b as vibratory trace 102 the frequency and intensity of which showing a significantly reduced.