excavated ground plane application_number US 22781302 A description referring now to the drawings wherein like reference numerals designate identical or corresponding parts throughout the several views fig2 and 3 illustrate a track trencher with an alternative excavation boom 100 as disclosed in co pending u s patent application ser no 10 227 838 aug 27 2002 the track trencher comprises track assemblies 20 frame 30 power unit 40 and excavating boom 100 including head unit 130 which supports excavation assembly 140 the orientation of the base machine is defined by the existing ground surface 180 the areas contacted by the two track assemblies 20 will define the effective ground plane 180 oriented at an angle relative to gravity the effective grade the location and orientation of the excavation assembly 140 will define the new ground surface 182 this location and orientation is controlled by several elements the position of the boom 100 relative to frame 30 is controlled with lift cylinders 62 which effectively rotate boom 100 about axis 114 defined by frame 30 as parallel to the existing ground surface 180 to effectively control the excavation depth relative to the track assemblies 20 the orientation of the excavation assembly 140 relative to the frame 30 is controlled with tilt cylinders 64 which rotate the head unit 130 about swivel axis 124 swivel axis 124 in this preferred embodiment is perpendicular to axis 114 allowing the orientation of the head unit 130 and excavation assembly 140 to be modified relative to axis 114 and the ground plane 180 alternatively a swivel axis not shown could be merely parallel with swivel axis 124 the excavation assembly 140 is designed to be in contact with the ground.