dropping of marker element 24 to the ground when marker arm 16 is extended in addition the vertical displacement of left marker arm 16 upon impact with an obstacle is damped by means of roller assembly 46b similar functions are performed by roller assembly 46a with respect to right marker arm 14 thus the impact force between the ground and the downward moving marker arm following marker element impact with an obstacle is reduced as is the possibility of damage thereto as a result of such impact finally right and left guide rollers 38 40 not only guide respective marker arms 14 16 during the extension lowering and retraction raising thereof but also facilitate the displacement of respective marker arms providing an easily moved surface along which the marker arms may be translationally displaced roller assemblies 46a and 46b not only facilitate the relative motion of the marker arms when in contact with one another in the overlaid configuration when retracted as shown in fig1 but also rotationally engage the upper inner surface of tool bar 12 in smoothly guiding the proximal ends of the marker arms thereacross during extension and retraction lateral roller assembly 48 and mounting plate roller 50 are shown in fig4 lateral roller assembly 48 is positioned adjacent the proximal end of left marker arm 16 and extends from a lateral surface thereof in rotationally engaging an adjacent lateral surface of tool bar 12 mounting plate roller 50 is also oriented vertically and rotationally positioned on left mounting plate 20 so as to rotationally engage a lateral surface of left marker arm 16 by positioning lateral roller assembly 48 and mounting plate roller 50 immediately adjacent facing lateral surfaces of tool bar 12 and left marker.