is spaced below the base to receive therebetween a horizontally disposed plate 35 associated with a positionable ground working blade 37 a pivot pin assembly at 34 extends through base 31 and lower plate 33 to pivotally secure triangular plate 35 therebetween blade construction includes blade 37 reinforced at points therealong by upwardly extending weldments 38 spaced along the backside of the blade to provide adequate blade clearance below supporting members 10 and 11 a central portion of the blade is recessed and thereat supports plate 35 which as aforesaid is in securement with blade attached weldments 38 end segments of the blades are of a height shown in fig1 a locking mechanism is carried by base 31 of the blade assembly as best shown in fig6 an extension 31a of the base outboard of support member 11 serves to receive the lock mechanism which includes a housing 38 within which a locking pin 39 is slidably mounted said pin is biased by a spring member 41 for sliding engagement with plate 35 and specifically plate apertures 35a therein a pin release arm at 42 journalled in housing 38 enables the operator to extract pin 39 from an aperture 35a to permit subsequent positioning of the blade about the upright axis of pivot assembly 34 raising and lowering of blade assembly 30 is as earlier mentioned by means.