now referring to the drawings and in particular to fig1 a grapple skidder 10 suited for transporting loads or drags of logs or trees and embodying the elements of the present invention will be described . the grapple skidder 10 generally comprises a log skidder or self - propelled tracked vehicle 12 supported on either side thereof by a track 14 . each track 14 is driven by a pair of wheels 16 operating in tandem . a rear chassis 18 is articulated to the front chassis of the vehicle 12 and is driven from the power take - off thereof . the rear chassis 18 is provided in the form of a structural frame 26 supported on either side thereof by a track 20 . each track 20 is driven by front and rear wheels 22 and 24 operating in tandem . it has been found that the use of such an articulated self - propelled tracked vehicle 12 in the context of a grapple skidder is advantageous , as compared to conventional four wheel drive grapple skidders , in that it provides increased traction , floatation , and stability , especially , on soft grounds . a base in the form of a turntable 28 is mounted on the frame 26 substantially between the front and rear wheels 22 and 24 for pivotal movement about a vertical axis . a boom mount 30 is secured on the top surface of the turntable 28 . as best seen in fig1 and 4 , a linear main boom 32 is pivotally mounted at a bottom end thereof to the boom mount 30 by means of a pair of hinge pins 34 extending along a horizontal pivot axis ( see fig2 ), accordingly , the main boom 32 is capable of swinging on the turntable bearings and pivoting about the hinge pins 34 relative to the boom mount 30 . likewise , a boom arm or boom stick 36 is pivotally mounted to the main boom 32 via a horizontal hinge pin 38 . a conventional grapple 40 is freely suspended from the free distal end of the arm 36 by an appropriate grapple coupling 42 that pivots about two axes perpendicular to one another . the grapple 40 includes a pair of opposed arcuate jaw members 44 which can be hydraulically operated to open and close tightly about a bundle of logs or trees piled on the ground in stacked relationship , as is well known in the art . as seen in fig1 and 2 , a pair of side - by - side hydraulic cylinders 46 are provided for pivoting the main boom 32 relative to the turntable 28 . each hydraulic cylinder 46 is connected at a first end thereof to the boom mount 30 via a hinge pin 48 located below the hinge pins 34 of the main boom 32 . each hydraulic cylinder 46 is connected at an opposed rod end thereof to a bracket 50 extending integrally away from a top surface 51 of the main boom 32 , at a pivot point 53 spaced from the main boom 32 itself to provide a lever arm for the movement of the main boom 32 . hydraulic lines ( not shown ) provide pressurized hydraulic fluid to retract and extend the boom cylinders 46 , as is well known in the art . as seen in fig2 and 4 , the proximal end of the arm 36 is received in a central rectangular cutout portion 52 provided in the main boom 32 at a location spaced lengthwise from the distal end thereof . a single central hydraulic arm cylinder 54 is provided on a top side of the arm 36 for pivoting the same relative to the main boom 32 . the arm cylinder 54 is pivotally connected at a first end thereof to the distal end of the main boom 32 via a hinge pin 56 . likewise , the arm cylinder 54 is pivotally connected at a second opposed end thereof to the boom arm 36 by means of a hinge pin 58 . the boom arm 36 has a proximal linear segment 60 and a distal arched segment 62 extending integrally downwardly from the linear segment 60 . the hinge pin 58 is located substantially adjacent the root of the arched segment 62 , i . e . at the end of the linear segment 60 near the junction of the linear and arched segments 60 and 62 . as to the hinge pins 38 connecting the boom arm 36 to the main boom 32 , they are disposed at a location between the opposed ends of the boom 32 below the point of pivot of the arm cylinder 54 relative to the main boom 32 and immediately above the point of connection of the boom cylinders 46 and the main boom 32 . the shape of the main boom 32 and the boom arm 36 in combination with the emplacement of the arm cylinder 54 between the boom arm 36 and the main boom 32 provide a boom assembly which is foldable upon itself , thereby allowing the overall dimensions of the boom assembly to be minimized for transport and storage purposes . furthermore , the fact the main boom 32 is displaceable to a forwardly inclined position , as illustrated in full line in fig1 also allows an elongated load , such as a bundle of trees or logs , to be dragged with the leading ends thereof elevated just over the rear wheel 24 of the grapple skidder 10 , thereby minimizing the couple transmitted to the boom 32 by the dragged load . the above described relative disposition of the various points of connection between the boom mount 30 , the main boom 32 , the boom arm 36 , the boom cylinders 46 and the arm cylinder 54 has been found suitable to maximize the freedom of movement of the main boom 32 and the boom arm 36 while preserving the lifting capacity thereof . as seen in fig1 the boom 32 and boom arm 36 are displaceable between a fully retracted position in which the boom 32 is inclined forwardly with respect to a direction of travel of the tracked vehicle 12 , while the arm 36 extends in a generally vertical direction with the grapple 40 hanging therefrom over the rear chassis 18 between the front and rear wheels 22 and 24 thereof , and a fully deployed position ( shown in dotted lines ) wherein the boom 32 is inclined rearwardly and the arm 36 extends downwardly therefrom in a generally vertical direction . according to a preferred embodiment of the present invention , the boom 32 extends at about 65 degrees from the vertical when displaced to its retracted position and about 55 degrees when displaced to its deployed position . as seen in fig3 the mobility of the main boom 32 and boom arm 36 allows the grapple skidder 10 to drag a bundle of trees t with the butt ends thereof elevated over the rear wheels 24 of the rear chassis 18 and the other ends of the trees remaining in contact with the ground . by skidding the bundle of trees t with the butt ends thereof lifted directly over the rear chassis 18 , the flexural forces exerted on the main boom 32 and the frame 26 by the trees t , which are being dragged , can be advantageously reduced . in the vehicle loaded and skidding position illustrated in fig3 the boom cylinders 46 and the arm cylinder 54 are respectively fully and partly retracted so as to position the grapple 40 substantially over the rear wheel 24 of the rear chassis 18 . the grapple 40 tightly grasped the bundle of trees t at an appropriate location adjacent the butt ends of the trees t . in the event that the grapple skidder 10 becomes stuck in a soft ground while dragging a bundle of trees t , the boom cylinders 46 can be extended from the retracted position thereof to a fully or partly extended position , as indicated by arrow 64 in fig3 to cause the main boom 32 to pivot in the direction indicated by arrow 66 in order to push the vehicle in a forward direction with the bundle of trees t . as seen in fig4 the grapple skidder 10 further includes a heel 68 which is mounted on the main boom 32 near the bottom end thereof . the heel 68 includes a pair of laterally spaced - apart parallel mounting plates 70 which are secured to the underside 72 of the main boom 32 . a square tube 74 extends at right angle from each mounting plate 70 for slidably receiving a corresponding one of a pair of legs 76 extending from a transversal arresting bar 78 . the opposed ends of the arresting bar 78 are curved inwardly to embrace the sides of the bundle of trees t which are being lifted by the grapple skidder 10 . transversal through bores 80 are distributed along the square tubes 74 and the legs 76 for receiving fasteners 82 in order to secure the legs 76 to the square tubes 74 in one of a selected positions . this thus allows to adjust the spacing between the arresting bar 78 and the underside 72 of the main boom 32 . the location of the heel 68 allows the grapple skidder 10 to completely lift a bundle of trees t above the ground over another bundle of trees t ′ lying on the ground ( see fig5 ). accordingly , the grapple skidder 10 can advantageously be used to stack a bundle of trees onto another . the stacking operation is accomplished by first extending the boom cylinders 46 while the arm cylinder 54 remains retracted so as to locate the grapple 40 behind the rear chassis 18 for embracing a bundle of trees t , which lies on the ground , at a location comprised between their butt ends and their center of gravity . the jaw members 44 of the grapple 40 are then caused to close tightly around the bundle of trees t . thereafter , the boom cylinders 46 are retracted to displace the main boom 32 in a generally vertical orientation so as to lift the butt ends of the bundle trees t , and the arm cylinder 54 is extended to locate the butt ends of the grasped bundle of trees t under the heel 68 . once the butt ends have been elevated directly under the heel 68 , further contraction of the boom cylinders 46 will first cause the butt ends of the bundle of trees to engage the arresting bar 78 , thereby preventing pivotal movement of the bundle of trees t due to gravitational forces , and then result in the grasped bundle of trees t being completely lifted off the ground . fig6 illustrates a second embodiment of the present invention which is similar to the first one with the exception that the turntable 28 ′ has been incorporated at the articulation between the boom arm 36 ′ and the main boom 32 ′ rather than being provided on the rear chassis 18 at the bottom of the main boom 32 ′. accordingly , the arm 36 ′ is adapted to pivot relative , to the main boom 32 ′ about two axes perpendicular to one another . the turn table 28 ′ is mounted on the main boom 32 ′ for pivotal movement about a pivot 102 . a pair of lateral cylinders 104 extend between the turntable 28 ′ and the main boom 32 ′ to pivot the turntable 28 ′ about the pivot 102 laterally relative to the main boom 32 ′. the boom arm 36 ′ is pivotally mounted to the turntable 28 ′ for pivotal movement about a pivot 106 . a boom arm cylinder 54 ′ is provided between the boom arm 36 ′ and the turntable 28 ′ to pivot the boom arm 36 ′ about its pivot axis 106 .