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FIELD AND BACKGROUND OF THE INVENTION 
     The present invention relates to ground surface cover systems used for erosion control, and more particularly to a ground surface cover system featuring interlocking elements flexibly locked by a flexible interlocking joint, used for erosion control, and a corresponding method. 
     Erosion is a process involving the movement of earthy or rock material along a ground surface as result of natural processes including rain, wind, earthquakes and related movements in the ground, or man made processes such as water redistribution or the formation of artificial bodies of water, which are capable of moving earthy or rock material along the upper surface of the ground. Ordinarily, it is desirable to control erosion at elevated or inclined locations such as along roadsides, edges around bodies of water, for example, reservoirs, rivers, and lakes, and bridge to ground connections, where erosion is known to cause structural and environmental damage. 
     Currently, commonly used methods of effectively controlling erosion involve the placement of a ground cover on top of and along the surface of interest, of an area extending the region of desired erosion control. The main objective of placing ground cover is to adequately control or minimize the movement of earthy or rock material along the surface of the ground, whatever the cause of the movement. In terms of functionality, there are several important properties for a ground surface cover system to have in order to be effective. Foremost, an effective ground surface cover system needs to be made of sufficient strength and long term stability to withstand one or more of the elements causing erosion processes such as water, water flow, and ground movement, over long periods of time, i.e., years. At locations where water flow is involved in the erosion process, it is desirable for a ground surface covering to withstand, and allow for, efficient patterns of water flow and water distribution along the covered surface or ground. At locations where ground movement is involved in the erosion process, for example, involving cavity or protrusion formations at the ground surface, it is desirable for the ground surface cover system to horizontally, vertically, and angularly self-adjust, in a flexible way, along with ground movement, otherwise damage to the ground surface cover system may take place, thereby decreasing the effectiveness of subsequent erosion control at such locations. Instead of, or, in addition to self-adjustment, for the same reason, it is desirable for a ground surface cover system to be manually adjustable, or flexible, according to need. Hereinafter, the terms flexible and flexibility refer to horizontal, vertical, and/or angular motion or movement, whereby such motion or movement is of a ground surface cover system in general, of interlocking elements of a ground surface cover system, or, of the interlocking joint of the elements, in particular. 
     An additional, but optional, desired attribute of a ground surface cover system relates to landscape, involving the presence of spaces throughout the ground surface cover system enabling botanic growth. This attribute may or may not have functional importance to the ground surface cover system, depending upon the actual causes and parameters of an erosion process at a particular location, i.e., the presence of botanic growth throughout a ground surface cover system can affect patterns of water flow, movement of ground, and movement of the ground surface cover system itself. Other important attributes of a suitable ground cover system are economic based, whereby manufacturing and installation need to be feasible, practical, and of reasonable costs. Other attributes include the extent to which a ground surface cover system is replaceable and reusable either at a same location, at a different location, or both. 
     Several different types of ground surface cover systems are in common use. In addition to simply partially or completely covering the selected area of ground surface requiring erosion control with a multitude of removable individual stones, four main categories are ordinarily referred to with respect to ground surface cover systems, i.e., single cast structures, multi-cast structures, ‘gabion’ structures, and combination structures. Single cast ground surface cover systems are based on permanently covering the selected area of ground surface requiring erosion control with a layer of concrete alone, or, with a layer of concrete containing a dispersion of stones. Optional metal reinforcements internal to the cover material may be used throughout selected portions of the ground surface cover system. Multi-cast ground surface cover systems are based on the placement of a multitude of, removable, individual, geometrically formed, elements or blocks, usually made from concrete, which partially or incompletely cover the selected area of ground surface requiring erosion control. Gabion ground surface cover systems are based on the placement of gabion structures, featuring a continuous or discontinuous network or web like structured system of metal baskets or cages of specified geometries, dimensions, and rigidity, filled with a chosen density of loose, non-cemented stones. Combination ground surface cover systems are based on the placement of a plastic matting featuring concrete casting modules, typically of a honeycomb like geometry, upon the ground, and casting, on-site, the concrete modules. Individual concrete modules are relatively near to, but are not in contact with, each other. 
     Multi-cast ground surface cover systems may be further classified into two different types, i.e., systems based on interconnecting elements or locks, and systems based on interlocking elements or blocks. Hereinafter, interconnecting refers to the state or configuration of elements or blocks placed side-to-side or adjacent to each other, thereby forming a larger non-flexible pattern of such elements or blocks, where the elements or blocks are connected, and not locked, even loosely, to each other via element to element or block to block male to female connection or mating of any sort. Hereinafter, interlocking refers to the state or configuration of elements or blocks which are placed in contact with each other via some sort of element to element or block to block male to female interlocking connection or mating, thereby forming a larger non-flexible or flexible pattern of such elements or blocks, where the elements or blocks are locked to each other. In this case, the interlocking connection or mating between any two elements or blocks forms a joint, where the joint is comprised of a male component structural feature such as a hook, protrusion, extension, barb, tongue, or nose, compatible with and interlocked to a corresponding female component structural feature such as a recess, opening, or related cutout structural feature. According to present usage, an interlocking element to element or block to block joint may be non-flexible or flexible, whereby flexibility refers to the capability of movement or turning in a horizontal or vertical direction without damaging or breaking the interlocking joint, or the elements or blocks. 
     In regard to multi-cast ground surface cover systems, current teachings of interlocking ground surface cover systems are based on individual elements interlocked by rigid or fixed, non-flexible joints between the elements, resulting in no degrees of freedom for vertical or horizontal movement. This characteristic of multi-cast interlocking element systems presents several significant limitations for application of such systems to erosion control. As will be shown, the system of the present invention overcomes many such limitations by featuring a flexible joint between interlocking elements of a multi-cast ground surface cover system for producing an effective erosion control system. There is a need for, and it would be useful to have a multi-cast interconnecting ground surface cover system which overcomes the limitation of non-flexibility of the system, in general, and non-flexibility of the joint of the interlocked elements, in particular, thereby resulting in a more effective erosion control system. 
     An ideal ground surface cover system for effective erosion control would feature all the above mentioned properties and attributes necessary for achieving the objective of adequately controlling or minimizing ground movement during a potential erosion process, including high strength and long term stability, patterns for efficient water flow and water distribution, flexible adjustment to ground movement, capability of including landscape, economic and feasible manufacturing and installation, replaceability, and reusability. It will be shown that incorporating the feature of flexibility into a ground surface cover system leads to significantly better achievement of having all of these properties and attributes of an effective erosion control system. In practice, each of the above categories of currently employed ground surface cover systems features varying degrees of limitations or shortcomings by lacking one or more of the above mentioned properties and attributes. Typically, multi-cast ground surface cover systems feature more of the above indicated properties and attributes for providing erosion control, especially with respect to the attribute of being non-permanent and removable, in contrast to single cast ground surface cover systems, and are thus more commonly employed for erosion control. Specific limitations of currently employed ground surface cover systems for erosion control follow. Each limitation is related, either directly or indirectly, to the absence of the feature of flexibility of the ground surface cover system as a whole, or to the absence of the feature of flexibility of the interlocking joint between the two elements. 
     For single cast ground surface cover systems, with respect to distribution of water flow, once a single cast ground surface cover system is installed on-site, the general characteristics of water flow are essentially fixed, i.e., random top to bottom flow, according to the single cast structure, and depend only upon variation in the influences causing erosion, for example, strength and velocities of rain and/or wind acting upon the ground surface cover. With respect to flexibility or adjustment to ground movement, by the very nature of a single cast ground surface cover system, there is none. That is, by sufficient forces in the ground causing cavity or protrusion formation at the ground surface, a single cast ground cover system becomes damaged, requiring on-site repair of the local and surrounding area of the single component ground surface cover which has either fallen into the cavity or protrudes from the surface. With respect to landscape, by the very nature of a single cast ground surface cover system covering the entirety of a given ground surface area, there is no space left for practically including any kind of ground landscape such as botanical growth. With respect to installation, inherently, single cast ground surface cover systems involve substantial on-site work relating to the placement of stones and casting of cement. With respect to reusability, inherently, single cast ground surface cover systems represent a one time installation, whereby, it would be extremely work intensive and economically unfeasible to remove or replace parts of the casted mixture of stones and cement. 
     For gabion structure ground surface cover systems, degree of limitation or shortcoming of a given property or attribute is directly related to the parameters of the system, including for instance extent or area, dimensions, and density, of the gabion structures lying on and rising above the ground surface. Gabion structures are generally rigid with respect to forces exerted by water flow or ground movement. As such, gabion ground surface cover systems provide limited control of water flow and distribution, which are based primarily on random top to bottom water flow through the stones contained within the metal baskets or cages. Depending upon stone density within the baskets or cages, over long periods of time, the stones contained within the baskets or cages of gabion structures are expected to shift, possibly leaving the baskets or cages, and may accumulate along an inclined area of potential erosion, due to gravity and influences of rainfall and wind shear, thereby causing changes in the overall gabion structure, possibly adversely affecting the efficiency of such an erosion control system. Installation of gabion structures for erosion control is ordinarily labor intensive and therefore costly, compared to installation of other erosion control systems. Moreover, as the baskets or cages of gabion structures are of metal, they are prone to corrosion following exposure to water, where the extent of corrosion depends upon the quality of metal used. Either using high quality corrosion resistant metal for the baskets or cages, or replacing baskets or cages as they corrode, clearly increases the cost of using gabion ground surface cover systems for erosion control. 
     Combination ground surface cover systems, based on the placement of a plastic matting, upon the ground, featuring a network of individual modules of casted concrete, is limited in several ways. Once cast, the network of concrete modules is essentially permanently fixed and non-flexible with respect to control of water flow, water distribution, and adjustment to ground movement. Moreover, since the system is based on having plastic matting covering the ground of interest, there is limited accommodation for the addition of botanic landscape. Combination erosion control systems are also significantly limited due to the need for on-site casting. In this case, typically, the quality of concrete and of the casted concrete modules are significantly less than that of multi-cast ground surface cover systems featuring concrete elements manufactured off-site and transported to the chosen site for installation. Moreover, the plastic matting and concrete modules of combination ground surface cover systems are not readily amenable to replacement or reuse. 
     Multi-cast interconnecting, i.e., not interlocking, element ground surface cover systems have the significant limitation of individual elements potentially being uplifted or submerged, in an unstable manner, during conditions of underground movements, i.e., cavity or protrusion formation, respectively. Under such conditions, there is the possibility of multiple elements of the interconnecting element system to move around, causing changes in patterns of water flow and water distribution, thereby, potentially adversely affecting effectiveness of erosion control. With respect to including landscape throughout an interconnecting element ground surface cover system by leaving spaces between elements, there is the limitation that, since the elements are not locked to each other, landscape spaces between elements must be maintained by a perimeter of elements. Moreover, future changes in landscape throughout such a system would require careful re-arrangement of several interconnecting elements, not simply by moving around one or two elements, in order to maintain overall system strength and stability for the purpose of providing erosion control. Related to this limitation of interconnecting element ground surface cover systems, is that of limited replaceability of individual elements. Again, since elements of an interconnecting element system are not locked to each other, moving any given element affects positioning and stability of its neighboring elements. 
     Multi-cast interlocking, i.e., not interconnecting, element ground surface cover systems, featuring non-flexible joints, have the potential of elements being damaged or broken under conditions of ground cavity or protrusion formation, due to the rigid nature of the fixed joints between the individual elements, especially for elements made of concrete. As a result of this, patterns of water flow and distribution are likely to change, thereby affecting erosion control effectiveness in an unpredictable manner. Additionally, with respect to water flow and distribution, as an example, placement of a rigid hollow honeycomb like or other hollow polygonal multi-cast interlocking structure at a location of erosion results in inefficient and poor control of water distribution and water flow during rainfall, whereby, water accumulates inside the honeycombs or polygonal structures, potentially leading to excessive wetting of the ground underneath the ground surface cover, with minimal possibility of water flow from top to bottom of the ground surface covering, except under flooding conditions of the individual honeycombs or polygonal structures. Another significant limitation of multi-cast interlocking element ground surface cover systems is that individual elements of such a systems are not readily replaceable, as several interlocked elements need to be removed one at a time before removing a particular element, due to the linked structure of interlocking element systems. 
     Based on limitations of currently employed ground surface cover systems, there is thus a need for, and it would be useful to have a ground surface cover system featuring interlocking elements flexibly locked by a flexible interlocking joint, used for erosion control, and a corresponding method. Such a system and corresponding method would overcome all of the above indicated limitations regarding effective erosion control. 
     Specific examples of multi-cast interconnecting ground surface cover systems currently available are those manufactured by Unglehrt GMBH &amp; Co., Gronenbach-Zell, Germany; Franz Carl Nudling, Fulda, Germany; and Kasper Rockelein KG, Wachenroth, Germany. Each of these currently available ground surface cover systems has the above described limitations with respect to erosion control. 
     The present invention relates to ground surface cover systems used for erosion control, and specifically to a ground surface cover system featuring interlocking elements flexibly locked by a flexible interlocking joint, and a corresponding method, used for erosion control. There is substantial prior art regarding elements, systems, and methods based on, or including, interlocking elements for construction of floors, panels, and load bearing surfaces such as roads or airplane landing mats. However, none of the following indicated prior art refers to erosion control of a ground surface, or includes the important feature of having directional, i.e., vertical or horizontal, flexibility of the system, or of interlocking elements flexibly locked by a flexible joint. Moreover, prior art relating to elements, systems and methods featuring interlocking elements teach about rigidity or non-flexibility of the interlocking element joints, thereby preventing vertical or horizontal movement of parts of an entire system or of the individual elements. Furthermore, interlocking elements and systems of interlocking elements taught about in the following prior art are preferably made from wood, metal, polymer, composite material, or combinations thereof, and not of concrete which is preferably used for making ground surface cover systems for erosion control. 
     One teaching, U.S. Pat. No. 5,580,191 issued to Egan, describes a retaining wall, preferably for marine use, featuring interconnecting and interlocking elements, used for erosion control along a vertical wall adjacent to a body of water. The following prior art relates to flooring or paneling elements, systems or methods based on, or including, non-flexible interlocking elements: U.S. Pat. No. 5,797,237 issued to Finkell, Jr.; U.S. Pat. No. 4,426,820 issued to Terback et al.; U.S. Pat. No. 4,037,377 issued to Howell et al.; and U.S. Pat. No. 2,740,167 issued to Rowley. The following prior art relates to elements, systems, and methods based on, or including, non-flexible interlocking elements for constructing load bearing surfaces such as roads and airplane landing mats: U.S. Pat. No. 3,859,000 issued to Webster; U.S. Pat. No. 3,572,224 issued to Perry; U.S. Pat. No. 3,385,182 issued to Harvey; U.S. Pat. No. 3,301,147 issued to Clayton et al.; and U.S. Pat. No. 1,371,856 issued to Cade. 
     SUMMARY OF THE INVENTION 
     The present invention relates to a ground surface cover system featuring interlocking elements flexibly locked by a flexible interlocking joint, and a corresponding method used for erosion control. 
     The ground surface cover system of the present invention introduces the important property of flexibility to the utilization of multi-cast interlocking elements for erosion control. The flexible interlocking joint of the present invention is featured with a corresponding preferred method of mechanically engaging two interlocking elements to each other, and is extended to a preferred method of forming a system of a ground surface cover featuring different patterns of interlocking elements to be used for ground surface erosion control. Several additional specific features of the interlocking elements, further enabling the ground surface cover system of the present invention for erosion control, are provided. 
     The ground surface cover system and method of the present invention serve as significant improvements over currently used ground surface cover systems and methods used for erosion control. The system and method of the present invention would result in overcoming each of the above indicated limitations regarding effective erosion control, by featuring properties and attributes necessary for achieving the main objective of effectively controlling or minimizing ground movement during a potential erosion process, including high strength and long term stability, patterns for efficient water flow and water distribution, flexible adjustment to ground movement, capability of including landscape, economic and feasible manufacturing and installation, replaceability, and reusability. 
     According to the present invention, there is provided a ground surface cover system for use in erosion control of a ground surface, the ground surface cover system comprising at least one layer upon the ground surface of a plurality of interlocking elements, wherein opposing ends of a pair of opposing interlocking elements are flexibly interlocked by a flexible interlocking joint, the flexible interlocking joint defining mechanical engagement of an interlocking element tongue transversely extending outward from one opposing end of a first interlocking element of the pair to an interlocking element channel transversely extending outward from one opposing end of a second interlocking element of the pair. 
     According to the present invention, there is provided a method of erosion control of a ground surface, the method comprising the steps of: (a) providing the ground surface to be erosion controlled; and (b) covering the ground surface with at least one layer of a plurality of interlocking elements, wherein opposing ends of a pair of opposing interlocking elements are flexibly interlocked by a flexible interlocking joint, the flexible interlocking joint defining mechanical engagement of an interlocking element tongue transversely extending outward from one opposing end of a first interlocking element of the pair to an interlocking element channel transversely extending outward from one opposing end of a second interlocking element of the pair. 
     According to the present invention, there is provided a flexible interlocking joint of interlocking elements for use in a ground surface cover for erosion control of a ground surface, the flexible interlocking joint comprising an interlocking element tongue transversely extending outward from one opposing end of a first interlocking element of a pair of the interlocking elements mechanically engaged to an interlocking element channel transversely extending outward from one opposing end of a second interlocking element of the pair of the interlocking elements. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Reference is now made to the drawings which illustrate the preferred embodiments the invention may take in physical form and in certain parts and arrangements of parts wherein: 
     FIG. 1A is a schematic close-up side view illustrating the flexible interlocking joint of the interlocking elements in a neutral position, in accordance with the present invention; 
     FIG. 1B is a schematic close-up side view illustrating the flexible interlocking joint of the interlocking elements following angular movement, in accordance with the present invention; 
     FIG. 1C is a schematic close-up side view illustrating the flexible interlocking joint of the interlocking elements following horizontal movement, in accordance with the present invention; 
     FIG. 1D is a schematic close-up side view illustrating the flexible interlocking joint of the interlocking elements following vertical movement, in accordance with the present invention; 
     FIG. 2A is a schematic side view illustrating part of the system featuring level top and bottom configured elements interlocked by the flexible interlocking joint, in accordance with the present invention; 
     FIG. 2B is a schematic side view illustrating part of the system featuring ridged top and bottom configured elements interlocked by the flexible interlocking joint, in accordance with the present invention; 
     FIG. 2C is a schematic side view illustrating part of the system featuring an elevated level top and level bottom configured element interlocked by the flexible interlocking joint, in accordance with the present invention; 
     FIG. 3 is a schematic side view illustrating part of the system featuring level top and bottom configured elements interlocked to a level top and bottom configured center element, via the flexible interlocking joint, in accordance with the present invention; 
     FIG. 4A is a schematic view illustrating one side of a level top and bottom configured interlocking element, in accordance with the present invention; 
     FIG. 4B is a schematic view illustrating the top of the level top and bottom configured interlocking element of FIG. 4 a , in accordance with the present invention; 
     FIG. 4C is a schematic side view illustrating alternative optional features of the level top and bottom configured interlocking element of FIG. 4A, in accordance with the present invention; 
     FIG. 4D is a perspective view of the level top and bottom configured interlocking element of FIG. 4A, featuring element or joint tongue pointing downward, and element or joint channel pointing upward, in accordance with the present invention; 
     FIG. 4E is a perspective view of the level top and bottom configured interlocking element of FIG. 4A, featuring element or joint tongue pointing upward, and element or joint channel pointing downward, in accordance Keith the present invention; 
     FIG. 5A is a schematic view illustrating one side of a ridged top configured interlocking element, in accordance with the present invention; 
     FIG. 5B is a schematic view illustrating the top of the ridged top configured interlocking element of FIG. 5A, in accordance with the present invention; 
     FIG. 6A is a schematic view illustrating one side of an elevated level top and level bottom configured interlocking element, in accordance with the present invention; 
     FIG. 6B is a schematic view illustrating the top of the elevated level top and level bottom configured interlocking element of FIG. 6A, in accordance with the present invention; 
     FIG. 7 is a schematic view illustrating one side of a level top and bottom configured center interlocking element, in accordance with the present invention; 
     FIG. 8 is a schematic sequential series of side views illustrating a method of interlocking the elements via the flexible interlocking joint, in accordance with the present invention; 
     FIG. 9A is a schematic top view of the system of interlocking elements, in a closed, non-staggered pattern, in accordance with the present invention; 
     FIG. 9B is a schematic top view of the system of interlocking elements, in a closed, staggered pattern, in accordance with the present invention; 
     FIG. 9C is a schematic top view of the system of interlocking elements, in an open, staggered pattern, in accordance with the present invention; 
     FIG. 10 is a side view diagram of the ground surface cover system of interlocking elements, as applied in practice to an exemplary single inclined ground surface featuring a cavity and a protrusion, illustrating flexibility of the system, in accordance with the present invention; and 
     FIG. 11 is a side view diagram of the ground surface cover system of interlocking elements, as applied in practice to an exemplary double inclined ground surface, in accordance with the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The present invention is of a ground surface cover system featuring interlocking elements flexibly locked by a flexible interlocking joint, and a corresponding method. The components and operation of the ground surface cover system featuring interlocking elements flexibly locked by a flexible joint, according to the present invention, are better understood with reference to the drawings and the accompanying description. For the purpose of providing logical flow of an appropriate description of the preferred embodiments of the present invention, the drawings and accompanying description are arranged in the following order: describing the flexible interlocking joint of the interlocking elements used for producing the ground surface cover system of this invention, describing exemplary parts of the system featuring different configurations of the interlocking elements and element components used for forming the flexible interlocking joint of this invention, describing different configurations of individual interlocking elements, describing a method of interlocking the elements via the flexible interlocking joint of this invention, describing different patterns of the ground surface cover system of this invention, and describing preferred methods for applying the ground surface cover system of this invention to realistic scenarios of erosion control. 
     It is to be noted that the drawings and accompanying description of the present invention shown here are for illustrative purposes only, representing preferred embodiments of the invention, and are not meant to be limiting. Throughout the drawings, same reference numbers represent same indicated features of the invention or parts of the invention shown and described in the figures. Typically, in addition to initial reference and description of features or components of the interlocking elements of the present invention, only those previously referenced and described same features or components relevant to understanding another indicated figure are repeated in that indicated figure. 
     Referring now to the drawings, FIG. 1A is a schematic close-up side view illustrating the flexible interlocking Joint of the interlocking elements in a neutral, i.e., non-flexed non-contact, position. The flexible interlocking joint, in a neutral position, generally referenced as  10 , is formed from interlocking, mechanically engaging or mating two interlocking elements, which are partially shown here and generally referenced as interlocking element end  12  and interlocking element end  14 . Element end  12  features a contour including element top surface segment  16  extending horizontally to bend  18 , further extending downward along an incline to bend  20 , further extending downward and around to bend  22 , further extending upward and around to bend  24 , further extending upward along an incline to bend  26 , further extending horizontally to bend  28 , further extending vertically downward to bend  30 , and further extending horizontally along element bottom surface segment  32 . That part of the contour of element end  12 , extending from bend  18  through bends  20  and  22 , and through bends  24 ,  26 , and  28 , forms a male type element or joint interlocking component, tongue  34 . 
     Element end  14  features a contour including element top surface segment  36  extending horizontally to bend  38 , further extending downward along an incline to bend  40 , further extending downward and around to bend  42 , further extending upward and around bend  43 , further extending upward and around to bend  44 , further extending downward along an incline to bend  46 , further extending vertically downward to bend  48 , and further extending horizontally along element bottom surface segment  50 . The contour of element end  14 , extending from bend  38  through bends  40  and  42 , and through bend  46 , forms a female type element or joint interlocking component, channel  52 . 
     Channel  52  of element end  14 , is contoured, of variable shape having variable dimensions, appropriate for insertion or mechanical engagement of tongue  34  of element end  12 , providing a joint for flexibly locking elements of a ground surface cover system for erosion control. Further illustration and description of preferred shapes and dimensions of tongue  34  and of channel  52  are provided in FIGS. 4D-4E. The presence of tongue  34  of element end  12 , inside of channel  52  of element end  14 , forms flexible interlocking joint  10 . Flexible interlocking joint  10  has dual functionality, enabling multi-directional and angular flexibility or movement of tongue  34  relative to channel  52 , following engagement of tongue  34  with channel  52 , simultaneous to enabling the corresponding elements to remain in an interlocked position. By design, disengagement of tongue  34  from channel  52  is limited to a small range of positions and angles of tongue  34  relative to channel  52 , according to actual relative shapes and dimensions of tongue  34  and channel  52 , in general, and, in particular, due to the presence of tongue surface segment extending along bend  22 , bend  24 , and bend  26  relative to the presence of channel surface segment extending along bend  43 , bend  44 , and bend  46 . This dual functionality is directly translated to the ground surface cover system of the present invention for the objective of providing a feasible and effective system of erosion control. 
     Tongue  34  of element end  12  includes tongue tip  58 , where tongue tip  58  features the region extending from bend  20  through bend  22 . Tongue tip  58  includes a tongue tip bottom  54 , with a corresponding tongue tip bottom tangent  56  drawn as reference, and a tongue tip side  60 , with a corresponding tongue tip side tangent  62  drawn as reference. Coordinate system  64 , featuring an x-axis positioned 90 degrees from, or perpendicular to, a y-axis, is included in FIG.  1 A as reference for the purpose of describing the positioning and flexibility of the flexible joint  10  of the interlocking elements of the present invention. For the flexible joint  10  illustrated in FIG. 1A in the neutral position, tongue tip bottom tangent  56  is parallel to the x-axis, and tongue tip side tangent  62  is parallel to y-axis, of coordinate system  64 , respectively. Moreover, for flexible joint  10  in the neutral position, tongue  34  is mechanically engaged, but not in physical contact with, element end  14 , whereby a gap exists between the contour of tongue  34  and the contour of channel  52 . For the preferred embodiment of the invention, bottom surface segment  32  of element end  12  lies parallel to and in the same plane as bottom surface segment  50  of element end  14 . 
     FIG. 1B is a schematic close-up side view illustrating the flexible interlocking joint of the interlocking elements following angular movement. The flexible interlocking joint, following angular movement, generally referenced as  66 , is formed by rotation of element end  12  with respect to element end  14 . In this illustration, element end  12  is rotated counterclockwise through an angle  68 , with coordinate system  64  as reference point of rotation. In practice, according to actual dimensions of tongue  34  and channel  52 , angle  68  is preferably less than sixty degrees. For flexible joint  66  illustrated in FIG. 1B in the flexed angular position, tongue tip bottom tangent  56  is rotated away from the x-axis, and tongue tip side tangent  62  is rotated away from the y-axis, of coordinate system  64 , respectively, through angle  68 . Moreover, for flexible joint  66  in the flexed angular position, tongue  34  may be in physical contact with element end  14 , and preferably, bottom surface segment  32  of element end  12  is positioned at an angle with respect to bottom surface segment  50  of element end  14 . 
     FIG. 1C is a schematic close-up side view illustrating the flexible interlocking joint of the interlocking elements following horizontal movement. The flexible interlocking joint, following horizontal movement, generally referenced as  70 , is formed by horizontal or lateral movement of element end  12  with respect to element end  14 . In this illustration, element end  12  is horizontally moved a distance  72 , along tongue tip bottom tangent  56 , where distance  72  is represented by the distance between new tongue tip side tangent  74  and neutral position tongue tip side tangent  62  (of FIG.  1 A), with coordinate system  64  as reference point of horizontal movement. For the horizontal movement of flexible joint  70 , tongue tip bottom tangent  56  is parallel to the x-axis, and new tongue tip side tangent  74  is parallel to y-axis, of coordinate system  64 , respectively. Moreover, for flexible joint  70  in this flexed position following horizontal movement, according to extent of horizontal movement, tongue  34  may be in physical contact with element end  14 . This is indicated by contact point  76 , where bend  24  of tongue  34  is in contact with the surface region of channel  52  of element end  14 . Preferably, following horizontal movement of element end  12  with respect to element end  14 , bottom surface segment  32  of element end  12  lies parallel to and in the same plane as bottom surface  50  segment of element end  14 . 
     FIG. 1D is a schematic close-up side view illustrating the flexible interlocking joint of the interlocking elements following vertical movement. The flexible interlocking joint, following vertical movement, generally referenced as  78 , is formed by vertical movement of element end  12  with respect to element end  14 . In this illustration, element end  12  is vertically moved up a distance  80 , along tongue tip side tangent  62 , where distance  80  is represented by the distance between new tongue tip bottom tangent  82  and neutral position tongue tip bottom tangent  56  (of FIG.  1 A), with coordinate system  64  as reference point of vertical movement. For vertical movement of flexible joint  78 , new tongue tip bottom tangent  82  is parallel to the x-axis, and tongue tip side tangent  62  is parallel to y-axis, of coordinate system  64 , respectively. Moreover, for flexible interlocking joint  78  in this flexed position following vertical movement, according to extent of vertical movement, tongue  34  may be in physical contact with element end  14 . This is indicated by contact point  84  and contact point  86 , where surface region of channel  52  of element end  14  extending from bend  40  to bend  38  is in contact with the surface of tongue  34  of element end  12 . Preferably, following vertical movement of element end  12  with respect to element end  14 , bottom surface segment  32  of element end  12  lies parallel to and in a different plane as bottom surface segment  50  of element end  14 . 
     It is to be noted that flexible interlocking joints  10 ,  66 ,  70 , and  78 , featured components, and different positions of movement or flexibility thereof, as illustrated in FIGS. 1A-1D, are representative of the interlocking elements forming the ground surface cover system of the present invention. Interlocking element top surface regions in continuity with, and extending from top surface segment  16 , or extending from top surface segment  36 , to the opposite element end (not shown in FIGS. 1A-1D) of the same corresponding interlocking element may be of variable configuration, including, but not limited to, level, ridged, or elevated, with variable dimensions. Likewise, interlocking element bottom surface regions in continuity with, and extending from bottom surface segment  32 , or extending from bottom surface segment  50 , to the opposite element end (not shown in FIGS. 1A-1D) of the same corresponding interlocking element may be of variable configuration, including, but not limited to, level, ridged, or elevated, with variable dimensions. 
     FIG. 2A is a schematic side view illustrating part of the system featuring level top and bottom configured elements interlocked by the flexible interlocking joint. The part of the system featuring level top and bottom configured elements interlocked by flexible joints  90  and  92 , is generally referenced as  88 . In this figure, interlocking element top surface segment  94 , in continuity with, and extending from element end top surface segment  16  to element opposite end top surface segment  36  is configured as level. Interlocking element bottom surface segment  96 , in continuity with, and extending from element end bottom surface segment  32  to element opposite end bottom surface segment  50  is also configured as level. In system  88 , flexible joints  90  and  92 , featuring element or joint tongue  34  mechanically engaged to element or joint channel  52  are variably positioned and flexible according to the description provided in FIGS. 1A-1D. 
     FIG. 2B is a schematic side view illustrating part of the system featuring ridged top and bottom configured elements interlocked by the flexible interlocking joint. The part of the system featuring optional ridged top and bottom configured elements interlocked by flexible joints  100  and  102 , is generally referenced as  98 . In this figure, interlocking element top surface segment  104 , in continuity with, and extending from element end top surface segment  16  to element opposite end top surface segment  36  is configured as ridged. Exemplary ridge  106  of ridged configured interlocking element top surface segment  104  may be of variable dimensions and frequency, as described in detail in FIG.  5 A. Interlocking element bottom surface segment  108 , in continuity with, and extending from element end bottom surface segment  32  to element opposite end bottom surface segment  50  is also configured as ridged. Exemplary ridge  110  of ridged configured interlocking element bottom surface segment  108  may also be of variable dimensions, as described in detail in FIG.  5 A. In system  98 , flexible joints  100  and  102 , featuring element or joint tongue  34  mechanically engaged to element or joint channel  52 , are variably positioned and flexible according to the description provided in FIGS. 1A-1D. 
     The presence of ridges along the top surface and/or bottom surface of one or more of the interlocking elements is functional with respect to hydrological, stability, and landscape properties of the ground surface cover system for erosion control. Ridged configured interlocking element top surface segment.  104  enables control of, and affects water flow and water distribution throughout the system of interlocking elements, based on interaction of flowing water with the ridges. Ridged configured interlocking element bottom surface segment  108  enables control of, and improves anchoring of the system of interlocking elements, based on interaction of the ground surface with the ridges  110 . This alternative feature of the interlocking elements of the invention results in a more stable erosion control system with respect to water flow and water distribution during possible ground movement due to an erosion process. Another result of increased stability is better preservation of botanic landscape which may be placed in spaces in between interlocking elements. 
     FIG. 2C is a schematic side view illustrating part of the system featuring an elevated level top and level bottom configured element interlocked by the flexible interlocking joint. The part of the system featuring an optional elevated level top configured element  118  interlocked to a level top and level bottom configured element  120  by flexible joint  114 ) which in turn is interlocked to another level top and level bottom configured interlocking element  122  by flexible joint  116 , is generally referenced as  112 . In this figure, element top surface region  124  of element  118  in continuity with, and extending from element end top surface segment  16  to element opposite end top surface segment  36 , is configured as elevated, and features level top surface segment  125 . Element bottom surface segment  126  of element  118  is shown as level configured, but may be configured as, including, but not limited to, level, ridged, or elevated. Exemplary elevated configured interlocking element top surface region  124  may be of variable dimensions, as described in detail in FIG.  6 A. In system  112 , flexible joints  114  and  116 , featuring element or joint tongue  34  mechanically engaged to element or joint channel  52 , are variably positioned and functional according to the description provided in FIGS. 1A-1D. 
     The presence of an elevated element top surface region of one or more of the interlocking elements is functional with respect to hydrological properties of the ground surface cover system for erosion control. Elevated configured interlocking element top surface region  124  enables control of, and affects water flow and water distribution throughout the system of interlocking elements, based on interaction of flowing water with the elevation. 
     FIG. 3 is a schematic side view illustrating part of the system featuring level top and level bottom configured elements interlocked to a level top and level bottom configured center element, via the flexible interlocking joint. As shown in FIG. 3, center interlocking element  130  features two identical element or joint channels  52  (FIG.  1 A), each being compatible for mechanical engagement via mating or interlocking to an element or joint tongue  34  (FIG. 1A) of another interlocking element. The part of the system featuring a level top configured center element  130  interlocked to a first, level top and level bottom configured element  132  by flexible joint  136 , and interlocked to a second, level top and level bottom configured interlocking element  134  by flexible joint  138 , is generally referenced as  128 . In this figure, center element top surface segment  140 , and center element bottom surface segment  142 , of center element  130 , are each configured as level, but each center element surface segment  140  or  142  may be configured as, including, but not limited to, level, ridged, or elevated, in accordance with the descriptions of FIGS. 2A-2C. Exemplary level configured interlocking element  130  may be of variable dimensions, as described in detail in FIG.  7 . In system  128 , flexible joints  136  and  138 , featuring element or joint tongue  34  mechanically engaged to element or joint channel  52 , are variably positioned and functional according to the description provided in FIGS. 1A-1D. 
     In addition to being another interlocking element of the ground surface cover system, center interlocking element  130  is uniquely functional with respect to enabling convenient and efficient installation of a series of interlocking elements along the bottom, and along both sides, of ground featuring a double incline, as illustrated and described in FIG.  11 . 
     FIG. 4A is a schematic view illustrating one side of a level top and level bottom configured interlocking element. Exemplary interlocking element  144  may be of variable overall element length  148  and of variable overall element height  146 . The contour of side  150  of interlocking element  144  includes element end level top surface segment  16 , extends outward and down past bend  18 , features element or joint tongue  34 , in continuity with, and extending down and around to element level bottom surface segment  32 , an element opposite end level top surface segment  36 , extends outward and down past bend  33 , features element or joint channel  52 , in continuity with, and extending down and around to element level bottom surface segment  50 , an element middle level top surface segment  94 , in continuity with, and extending between element end level top surface segments  16  and  36 , and an element middle level bottom surface segment  96 , in continuity with, and extending between element end level bottom surface segments  32  and  50 . Side  150  of element  144  features element level top surface segments  36 ,  94 , and  16 , all positioned in a same plane, and element level bottom surface segments  50 ,  96 , and  32 , all positioned in a different same plane, whereby the plane of element top surface segments is parallel to the plane of element bottom surface segments, with coordinate system  64  as reference. 
     FIG. 4B is a schematic view illustrating the top of level top and bottom configured interlocking element  144  of FIG.  4 A. Top  152  of interlocking element  144  includes element level top surface regions  36 ,  94 , and  16 , and top profiles of surface regions of element or joint tongue  34  and element or joint channel  52 , corresponding to side  150  of FIG.  4 A. Top  152  of exemplary interlocking element  144  features element width  154 , element half-length  147 , each of variable dimensions, and element side  156  opposite to element side  150  shown in FIG.  4 A. 
     In a preferred alternative embodiment of the level top and level bottom interlocking element of the present invention, an element side, for example, element side  150  as shown in FIG. 4B, features optional pin groove  158 , preferably located along the center of element side  150  at element half-length  147 , of variable geometry and dimensions, preferably configured as an open trapezoid, spanning element volume vertically along entire element height  146  of element side  150  of level top and level bottom interlocking element  144 . Pin groove  158  provides space for optional insertion of a pin (not shown), starting from the top opening of pin groove  158  and positioned vertically downward along the side of one interlocking element, or starting from the top opening of pin groove  158  and positioned vertically downward between the sides of two adjacent interlocking elements, respectively, of the ground surface cover system. The optional use of pins along the interlocking elements is primarily for increased holding strength and stability of those elements positioned at the top, bottom, or critical locations, of inclined ground, where such elements maintain a larger load of other interlocking elements of the system, as is further illustrated and described in FIGS. 9-11. 
     In another preferred alternative embodiment of the interlocking elements of the present invention, an element side, for example, element side  150 , as shown in FIG. 4B, features optional water channel  160 , preferably located along the center of element side  150  at element half-length  147 , of variable geometry and dimensions, preferably configured as an open half donut, spanning element volume along part of element height  146  along element side  150  of level top and level bottom interlocking element  144 . Water channel  160  functions to channel or trap water, enabling additional control of water flow and distribution throughout the erosion control system of interlocking elements during conditions of rainfall. 
     FIG. 4C is a schematic side view illustrating alternative optional features of the level top and bottom configured interlocking element  144  of FIGS. 4A-4B. Optional pin groove  158 , and optional water channel  160  are shown configured as part of element side  150 . Optional pin groove  158  spans element volume vertically along entire element height  146 , and optional water channel  160  spans element volume along part of element height  146  of level top and level bottom interlocking element  144 . Element level bottom surface segment  162  of element side  150  corresponds to element level bottom surface segments  50 ,  96 , and  32 , of element  144  (FIG.  4 A). 
     FIG. 4D is a perspective view of level top and bottom configured interlocking element  144  of FIGS. 4A-4C, featuring element or joint tongue  34  pointing downward, and element or joint channel  52  pointing upward. The upper outer surface contour of joint tongue  34 , extending outward and sloping downward from element surface bend  18  (FIG. 1) to joint tongue side lip  60  (FIG.  1 ), is of variable geometry, preferably, but not limited to, polygonal stepped, but may also be curved and smooth. Of polygonal stepped geometry, joint tongue surface steps  166 , separated and bordered by joint tongue surface step edges  168 , are preferably level and rectangular in shape having variable step width  170  and variable step number, e.g., shown here are three joint tongue surface steps  166 , extending parallel to and along entire element width  154 , from element surface bend  18  to element surface bend  164 , of element  144 . Optional pin, groove  158 , and optional water channel  160  are shown as part of side  150  of element  144 . 
     FIG. 4E is a perspective view of level top and bottom configured interlocking element  144  of FIGS. 4A-4C, featuring element or joint tongue  34  pointing upward, and element or joint channel  52  pointing downward. FIG. 4E shows element  144  of FIG. 4D turned over. The upper outer surface contour of joint channel  52 , extending outward and sloping downward from element surface bend  48  (FIG. 1) to joint channel bend  46  (FIG.  1 ), is of variable geometry, preferably, but not limited to, polygonal stepped, but may also be curved and smooth. Of polygonal stepped geometry, joint channel surface steps  172 , separated and bordered by joint channel surface step edges  174 , are preferably level and rectangular in shape having variable step width  176  and variable step number, e.g., shown here are three joint channel surface steps  172 , extending parallel to and along entire element width  154 , from element surface bend  48  to element surface bend  178 , of element  144 . Perspective side views of optional pin groove  158 , and optional water channel  160  are shown as part of side  150  of element  144 . 
     The functionality of the downward sloping surface contours of element or joint tongue  34  and element or joint channel  52  is for enabling water drainage down and along the outer surfaces of the interlocking elements. For a ground surface cover system featuring a pattern of several interlocking elements of the present invention, the downward sloping contours of a multitude of interlocked flexible joint tongues  34  and joint channels  52  forms extended lanes for which water can freely flow, in a guided manner according to the particular system geometric pattern and ground topography. 
     The perspective views of element  144  described and shown in FIGS. 4D and 4E are exemplary, whereby features, components, configurations, geometries, and relative positioning thereof, relating to element or joint tongue  34 , element or joint channel  52 , sides  150  and  156 , optional pin groove  158 , and optional water channel  160 , are applicable to the other interlocking elements of the present invention. 
     FIG. 5A is a schematic view illustrating one side of a ridged top and bottom configured interlocking element. Exemplary ridged top and ridged bottom interlocking element  180  may be of variable overall element length  198 , element half-length  197 , and of variable overall element height  200 . The contour of element side  194  of ridged interlocking element  180  includes element end level top surface segment  16 , extends outward and down past bend  18 , features element or joint tongue  34 , in continuity with, and extending down and around to element level bottom surface segment  32 , an element opposite end level top surface segment  36 , extends outward and down past bend  38 , features element or joint channel  52 , in continuity with, and extending down and around to element level bottom surface segment  50 , an element middle ridged top surface segment  104 , in continuity with, and extending between element end level top surface segments  16  and  36 , and an element middle ridged bottom surface segment  108 , in continuity with, and extending between element end level bottom surface segments  32  and  50 . Side  194  of ridged element  180  features element top surface segments  36 ,  104 , and  16 , all positioned in a same plane, and element bottom surface segments  50 ,  108 , and  32 , all positioned in a different same plane, whereby the plane of element top surface segments is parallel to the plane of element bottom surface segments, with coordinate system  64  as reference. 
     In FIG. 5A, element middle ridged top surface segment  104  is of variable length extending between element level top surface segment  36  to element level top surface segment  16 . Ridged top surface segment  104  features ridges  106  of variable dimensions, including ridge upper segment length  182 , ridge lower segment length  184 , ridge height  186 , and ridge segment angles  188 ,  190 , and  192 . Oppositely positioned element middle ridged bottom surface segment  108  is of variable length extending between element level bottom surface segment  32  to element level bottom surface segment  50 . Ridged bottom surface segment  108  features ridges  110  of variable dimensions (not referenced), similar to the dimensions of ridged top surface segment  104 , including ridge upper segment length, ridge lower segment length, ridge height, and ridge segment angles. Preferably, element top surface ridges  106 , and element bottom surface ridges  110 , are parallel to each other, along the x-axis of reference coordinate system  64 , throughout length  198  of ridged element  180 . 
     In a preferred alternative embodiment of the present invention, ridged top and ridged bottom interlocking element  180  features optional pin groove  196  (shown in FIG. 5A as dashed lines, representing position of the pin groove in the plane of the page, as part of element side  193  located opposite to element side  194 , shown in FIG.  5 B), preferably located along the center of element side  193  at element half-length  197 , of variable geometry and dimensions, and preferably configured as an open trapezoid, spanning vertically along element height  200  of ridged interlocking element  180 . Similar to the preferred alternative embodiment of level top and level bottom interlocking element  144  of FIG. 4B, pin groove  196  provides space for optional insertion of a pin (not shown), starting from the top opening of pin groove  196  and positioned vertically downward along the side of one interlocking element, or starting from the top openings and positioned vertically downward between the sides of two adjacent interlocking elements, respectively, of the ground surface cover system. 
     FIG. 5B is a schematic view illustrating the top of the ridged top and bottom configured interlocking element  180  of FIG.  5 A. Top surface of exemplary ridged interlocking element  180  includes element ridged top surface region  104  featuring ridges  106 , element level top surface regions  36  and  16 , and top profiles of surface regions of element or joint tongue  34  and element or joint channel  52 , corresponding to side  194  of FIG.  5 A. Preferably, element top surface ridges  106  are parallel to each other, along the x-axis of reference coordinate system  64 , throughout length  198  of ridged element  180 . Top surface of ridged interlocking element  180  features element width  204  of variable dimension, and element opposite side  193  featuring pin groove  196 , located opposite to element side  194 . 
     FIG. 6A is a schematic view illustrating one side of an elevated level top and level bottom configured interlocking element. In this alternative preferred embodiment, exemplary interlocking element  208  may be of variable overall element length  210 , element half-length  209 , and of variable overall element height  212 . The contour of side  214  of interlocking element  208  includes element end level top surface segment  16 , extends outward and down past bend  18 , features element or joint tongue  34 , in continuity with, and extending down and around to element level bottom surface segment  32 , an element middle level bottom surface segment  96 , in continuity with, and extending between element end level bottom surface segments  32  and  50 , extends up and around element or joint channel  52 , in continuity with, and extending up and around bend  38  to element opposite end level top surface segment  36 , extends around bend  216 , Up and around bend  218 , along element elevated level top surface segment  125 , around bend  220 , down and around bend  222 , and extends back to element level top surface segment  16 . 
     In FIG. 6A, element elevated level top surface segment  125  extends between element end level top surface segments  16  and  36 . Element elevated level top surface region  124  is of variable geometry with variable dimensions. Element elevated level top surface region  124  is preferably, but not limited to, a rectangle of elevated top length  224  and elevated top height  226 . Element side  214  of element  208  includes element level bottom surface segments  32 ,  96 , and  50 , positioned in a first same plane, element level top surface segments  16  and  36 , positioned in a second same plane, and element elevated level top surface segment  125  positioned in a third plane, whereby all three planes of surface segments are parallel to each other, with coordinate system  64  as reference. 
     In a preferred alternative embodiment of the elevated level top and level bottom interlocking element of the present invention, element  208  features optional pin groove  226  (shown in FIG. 6A as dashed lines, representing position of the pin groove in the plane of the page, as part of element side  213  located opposite to element side  214 ), preferably located along the center of element side  214  at element half-length  209 , of variable geometry and dimensions, and preferably configured as an open trapezoid, spanning vertically along element height  212  of elevated level top interlocking element  208 , and having the same function of providing space for optional insertion of a pin (not shown), starting from the top opening of pin groove  226  and positioned vertically downward along the side of one interlocking element, or starting from the top opening of pin groove  226  and positioned vertically downward between the sides of two adjacent interlocking elements, respectively, of the ground surface cover system, as described for level top and level bottom interlocking element  144  of FIG. 4B, and for ridged top and ridged bottom interlocking element  180  of FIG.  5 A. 
     FIG. 6B is a schematic view illustrating the top of elevated level top and level bottom configured interlocking element  208  of FIG.  6 A. Top surface of exemplary elevated level top and level bottom interlocking element  208  includes elevated level top surface region  125 , top profiles of surface segments extending from bend  216  to bend  218 , and extending from bend  220  to bend  222 , element level top surface regions  36  and  16 , and top profiles of surface regions of element or joint tongue  34  and element or joint channel  52 , corresponding to side  214  of FIG.  6 A. Top surface of elevated level top interlocking element  208  features element width  228  of variable dimension, and element opposite side  213  featuring optional pin groove  226 , located opposite to element side  214  as shown in FIG.  6 A. Preferably, element top surface segments  216 ,  218 ,  220 , and  222 , and element top surface segments formed by extension of each bend  38  and bend  18  across width  228  of element  208 , are parallel to each other, along the x-axis of reference coordinate system  64 , throughout element length  210  of elevated level top element  208 . 
     FIG. 7 is a schematic view illustrating one side of a level top and bottom configured center interlocking element. Exemplary center interlocking element  130  features two identical element or joint channels  52 , each being compatible for mechanical engagement via mating or interlocking to an element or joint tongue  34  of another interlocking element. Center element  130  may be of variable overall element length  232  and of variable overall element height  234 . The contour of element side  236  of center element  130  includes two element end level top surface segments  36 , each extending outward and down past bend  38 , features two element or joint channels  52 , each in continuity with, and extending down and around to element level bottom surface segment  50 , an element middle level top surface segment  94 , in continuity with, and extending between element end level top surface segments  36 , and an element middle level bottom surface segment  96 , in continuity with, and extending between element end level bottom surface segments  50 . Element side  236  of element  130  features element level top surface segments  36  and  94  positioned in a same plane, and element level bottom surface segments  50  and  96  positioned in a different same plane, whereby the plane of element top surface segments is parallel to the plane of element bottom surface segments, with coordinate system  64  as reference. 
     In a preferred alternative embodiment of the present invention, level top and level bottom center interlocking element  130  features optional pin groove  238  (shown in FIG. 7 as dashed lines, representing position of the pin groove in the plane of the page, as part of element side located opposite to element side  236 ), preferably located along the center of element side  236  at element half-length  231 , of variable geometry and dimensions, and preferably configured as an open trapezoid, spanning vertically along element height  234  of level top and level bottom center interlocking element  130 , and having the same function of providing space for optional insertion of a pin (not shown), as previously described and shovel in FIGS. 4B-6B. 
     FIG. 8 is a schematic sequential series of side views illustrating a method of interlocking the elements via the flexible interlocking joint. In the sequential series of side views  240 A through  240 E illustrating a preferred method of interlocking the elements via the flexible joint of the present invention, exemplary level top and level bottom interlocking element  242  featuring tongue  34  is to be mechanically engaged or interlocked to exemplary level top and level bottom interlocking element  244  featuring channel  52 . Channel  52  of element  244  is appropriately contoured for insertion or mechanical engagement of tongue  34  of element  242 . Insertion or engagement of tongue  34  into channel  52  is limited to a small range of positions and angles of tongue  34  relative to channel  52 , according to actual relative shapes and dimensions of tongue  34  and channel  52 . In particular, the objective is to insert tongue  34 , having a configuration featuring tongue surface region contour extending along bends  26 ,  24 ,  22 , and  19 , with a widest chord  246  extending between bend  24  and bend  19 , into channel  52 , having a configuration featuring channel surface region contour extending along bends  46 ,  44 ,  43 , and  38 , with an opening chord  248  extending between bend  44  and bend  38 . 
     The method of insertion of tongue  34  of element  242  into channel  52  of element  52  is straightforward and is based on positioning element  242  through a sweeping range of decreasing angles  250  such to enable mechanical engagement of the elements, where angle  250  is the angle formed between tongue tip bottom tangent  56  (FIG. 1) and line  57 , where line  57  is parallel to the x-axis of reference coordinate system  64 . The process of inserting tongue  34  of element  242  into channel  52  of element  244  until mechanical engagement is attained, is continued until angle  250  is approximately zero, where in such position, tongue tip bottom tangent  56  is parallel to and in the same plane as line  57 . The process of mechanical engagement or interlocking opposing ends of a pair of opposing interlocking elements is completely reversible i.e., mechanical disengagement or unlocking opposing ends of a pair of interlocked interlocking elements is readily accomplished by reversing the above process, with reference to the reverse of the sequence illustrated FIG.  8 . This reversible process is sequentially illustrated in the series of side views  240 A through  240 D. 
     Side view  240 E illustrates an extreme horizontal position of element  242  interlocked to element  244  via the flexible joint. Following completion of initial engagement of the elements, side view  240 D, elements  242  and  244  are flexed or moved horizontally with respect to each other along the plane of the x-axis of reference coordinate system  64 , such that surface segment of tongue  34 , extending between bends  24  and  26 , is in substantial physical contact with surface segment of channel  52 , extending between bends  43  and  44 , as described and illustrated in FIG.  1 C. This method of mechanical engagement or interlocking of elements is applicable to all elements featured in this invention. 
     FIGS. 9A through 9C are schematic top views of different preferred embodiments of patterns of the system of interlocking elements featuring flexible interlocking joints, and corresponding methods of forming the different patterns. Patterns  262  through  266  could feature any combination of the various configurations of level, ridged, elevated, or center, interlocking elements already described and illustrated in this invention, however, for illustrative purposes, exemplary level top and level bottom interlocking elements  144  described and illustrated in particular, in FIGS. 2A, and  4 A- 4 E, are referred to here. Optional pin groove  158  of interlocking element  144  is shown throughout the different patterns of the system in FIGS. 9A-9C for illustrative purpose only, and its presence is not meant to be limiting with respect to the present invention. The detailed method of mechanically engaging or interlocking individual elements is applicable here, and includes the description and illustrations related to FIG.  8 . Moreover, the method of mechanically engaging or interlocking individual elements is completely reversible, whereby, patterns of the system of interlocked elements can be partly, or completely, taken apart by mechanically disengaging or unlocking the interlocked elements via the flexible interlocking joint. 
     FIG. 9A is a schematic top view of the system of interlocking elements, and the method of forming a closed, non-staggered pattern. Exemplary closed pattern  262  of interlocking elements features rows  252 A through  252 D of interlocking elements, and columns  254 A through  254 D of interlocking elements. 
     According to relative directions and geometries of the features and components of interlocking elements  144  shown in the side, top, and perspective views of FIGS. 2A, and  4 A- 4 E, using coordinate system  64  as reference, right end row  252 A features element tongues  34  exposed and non-interlocked and element channels  52  interlocked to element tongues  34  of adjacent row  252 B, middle rows  252 B and  252 C feature element tongues  34  and element channels  52  interlocked and mechanically engaged to corresponding element channels  52  and element tongues  34 , of corresponding adjacent rows, and end row  252 D features element tongues  34  interlocked to element channels  52  of adjacent row  252 C and element channels  52  unoccupied and non-interlocked. In pattern  262 , rows  252 A through  252 D are adjacent to each other in that row interfaces  256  feature tongues  34  interlocked to channels  52 , thereby, forming rows of the flexible interlocking joint of the present invention. 
     A preferred method of forming the system featuring closed, non-staggered pattern  262  is by initially forming row  252 A, featuring tongues  34  facing outside, and exposed and non-interlocked, by placing element sides  150  and  156  of interlocking elements  144  immediately adjacent to each other, leaving no space between them. Following completion of closed pattern row  252 A, additional rows  252 B through  252 D are formed by interlocking or mechanically engaging entire width  154  (FIG. 4D) of tongue  34  of each added interlocking element  144  to an entire width  154  of channel  52  of one other interlocking element  144  of a previous row, until a new row is complete, thereby forming columns  254 A through  254 D, such that all element side to element side interfaces  258  of a given column of interlocking elements  144  are positioned parallel to each other and in the same vertical plane, with respect to reference coordinate system  64 . 
     FIG. 9B is a schematic top view of the system of interlocking elements, and the method of forming a closed, staggered pattern. Pattern  264  shown in FIG. 9B is closed, as described and shown for pattern  262  in FIG. 9A, whereby rows  252 A through  252 D feature element sides  150  and  156  of exemplary interlocking elements  144  immediately adjacent to each other, without space between them, thereby forming element to element interfaces  258  extending across each row. 
     A preferred method of forming the system featuring closed, staggered pattern  264  is by initially forming row  252 A, featuring tongues  34  facing out, exposed and non-interlocked, by placing element sides  150  and  156  of interlocking elements  144  immediately adjacent to each other, leaving no space between them. Following completion of closed pattern row  252 A, additional rows  252 B through  252 D are sequentially formed by interlocking or mechanically engaging each tongue  34  of interlocking elements  144  to two separate channels  52  of two adjacent interlocking elements  144 . According to this method, separate and distinguishable sets of columns, i.e., columns  254 A through  254 D, and columns  255 A through  255 C, are formed such that element side to element side or column interfaces  258  of each of the two formed sets of columns of interlocking elements  144  are horizontally located in alternate rows  252 A through  252 D, positioned parallel to each other, and in the same plane, with respect to reference coordinate system  64 . 
     Staggered pattern  264  illustrated in FIG. 9B is periodic, whereby staggered positions of interlocking elements  144  are periodic in alternating rows, e.g., positions of element sides  150  and  156 , and interfaces  258  in row  252 A are in the same x-axis planes as positions of element sides and interfaces in row  252 C, and likewise for alternating rows  252 B and  252 D. This represents a special case of staggered patterns of the system of interlocking elements of the present invention, where, in general, the staggered patterns of interlocking elements need not be periodic. 
     FIG. 9C is a schematic top view of the system of interlocking elements, and the method of forming an open, staggered pattern. Pattern  266  shown in FIG. 9C is open, whereby rows  252 A through  252 D feature a number of interlocking elements and corresponding element sides  150  and  156  of exemplary interlocking elements  144  spaced apart, forming variable sized rectangular regions  268  surrounded by a variable number of elements, according to specific location of a given region  268 . As a result of featuring openings in the system of interlocking elements, pattern  266  is staggered, in accordance with the description of FIG.  9 B. 
     A preferred method of forming the system featuring open, staggered pattern  266  is by placing element sides  150  and  156  of interlocking elements  144  at variable distances from each other, leaving variable spaces between selected elements  144 , and interlocking or mechanically engaging each tongue  34  of interlocking elements  144  to two separate, not necessarily equal parts of channels  52  of two interlocking elements  144 , thereby forming non-periodic rows  252 A through  252 D, of elements, where elements  144  are positioned in variable x-axis planes with respect to reference coordinate system  64 . 
     Features and capabilities of flexibility and directional movement of the flexible interlocking joint of the present invention (FIGS. 1A-1D) are all applicable to the interlocking elements and joints formed thereof in the different preferred embodiments of patterns of the ground surface cover system of erosion control described and illustrated in FIGS. 9A-9C. Applying the property of flexibility of the interlocking joints to the installation and use of the interlocking elements featured in the different system patterns provides significant capability of custom designing an effective ground surface cover system for erosion control for a wide variety of erosion prone ground surface topographies. This translates to achieving the main objective of effectively controlling or minimizing ground movement during a potential erosion process, by designing an erosion control system which provides high strength and long term stability, patterns for efficient water flow and water distribution, flexible adjustment to ground movement, capability of including landscape, economic and feasible manufacturing and installation, replaceability, and reusability. 
     Flexibility of individual pairs of interlocked elements is directly scalable to flexibility of an overall ground surface cover erosion control system. For example, system pattern  262  of FIG. 9A should be well suited to erosion prone ground surface requiring a firm, closed, non-staggered, but flexible surface cover, whereas, system pattern  266  of FIG. 9C should be well suited to erosion prone ground surface featuring botanic landscape, where, in addition to providing space for inclusion of botanic landscape in between the interlocking elements of the ground surface cover system, it is desirable that at least part of the water flow be directed into the ground in the regions of botanic growth. 
     FIG. 10 is a side view diagram of the ground surface cover system of interlocking elements, as applied in practice to an exemplary single inclined ground surface featuring a cavity and a protrusion, illustrating flexibility of the system. Ground surface cover system  270  features ground region  272  with ground surface region  274  requiring erosion control. FIG. 10 shows a side view diagram, with coordinate system  64  as reference, of ground surface region  274  spanning along xz-planes of a single continuous incline of incline height  276  in the y-direction, initially absent of any noticeable cavity or protrusion along the xz-plane or y-direction. System  270  of a layer of exemplary level top and level bottom interlocking elements  144  featuring flexible joints  10 ,  66 ,  70 ,  78  (FIGS. 1A-1D) of the present invention covers ground surface region  274  spanning xz-planes along the y-direction incline. System  270  includes a bottom row, in the z-direction, of bottom end interlocking elements  278  attached by element tongues  34  to bottom end rigid non-mobile foundation  280 , where foundation  280  is preferably made of, but not limited to, concrete, metal, or a combination thereof, both  278  and  280  being in contact with bottom end ground surface region  282 , and system  270  includes a top row, in the z-direction, of top end interlocking elements  284  attached by element channels  52  to top end rigid non-mobile foundation  286 , where foundation  286  is preferably made of, but not limited to, concrete, metal, or a combination thereof, both  284  and  286  being in contact with top end ground surface region  288 . Interlocked elements form a continuous series of interlocked rows, positioned in xz-planes, spanning the y-direction incline of ground surface region  274 , in between bottom end interlocking elements  278  and top end interlocking elements  284 , featuring at least one selected pattern, for example, closed non-staggered, closed staggered, or open staggered, in accordance with the description and illustrations of FIGS. 9A-9C. 
     In FIG. 10, in an alternative preferred embodiment of ground surface cover system  270 , optional pins  290  are positioned through pin grooves  158 , in accordance with description and illustrations of FIGS. 4A-4E, in between the sides of selected interlocking elements, for example, bottom end interlocking elements  278 , top end interlocking elements  284 , and intermediate interlocking elements  292  and  294 , through ground surface region  274  and ground region  272 , located at selected positions along ground surface region  274 , for example,  282 ,  288 ,  296 , and  298 , respectively, requiring additional stability of ground surface cover system  270  for effective erosion control. 
     In FIG. 10, in another alternative preferred embodiment of ground surface cover system  270 , optional botanic landscape (not shown) is positioned in spaces, in between the sides of selected interlocking elements, along ground surface region  274 , in accordance with description and illustration of open pattern  266  in FIG.  9 C. 
     With reference to FIG. 10, following is a preferred method of establishing ground surface cover system  270  of a layer of exemplary level top and level bottom interlocking elements  144  featuring flexible joints  10 ,  66 ,  70 ,  78  (FIGS. 1A-1D) of the present invention. System  270  is constructed, on-site, upon ground surface region  274 , preferably starting at bottom end ground surface region  282 , as part of ground region  272  requiring erosion control. First row of a layer of interlocking elements  278  is placed on bottom end ground surface region  282  in the z-direction, and these interlocking elements are attached to foundation  280 , where foundation  280  is preferably made of, but not limited to, concrete, metal, or a combination thereof, both  278  and  280  placed in contact with bottom end ground surface region  282 . First row interlocking elements are preferably placed with element tongues  34  attached to foundation  280 , enabling first row element channels  52  to be flexibly interlocked to element tongues  34  of second row of interlocking elements  300 . Second row of a layer of interlocking elements  300  is flexibly interlocked to first row  278 , by using the preferred method of interlocking elements with flexible joints in accordance with the description and illustration of FIG.  8 . Henceforth, in similar manner, a continuous series of rows, featuring at least one selected pattern, for example, closed non-staggered, closed staggered, or open staggered, positioned along xz-planes, along the y-direction incline of ground surface region  274 , in accordance with the description and illustrations of FIGS. 9A-9C, of interlocking elements is constructed until reaching top end ground surface region  288 , at which a last row of interlocking elements  284  is attached to rigid and non-mobile foundation  286 , where foundation  286  is preferably made of, but not limited to, concrete, metal, or a combination thereof, both  284  and  286  placed in contact with top end ground surface region  288 . 
     With reference to FIG. 10, in an alternative preferred embodiment of the method of forming ground surface cover system  270 , optional pins  290  are placed in between and along the sides of selected interlocking elements, for example, bottom end interlocking elements  278 , top end interlocking elements  284 , and intermediate interlocking elements  292  and  294 , through ground surface region  274  and ground region  272 , located at selected positions along ground surface region  274 , for example,  282 ,  288 ,  296 , and  298 , respectively, requiring additional stability of ground surface cover system  270  for effective erosion control. 
     With reference to FIG. 10, in another alternative preferred embodiment of the method of forming ground surface cover system  270 , optional botanic landscape (not shown) is placed in spaces in between the sides of selected interlocking elements, along ground surface region  274 , in accordance with description and illustration of pattern  266  in FIG.  9 C. 
     FIG. 10 also illustrates different realistic scenarios of the functionality of ground surface cover system  270  following topological changes of ground surface region  274  due to localized movement of ground region  272 . In the event of formation of cavity  302  and/or protrusion  304  at localized places  306  and/or  304 , respectively, in ground surface region  274 , system  270  of interlocking elements remains intact, in a flexible, interlocked mode. Interlocking elements  308  and/or interlocking elements  310 , in the immediate vicinity of cavity  302  and/or protrusion  304 , respectively, undergo directional movement, including angular, horizontal, and/or vertical, in accordance with descriptions and illustrations of FIGS. 1A-1D, according to the particular nature, directionality, and dimensions of formation of cavity  302 , and/or protrusion  304 . Interlocking elements  308  and/or interlocking elements  310 , respectively, of ground surface cover system  270  in the immediate vicinity of cavity  302  and/or protrusion  304 , respectively, are amenable to adjustment, via addition or subtraction, of interlocking elements. Alternatively, localized places  306  and/or  304  of cavity and/or protrusion formation, respectively, may be adjusted by addition of ground or ground filler material, and/or subtraction of ground, thereby, returning ground surface region  274  to its original level inclined form absent of cavities or protrusions, enabling re-establishment of stable and effective erosion control ground surface cover system  270 . 
     FIG. 11 is a side view diagram of the ground surface cover system of interlocking elements, as applied in practice to an exemplary double inclined ground surface. Ground surface cover system  312  features ground surface region  314  requiring erosion control. FIG. 11 shows a side view diagram, with coordinate system  64  as reference, of ground surface region  314  spanning along xz-planes of a continuous double incline with first incline ground surface region  314 A and second incline ground surface region  314 B, featuring first and second incline ground surface region heights  316  and  318 , respectively, each in the y-direction, and double incline bottom ground surface region  314 C, where ground surface region  314  is absent of any noticeable cavity or protrusion along the xz-plane or y-direction. 
     In FIG. 11, system  312  of a layer of exemplary level top and level bottom interlocking elements  144  featuring flexible joints  10 ,  66 ,  70 ,  78  (FIGS. 1A-1D) of the present invention covers ground surface region  314  spanning xz-planes along the y-direction of the continuous double incline. System  312  includes, in the z-direction, double incline bottom row of center interlocking elements  320  (refer to center interlocking element  130 , as described and illustrated in FIGS. 3 an  7 ) attached from first channels  52 A of double incline bottom center interlocking elements  320  to tongues  34 A of first row of first incline interlocking elements  322 , and attached from second channels  52 B of same double incline bottom center interlocking elements  320  to tongues  34 B of first row of second incline interlocking elements  324 . System  312  also includes, in the z-direction, last row of first incline interlocking elements  326  attached from element channels  54 C to first incline rigid non-mobile foundation  328 , where foundation  328  is preferably made of, but not limited to, concrete, metal, or a combination thereof, both  326  and  328  being in contact with first incline ground surface region  314 D, and system  312  includes, in the z-direction, last row of second incline interlocking elements  330  attached from element channels  52 D to second incline rigid non-mobile foundation  332 , where foundation  332  is preferably made of, but not limited to, concrete, metal, or a combination thereof, both  330  and  332  being in contact with second incline ground surface region  314 E. Interlocked elements form a continuous series of interlocked rows, positioned in xz-planes, spanning the y-direction of first incline ground surface region  314 A and the y-direction of second incline ground surface region  314 B of ground surface region  314 , in between last row of first incline interlocking elements  326  and last row of second incline interlocking elements  330 , featuring at least one selected pattern, for example, closed non-staggered, closed staggered, or open staggered, in accordance with the description and illustrations of FIGS. 9A-9C. 
     In FIG. 11, in an alternative preferred embodiment of ground surface cover system  312 , optional pins  290  are positioned through pin grooves  158 , in accordance with description and illustrations of FIGS. 4A-4E, in between the sides of selected interlocking elements, for example, first incline row of interlocking elements  334 , second incline rows of interlocking elements  336  and  338 , through first incline ground surface region  314 A and second incline ground region  314 B, respectively, located at selected positions along double incline ground surface region  314  requiring additional stability of ground surface cover system  312  for effective erosion control. 
     In FIG. 11, in another alternative preferred embodiment of ground surface cover system  312 , optional botanic landscape (not show) is positioned in spaces, in between the sides of selected interlocking elements, along double incline ground surface region  314 , in accordance with description and illustration of open pattern  266  in FIG.  9 C. 
     With reference to FIG. 11, following is a preferred method of establishing ground surface cover system  312  of a layer of exemplary level top and level bottom interlocking elements  144  featuring flexible joints  10 ,  66 ,  70 ,  78  (FIGS. 1A-1D) of the present invention. System  312  is constructed, on-site, upon double incline ground surface region  314 , preferably starting at double incline bottom ground surface region  314 C, as part of ground surface region  314  requiring erosion control. Double incline bottom row of center interlocking elements  320  is placed on level ground surface  340  along double incline bottom ground surface region  314 C, in the z-direction. Double incline bottom row of center interlocking elements  320  is attached from first channels  52 A of double incline bottom center interlocking elements  320  to tongues  34 A of first row of first incline interlocking elements  322 , by using the preferred method of interlocking elements with flexible joints in accordance with the description and illustration of FIG.  8 . Henceforth, in similar manner, a continuous series of rows, featuring at least one selected pattern, for example, closed non-staggered, closed staggered, or open staggered, positioned along xz-planes, along the y-direction of first incline ground surface region  314 A, in accordance with the description and illustrations of FIGS. 9A-9C, of interlocking elements is constructed until reaching last row of first incline interlocking elements  326  of first incline ground surface region  314 D, at which last row of first incline interlocking elements  326  is attached to first incline rigid and non-mobile foundation  328 , where foundation  328  is preferably made of, but not limited to, concrete, metal, or a combination thereof, both  326  and  328  placed in contact with first incline ground surface region  314 D. 
     Then, double incline bottom row of center interlocking elements  320  is attached from second channels  52 B of double incline bottom center interlocking elements  320  to tongues  34 B of second row of first incline interlocking elements  324 , by using the preferred method of interlocking elements faith flexible joints in accordance with the description and illustration of FIG.  8 . Henceforth, in similar manner, a continuous series of rows, featuring at least one selected pattern, for example, closed non-staggered, closed staggered, or open staggered, positioned along xz-planes, along the y-direction of second incline ground surface region  314 B, in accordance with the description and illustrations of FIGS. 9A-9C, of interlocking elements is constructed until reaching last row of second incline interlocking elements  330  of second incline ground surface region  314 E, at which last row of second incline interlocking elements  330  is attached to second incline rigid and non-mobile foundation  332 , where foundation  332  is preferably made of, but not limited to, concrete, metal, or a combination thereof, both  330  and  332  placed in contact with second incline ground surface region  314 E. 
     The preferred embodiment of the method of forming ground surface cover system  312  with reference to FIG. 11, clearly illustrates the advantageous functionality of center interlocking elements  320 , whereby center interlocking elements  320  feature two element channels  52 A and  52 B. In the case of a ground surface region featuring a double incline, such as ground surface region  314 , two ground surface inclines  314 A and  314 B are covered by interlocking elements. Interlocking elements  144  of the present invention feature one tongue  34  and one channel  52 . If, instead of double incline bottom center elements  320 , interlocking elements  144  were used, such that double incline bottom elements featured one end having an element channel and another end having an element tongue, formation of one of the two first rows of interlocking elements interlocked to the double incline bottom elements would begin with tongues  34  of one of the ends of the double incline bottom elements interlocked to the channels  52  of the interlocking elements of one of the two first rows. Accordingly, due to the topography of the bottom of the double incline of ground surface region  314 , in order to interlock or mechanically engage one of the two first rows of interlocking elements to the row of double incline bottom interlocking elements, on-site at the bottom of the double incline, it would be necessary to remove or dig out ground from underneath tongues  34  of the double incline bottom elements for proper angular positioning of channels  52  for interlocking to tongues  34  of the corresponding opposing interlocking elements, thereby forming flexible interlocking joints, in accordance with the preferred method of interlocking elements of the present invention. Using center interlocking elements  320  precludes the need for ground removal and therefore bypasses this limitation of forming ground surface cover system  312  of the present invention, for effective erosion control. 
     With reference to FIG. 11, in an alternative preferred embodiment of the method of forming ground surface cover system  312 , optional pins  290  are placed in between and along the sides of selected interlocking elements, for example, next to last row of first incline interlocking elements  334 , next to last row of second incline interlocking elements  338 , and intermediate row of second incline interlocking elements  336 , through first incline ground surface region  314 A and second incline ground surface region  314 B, respectively, located at selected positions along ground surface region  314  requiring additional stability of ground surface cover system  312  for effective erosion control. 
     With reference to FIG. 11, in another alternative preferred embodiment of the method of forming ground surface cover system  312 , optional botanic landscape (not shown) is placed in spaces in between the sides of selected interlocking elements, along ground surface region  314 , in accordance with description and illustration of pattern  266  in FIG.  9 C. 
     While the invention has been described with respect to one embodiment, it will be appreciated that many variations, modifications and other applications of the invention may be made.

Summary:
A flexible interlocking element ( 10 ) having opposing, interlocking ends ( 12, 14 ). Element end ( 12 ), features a contour, including element top surface segment ( 16 ), extending to bend ( 18 ), then defining a tongue element ( 34 ) extending downwardly along an incline ( 19 ), to inverted bend ( 20 ), which defines a tongue side ( 60 ) having a tangent ( 62 ). The contour of the tongue, further extending downward and around to bend ( 22 ), which defines a tongue tip ( 58 ). The tongue tip ( 58 ) having a bottom ( 54 ) with a tangent ( 56 ). The tongue further extending upward along incline ( 26 ), then further extending horizontally to bend ( 28 ), then further extending vertically downward to bend ( 30 ). Bend ( 30 ) defines the lower edge of bottom surface segment ( 32 ). Element end ( 14 ) features a contour including element top surface segment ( 36 ) extending horizontally to bend ( 38 ). A channel ( 52 ) is then defined by forming a side face of the element ( 14 ) to include, an upwardly extending bend ( 43 ), which further extends upward and around to bend ( 44 ), then downwardly along an incline to bend ( 46 ), which further extends vertically downward to bend ( 48 ), which defines the edge of a bottom surface segment ( 50 ).