Abstract: An interlocking block and a retaining wall system derived from such blocks wherein the blocks when arranged to extend in horizontal courses in end to end relation and one upon the other in interlocking overlapping wall defining relation extend upwardly as such wall structure is erected, such interlocking block having an axis terminating in spaced apart end walls and bounded by generally flat top and bottom walls arranged in parallel relation to each other and by front and rear facings extending from end wall to end wall and so spaced apart as to provide a substantially uniform cross-section throughout the axial extent of same, the block having an axially extending projection upstanding from the top wall and the bottom wall having an axially extending recess formation of a configuration and extent to match the projection, the projection being spaced inwardly from the front facing a distance substantially equal to the distance the recess formation is spaced inwardly from the rear facing and the projection is sp