the ribs have preferably flat sides angled to the medial central longitudinal plane q of the rib the included angle between the flat sides may be 30 to 38 and preferably 34 u00b0 the spacer stud is optional and may be omitted when used its presence assists in giving the wall a pleasant appearance when seen from the front in the most preferred embodiment of the invention the major part of the upper surface indicated 25q is shaped as the curved surface of a cylinder of diameter 300 mm the axis of the cylinder being shown at ac1 and the major part of the undersurface 25c is shaped as the curved surface of a cylinder of the same diameter the cylinder axis being at ac2 the angular spacing of the ribs is preferably 7 5 degrees the distance between axes ac1 and ac2 is preferably substantially equal to the block thickness and more preferably as in the case of the illustrated fig2 block substantially 150 mm the overall front back dimension of the block may be about 350 mm as stated seen in plan the block is preferably rectangular with its overall fore and aft dimension from 80 to 200 of its overall width dimension fig3 and 7 illustrate two versions of a substantially vertical retaining wall in which the retained earth and.