invention where unit 50 is identical to unit 60 except that unit 60 has been overturned this embodiment is used for structures of two layers and to shape the end layers of structures of three or more layers as will be seen further on for explanation purposes each unit can be understood to have a generally flat top side and a bottom side in fig6 unit 50 has a flat top side 51 and a bottom side 52 unit 60 has been flipped over and its top side 61 is facing downwardly while its bottom side 62 is facing upwardly as will be seen the terms 34 top 34 and 34 bottom 34 are used for identification purposes and do not necessarily mean that on any given unit the 34 top 34 side will be facing upwardly and the 34 bottom 34 side will be facing downwardly as can be further noted in fig6 unit 60 has an end t shaped portion 150 and a spanning portion 154 while the top side 61 of unit 60 is generally flat and smooth bottom side 62 has an uneven contour the uneven contour is the result of raised and lowered sections designed to create cavities when the units are placed upon one another on unit 60 the surfaces 170 and 172 of shoulder sections 156 and 158 and shoulder sections 157 and 159 are raised above the bottom side surfaces 174 and 178 of end lobes 160 and 162 and the bottom side surface 176 of spanning portion 154 further fig6 illustrates that steps 180 and 182 slant inwardly from surface 170 to surface 174 and from surface 172 to surface 178 in the.