More specifically, the invention relates to {HEAD ~ concrete masonry blocks} which are useful in forming {TAIL ~ various retaining structures} 
are useful in forming
More specifically , the invention relates to { HEAD ~ concrete masonry blocks } which are useful in forming { TAIL ~ various retaining structures }
OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD OTH REL REL REL REL TAIL TAIL TAIL TAIL TAIL TAIL TAIL