The concavity has a minimum depth at its leading end and {HEAD ~ a maximum depth} at {TAIL ~ its trailing end} 
at
The concavity has a minimum depth at its leading end and { HEAD ~ a maximum depth } at { TAIL ~ its trailing end }
OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD REL TAIL TAIL TAIL TAIL TAIL TAIL TAIL