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