On Monday in the early days the Abbotsgrange Dredged the Bellmouth of the Locks then ran back and forward to inchkeith with the spoil. At this time the MOD had there own Dredger at Rosyth.
Cost became a factor in running the Abbotsgrange back an forward to Inchkeith so the spoil from Grangemouth started to be dumped at Bo'ness. Later Rosyth lost it's Dredger Abbotsgrange took
over Rosyth Dredging duties. As you said spoil from Leith and Rosyth was dumped South of the Oxcars light.