A sort of shelf sticking out about 30 foot from the shore on which the old barges used to sit upright rather than sloping with the river bed. A wooden or iron box filled with gravel/stones /mud. Most are now in a dilapidated condition but some surviving examples can be seen above Tower Bridge at low water. If you don't use the outermost arches you'll be alright.
-------------------- "Treat every day as if it was going to be your last. 'Cos one day you'll be right" - Benny Hill