Known as Dialogue at the Dogwatch, they were commissioned from David Brown in 1994 by a UCT alumnus, Charles Diamond. The pieces are made from bronze, copper, brass, Cor-ten and stainless steel, and together weigh about 7 tons.
Dogwatch, fwiw, is a navy term that refers to the watch time between late afternoon and night time, which is probably a time of day when the low angle of the sun and the increasingly-dim light makes the watch particularly difficult. Read a little more about the term and why it was coined over here on Wikipedia.