Apologies for being a touch late in posting the answer to the latest Photo Quiz. I note that Dr Grumpy and Cllr Adrian Page both knew the answer. Bright sparks, both.
The steps and terrace in the photo are know as Queen Mary’s Steps. They provided access for Queen Mary II and her officials to reach the State Barge on the Thames. They were part of the Palace of Whitehall, the official London home of the royal family from 1538 to 1698. The palace burnt down by fire. British History Online describes the history of the steps.
The steps can be seen on the Thames embankment side of the Ministry of Defence, not far from the Houses of Parliament. The descriptive plaque by the steps says,
In 1691, Sir Christopher Wren designed for Queen Mary II a terrace overlooking the Thames in front of the old river wall of Whitehall Palace built by Henry VIII. This terrace, projecting about 70 feet into the bed of the river, was about 280 feet long. As it involved the destruction of an earlier private landing stage a curving flight of steps was made at each end to give access from the Royal Apartments to the State Barge.
In 1939 excavations for the new Government Building revealed the river wall of the Tudor Palace, the later terrace wall and the Northern flight of steps. The upper portion of the steps has been repaired and can be seen. A reconstructed length of the terrace can be seen immediately to the left of the steps and a rebuilt section of the river wall behind and above the terrace.
Here’s my photo of the steps, and a photo of the steps being uncovered in 1939, [click on images to expand]