Kedleston Hall (National Trust, Derbyshire) has a very impressive ha-ha – I estimate about 3km long. It was part of the garden design by Robert Adam c.1759, and still stands, looking incredibly good for its age. Obviously very well cared for over the years.

A ha-ha is a very clever, very simple and very ornate device used to keep livestock out of the garden without the need for hedges, walls or fences that will block or detract from the view. It makes the garden seem far bigger than it is by visually linking it to the surrounding fields and parkland. Which in their day would have been pastureland with cows, sheep or deer. 

It works with the use of a wide trench: about 2m+ wide. The garden side of this trench is a vertical drop, faced with a stone (usually, or brick) wall which is keyed into the bank behind with large stones to hold it in place, and topped with large capping stones. This is backfilled with free-draining material to ensure that ground water doesn’t erode the wall away from behind. The field side of the trench has a gentle slope up to original ground level. The result is that you can be standing on the garden side at the same level as a cow in the field, but as the cow gets closer, they are met by a head height vertical wall that they can’t climb, while you have an unobstructed view across the parkland.

Genius.