This ha-ha gives the illusion, seen from the big house right of shot, of an unbroken lawn and field while the hidden ditch is a barrier for animals.
“Originally a feature of formal French gardens of the early eighteenth century, the ha-ha was first described in print in 1709 by the gardening enthusiast, Dezallier d’Argenville in his La Theorie et la Practique du jardinage (The Theory and Practice of Gardening).
According to d’Argenville - and his first English translator, John James - the ha-ha derived its name from the success of the optical illusion it created from a distance on viewers of the garden: the hitherto concealed ditch and wall would ‘surprise the eye coming near it, and make one cry, “Ah! Ah!”’ “
Ian