There are several large, partially buried sarsens about, and a bit of a mess left by Brighton & Hove Archaeological Society! (tsk, tsk).
There's been a fair amount of archaeological work done up here with a burial site and lots of Iron age/ Roman stuff found. There was definitely a lot of activity going on at Rocky Clump at that time.
There are a lot of sarsen stones in the area (Stanmer means 'Stony Pond'), and there are rumours of a destroyed stone circle at a copse nearby called 'Grannys Belt'. There are stones there, but any circle is unsubstantiated. More likely field clearance, but who knows?
There are a few big sarsens in Stanmer Park and environs, but most aren't of 'megalith' size.
Nice spot this, and always unusual to visit barrows and cross dykes that have had woodland planted around them (this site would have originally been on open downland)