You're actually looking at an old excavation. These are the Wyke Down henges, as featured in Martin Green's 'A Landscape Revealed...' (pages 85-90) and in more detail in Charles French et al (2007) 'Prehistoric Landscape Development and Human Impact in the upper Allen valley, Cranborne Chase, Dorset' (MacDonald Insititute Monographs, Cambridge). The latter features the full excavation reports of these and several other sites on the Chase. It'll cost you though...
Anyone else noticed that the 'most recent' Google Earth imagery isn't necessarily that recent?