Rodney Castleden’s book ‘Neolithic Britain’* says that half this ritual enclosure still exists unexcavated – the rest was lost when the Dorchester bypass was built. He says: “The most striking thing about Flagstones has been the discovery of neolithic chalk engravings, carved in the near-vertical walls of the ditch.” The enclosure was constructed on the same east-west ridge where Mount Pleasant and Maumbury Rings would later be sited.
I was in Dorchester’s museum today, where you can visit one of the carvings.
*What you probably really want is PJ Woodward’s 1988 Antiquity article about the discoveries, ‘Pictures of the Neolithic’. I haven’t seen that though.