He pieces together evidence about it from excavations by Atkinson, Hawley and Gowland and concludes it dates to 2440-2100 BC, after the Great Trilithon was erected and that it was filled back up in prehistory because a bluestone was set into it as part of the inner oval of bluestones.
Atkinson thought it was the ramp for the erection of Stone 56 - photos below - but MPP shows it wasn't. The dark area of the ramp can be seen on the side of the excavation leading down towards the base of 56 but it is too shallow to be of use and doesn't join up to Gowland's excavation around the base of 56.
MPP declares the purpose of this huge pit, estimated as 12 metres long, 5 metres wide and 2.4 metres wide to be a "complete mystery". The size is very approximate as edges haven't been found.
Let me indulge in proposing a hypothesis of what the pit might have been for. I think it fits all the scant evidence but that any further proof is unlikely to be possible.
I think the pit isn't for erecting something or burying something, it looks more like a quarrying pit used to extract something heavy that was already buried in front of the Great Trilithon. To bury or erect a stone a steep sided hole fits the purpose, but to drag something out a shallow slope is needed, in the absence of lifting gear,
Any number of stones could be the candidate for having being in this central position but my hypothesis is that it was the Lake House Meteorite .
Follow the link for more information but we know this 92.75kg stone landed about 30,000 years ago, no crater has been found so it may have been onto the Ice Age ice at that time, and may not have been anywhere near Stonehenge. About 10,000 years ago it was exposed to the elements and started weathering. About 4000 years ago it was buried in the local chalk before being dug up in probably Victorian times by amateur barrow diggers.
The fantasy history runs that this strange stone was brought to Stonehenge very early on, it was buried in the holiest central position in recognition of its unique qualities. A generation or so later it was decreed that some leader's tomb, or other place was more fitting for it and so like the Stone of Scone it was moved to please the leader. This involved excavating the pit to find and remove it. The pit was then filled in and the bluestone horseshoe erected.
Our Victorian diggers then found it in its secondary burial place in a barrow.