I'm happy to draw attention to the 2009 report by Darvill and Wainwright -- dealing with the main findings from the 2008 Stonehenge dig. Interesting material, written in a lively and accessible style.
Much of the report is about the Stonehenge Layer, but this is what the authors say about the two bluestones which they encountered.
Let us move hastily along to the next period of Stonehenge, where three sockets relate to stones that are still visible above the ground surface. Stone 35a and Stone 34 are both part of the Bluestone Circle that is such a striking part of all the later phases of the monument. Both stones are still above ground. Stone 35a is a massive block, but projects only a few centimetres above ground level (fig 9). Petrologically speaking, this one is very close to the material from Carn Menyn to which Geoff referred earlier. I think that you can see straightaway that this is the natural patina on the rock and it has been smashed up in relatively recent times, sufficient that no new patina has developed on the exposed faces. As we shall see a little later, that is not surprising. But there it is in its place, going down the best part of a metre into the ground.
The next one along is Stone 34. It is a beautiful stone that extends into the ground more than a metre, so that less than one-third of it is sticking up above the ground and the rest is now under the ground. You can also just make out a massive hole next to the stone, which is partly filled with concrete, put there when Atkinson refilled the trench, probably to give it support. It is fairly certain that when Atkinson was digging here Stone 34 was loose and you could move it, and, given the size of the hole, there is no question that material could get into the ground alongside that stone.
The pic of stone 34 is a nice one too. This one is apparently complete -- although its base was not fully excavated by Darvill and Wainwright. As fine a weathered and battered glacial erratic as one is ever likely to see.