Over and again, in books and articles, and in exhibits galore, we are told that there was a "bluestone arrival date" at Stonehenge. That date varies a bit from one source to another, but the essence is that before 4,600 yrs BP the stones were not there, and then the bluestones were fetched from Wales specifically for use in the monument. Suddenly (or maybe over a few decades) they were there, available to be incorporated into one bluestone setting after another. The most common view is that the first setting of the bluestones was in the Q and R holes, in Stage 2 of the monument's history -- but Parker Pearson (2012) subscribes to the view that the bluestones arrived on site earlier, around 5,000 yrs BP, and were first used in the Aubrey Holes. he thinks this partly because og the frequency of bluestone fragments in the holes, and partly because of the presence of "compacted sediments" deemed to have carried the weight of standing stones.
The Bluestones Arrive (2,600 – 2,500 B.C.)
With the exception of evidence of human burials, Stonehenge remained largely untouched from its initial stages of construction for around 500 years. Then suddenly, around 2,500 B.C., the smaller ‘bluestones’ started to arrive. Around 82 bluestones arrived from the Preseli Hills in Pembrokeshire, Wales – around 140 miles (225km) away.
When your interpretations are driven by powerful assumptions, even your "accurate recording" of exposures can be influenced. In your diagrams, where you encounter complex and subtle stratigraphy, it is tempting to locate your "bluestone-rich layers" and to draw a line beneath them. This line then becomes the dividing line between a "pre-bluestone" layer below and a "post-bluestone layer" above, always interpreted as a secondary or tertiary fill of some sort. So the graphic representation is falsified or corrupted, not because of scientific malpractice but because of unconscious bias. I think this has happened time and again, and can be seen in many of the diagrams in the pages of the Cleal et al volume.