An Erratic Source

In: KELLAWAY, G.A.. (Ed.), Hot Springs of Bath. Bath City Council, Bath a small rock is pictured:

This small lump of "bluestone" found at "Stonehenge" in 1924, known as RSN18, was considered by Kellaway as being an important indicator of glacial transport of erratics to the area. The lack of a recorded find spot has been problematic.

Transport of the Stonehenge Bluestones: Testing the Glacial Hypothesis by JD SCOURSE · 1997 http://publications.thebritishacademy.ac.uk/pubs/proc/files/92p271.pdf highlights this: 

In Hawley's 6th Report Jan 1926 Vol V1 No.1 The Antiquaries Journal which is of his 1924 excavations he describes finding a foreign stone and also a photo of the excavation.

I think the stone pictured in the middle of the very left hand side of the excavations around stone 8, judging by the scale, is about the right depth and size to be the foreign stone he mentions. This provides the original context for the stone.  (Click to embiggen)

Kellaway's description seems to match it very well:

Now we need expert analysis as to whether the striae are glacial marks or simply Rhyolite layers as originally suggested.