The Altar Stone -- if it looks like a glacial erratic, that's probably what it is........

Source:  Historic England Archive

Tim Daw's annotated diagram of the surface scratches, published on Twitter (now called X)

Ths scratches on the flank of the Altar Stone (courtesy Historic England)

According to Simon Banton, the Altar Stone lies "beneath the collapsed upright of the Great Trilithon (Stone 55b) and its lintel (Stone 156), sunk into the grass. The stone itself was broken by the fall of the Great Trilithon's upright and is in two pieces."

There is an interesting post on Tim Daw's blog -- thanks to him and Simon Banton -- regarding an assortment of new images of the Altar Stone -- unearthed in the Historic England archive. They all date from the 1958 Atkinson excavation, including a number of photos of blocks that were recorded as "unidentified stones".........

Here is the link:

https://www.sarsen.org/2025/02/the-archive-excavation-of-altar-stone.html

Tim refers to the "fine working of the stone" and scratches which have "obvious similarity to other neolithic stone markings."  The so-called "engraved lines" shown on the side of the Altar Stone in photo P50107 are very sharp, and I think they were most likely made with metal tools.  They look to be remarkably fresh -- and it will be interesting to read more about the stratigraphic context in due course.

Could the marks be glacial striations?  It's possible, but they appear to be too straight and regular.  I would dearly like to look at the whole of the smoothed SW flank of the slab which appears to be very different from the other surfaces.  There appear to be other scratches as well -- less distinct but in my view even more interesting........

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Overall, my impression from looking at the photos is that the Altar Stone is a broken elongated slab which is heavily weathered and abraded.  The edges are for the most part rounded off, and I think I see at least two scoop-shaped glacial facets.  There are abundant fracture scars, some quite fresh and others rather old and degraded.  

The big break at the SE end of the slab looks to be quite fresh, and was probably the result of the accident which brought Sarsen Stone 55B crashing down on top of it.  In one photo we can see some loose fragments associated with the breakage.  It's possible that other debris might have been removed during excavations.

All in all, these latest images convince me that this is not a quarried slab but a glacial erratic with a complex history............