The Wren's Egg glacial erratics

Thanks to Corbred Galdus and Northern Antiquarian for drawing attention to the Wren's Egg and Nest, in Galloway, Scotland.  Posted on Facebook -- nice photos!!  But is the stone in the top photo a glacial erratic and are the other two something else?   

There are apparently four boulders here, but I am intrigued that in all of the official documentation only one glacial erratic is mentioned -- namely the one in the top photo.  The boulders are apparently quite small -- less than a metre in diameter.  They are made of granite, and the source is unknown. I have looked at many photos of these stones, and they have the key characteristics of glacial erratics -- rounded off edges, facets and a weathering crust which is peeling off in places. Since we are dealing with granite, I doubt that there are striations -- but I may be wrong.

What intrigues me here is that boulders 2, 3 and 4 are NOT referred to as erratics, presumably because they are deemed to be in rather crude and simple stone settings -- ie they have been moved a short distance from where they were found.  There is no logic at all in that.  I suspect that the majority of megalithic monuments in the glaciated parts of the British Isles are made with glacial erratics, moved relatively short distances (ie. metres, rather than kilometres) from the places in which they were discovered.

As pointed out by Olwen Williams-Thorpe and her colleagues many years ago, the most powerful factor in the determination of location for standing stones and burial chambers was probably the ready availability of stone -- and I agree with them.

 https://www.historicenvironment.scot/archives-and-research/publications/publication/?publicationId=015d347c-65bd-4955-a456-a6cc009cd744

https://darkgalloway.wordpress.com/

Thanks to Tony for drawing my attention to this topic.