Noticeably overgrown and 'let go' since Greywether's posts, I had a bit of difficulty trying to reconcile what I saw here..... too wide for a long cairn, etc.... until I caught on that it represented the remains of a multi-chambered round cairn.
The siting, alongside a dusty foresty track with grafitti-defiled signage, within close proximity of the A714, isn't exactly the most salubrious, but that's not the monument's fault, is it?
Perhaps best to visit prior to seeing the nearby White Cairn - an aperitif, if you like - I nevertheless found the site to be a 'grower' and well worth a short visit before going to find a wild camp in Glen Trool for the night.
This is another Bargrennan-style tomb (third today!). It is in the process of detailed excavation and has been backfilled since my last visit making the main features easier to make out. Further work will continue this year.
There are three chambers (S with a capstone, E and N) in a round cairn. What looks like the remains of a kerb can also be seen in the W.