I admit I’m not certain, but I believe this is a collapsed former natural arch (or part of a cave). It can be found on the coast east of Sanaigmore.
Collapsed arch near Sanaigmore, Isle of Islay
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A view southwest from the top of Mala Bholsa (127m) in the north of Islay. An amazing coastline with raised beaches, cliffs, beaches, natural arches, caves and more. In the distance Loch Gruinart can just be seen cutting deep into Islay. On the right Ardnave Point with Nave Island off it. In the far distance the hills at Sanaigmore can be seen. Just below some of the deers living in this remote part of Islay.