In the hills above Glen Falloch, just north of Loch Lomond, a grassy mound and a pile of boulders stand at what is commonly thought to have been (around 1500 years ago) the boundary between the ancient kingdoms of Strathclyde to the south, Dal Riata to the northwest, and Pictland to the northeast.
There's no agreement on whether the boulders in the mound are natural or placed there, and there's debate about how much of the mound itself is man-made. But it appears to be widely accepted that the large megalith resting on top was deliberately placed there.
There's also a tradition that a battle between the Britons and the Dalriads was fought near a large stone called Minvirc in this area, and it has been suggested that this is the stone. There's no direct evidence for that, though.
You might like to read the entry about this stone on the Senchus blog, which is where I first learned about it.