Yes, stone is continually recycled and there is no reason why a glacial erratic erected as a standing stone four and a half thousand years ago shouldn't be refaced and reused as a gatepost more recently. Is it a gatepost now or a former standing stone ? What about the pairs of standing stones (Burl calls them a type 'a' Stone Row) that have just been drilled for hinges and have had a hedge planted up and down from them ? Are they standing stones, gateposts or a small stone row ? (These are rhetorical questions).

I've got loads of these kind of stones in Smithills, both upright and fallen - I posted some of them, then there were substantial objections (I'd called the most bulbous one the Bo Stone or something) and I pulled them out. Boundary stones are often ancient simply as the boundaries are also ancient.

Ermm.