The small circle on Porlock Common/Allotment next to the sheep fold?
If it is, I know it well. Very small recumbent stones, but a very special place. I've been up there under a full moon and the moor is an eerie-a-place as can be, with shrieks, hoots, wild horses and miles of person-less space, shining brightly like silver.
There's also Dunkery Beacon (cairn), and Countisbury Hillfort, Chapman Barrows, Culbone Hill and the Tarr Steps clapper bridge. Also Culbone Hill (above Porlock) is worth checking out too for the stone row and barrows.
Here's a small but good illustrated list of interesting prehistoric sites on Exmoor:
http://www.celiahaddon.co.uk/standing%20stones/exmoor.html
Wishing you safe travels and good fortune.
morfe