http://home3.inet.tele.dk/mcamara/stones.html gives a date of 750AD plus or minus 110 years on the Senegambian stone circles "as a result of Laboratory tests at the University of Dakar". Disappointing. I'd like more information on the tests before giving up on the stone circles being prehistoric.
There's an unusually large amount of continental shelf off the African coast near Senegal, Gambia and Guinea. Don't know how deep it is. If shallow, then it may have been dry land a few thousand years ago, which would be interesting. Also the Cape Verde islands, in the Atlantic opposite Senegal, seem to be on a horseshoe of shallow shelf - once again, potentially interesting.
None of this adds up to much so far. Worth keeping in the back of the mind, though, in case any more information comes up.
Anyone got any?