Sorry for the late add to this post, But I have come across what looks to be the same thing, a Very large (solid) stone brick wall 16 blocks wide and at Y level 122 (along the walkway) and has been going on for 9,000 blocks (along the x co-ordinate) with no visible end.
Can anyone shed some light on this peculiarity?
(note this is on a public server, and I found it about 9,000-11,000 blocks from spawn