This boundary is marked by the Patcham Pylons, a pair of stone towers either side of the southbound A23 that were erected in 1923. If you weren't zipping between them at 70mph you'd be able to read the inscription, which says:

HAIL GUEST. WE ASK NOT WHAT THOU ART.
IF FRIEND. WE GREET THEE. HAND & HEART:
IF STRANGER. SUCH NO LONGER BE:
IF FOE. OUR LOVE SHALL CONQUER THEE.