A cap assembly is shown for a right of way line marker which includes a cap body with a conical exterior and an end opening. A mounting assembly including a lower, stationary disk and an upper, sliding disk are received within the end opening of the cap body. The lower disk has a T-shaped aperture therein for receiving an upper exposed end of a T-shaped vertical support member. The upper sliding disk is positionable within the cap open interior upon contact with the upper end of the support member to thereby securely mount the cap assembly about the right of way line marker.