The monumnet comprises the Old Beacon at Dennis Head, North Ronaldsay, and represents the remains of the earliest surviving purpose-built lighthouse tower in Scotland. It was built for the Commissioners of Northern Lighthouses in 1788/9 by Thomas Smith, engineer, and Ezekiel Walker, lighthouse designer, and was first lit on 10 October 1789. Materials and workmen were brought from Leith, and the masons were John White and James Sinclair. The total cost was 199.12s.6d.
The tower was cylindrical, standing 70ft (21.35m) high with the house for the keeper, one James Smith, abutting it on the west side. The light system designed by Thomas Smith was of the catrophic or reflecting kind, in which light from a cluster of oil lamps was reflected by means of copper reflectors covered with facets of mirror glass.
Unfortunately, one immediate result of the provision of a light to guide mariners around the northernmost part of Orkney was an increase in the number of shipwrecks; for ships which had formerly kept to the open sea were now encouraged to sail closer to land, often coming to grief off the coast of Sanday. In 1802, an unlit beacon tower was therefore erected at Start Point on Sanday. This was provided with a revolving light in 1806. Three years later, in 1809, the North Ronaldsay light was extinguished, its lantern being replaced by the stone ball removed from Start Point. At the same time the keeper's house, to which another had meanwhile been added, was unroofed and abandoned.
The tower is built close to the water line, and although its base is now surrounded by shingle it is apparently founded on bedrock. The original cylindrical tower stands entire up to the corbel course that would have supported the external gallery enclosing the lantern. Except for the corbels and an internal spiral stair, it is built in undressed stone. There is a plain lintelled door at the base on the west, and two rectangular windows with timber rear-lintels (the lower one now blocked) on the south. The stair, which was built in yellow sandstone, has collapsed inside the tower, leaving some of the stubs projecting from the walls.
The ball finial that surmounts the tower is made of finely jointed ashlar, and is carried on a corbelled stone roof with a low rounded profile. This incorporates some timber work internally; but it is uncertain whether this represents a survival of the original timber superstructure enclosing the lamp, or is simply part of the form work used in constructing the corbelling.
The keepers' houses are built against the western side of the tower, in an elongated block with a pitched roof. This was originally roofed with Welsh slates, some of which still survive in the raggle cut into the wall of the tower. The masonry is undressed, and set in a hard lime mortar, which includes beach sand and small pieces of coal. The main dwelling, in the middle of the block, had two rooms, with a fireplace and chimney in each crow-stepped gable.
The second dwelling represents a subsequent addition to the west; it consisted of a single room with a west gable containing a fireplace similar to those in the first dwelling. Between the main dwelling and the tower is a passage or vestibule leading to the door to the tower. To the left of the outer entrance to this, in the angle betwen it and the tower, is a small rounded closet with an external door, which may have been used as a store room or possibly as a latrine.
The tower and ruined keepers' dwellings are now incorporated into a system of drystone dykes; some may be contemporary with the tower's period of use, when the keeper had grazing for a cow, while others are evidently later and associated with sheep management.
The monument to be scheduled includes the tower, the keepers' accommodation block, the associated drystone dykes and an area of ground encompassing these structures and measuring some 90m E-W by 90m N-S, as shown in red on the accompanying map.