The monument includes the remains of part of an ore hearth smelt mill and a reverberatory smelt mill, as well as the extensive system of flues which includes two chimneys, several ramped access ways and part of a contemporary trackway. It is in six separate areas of protection. The smelt mill is situated on the valley floor on the south side of the River East Allen; the flues emerge from the rear of the smelt mill and run in a south westerly direction to the open fells above, terminating in a stone built chimney near the watershed at Flow Moss approximately 3.5km away. Ore smelting in Allendale is well documented and a smelt mill is recorded in AD 1692 which belonged to the Bacon family. During the 18th century the smelt mill was leased from Sir William Blackett by Lancelot Algood who used it to smelt ore from his Alston Moor mines. From 1786 the mill was owned by the Beaumont Company which, during the 19th century, carried out improvements to the smelt mill. In 1847 a document and an accompanying plan of the mill shows that there were five roasting furnaces, eight ore hearths, a refining furnace, two reducing furnaces, two calcining furnaces, two reverberatory furnaces, one slag hearth and a separating house with 18 pots. The Allen smelt mill ceased production in 1896. Much of the smelt mill has been levelled, but a broad retaining earthen bank at the rear of the smelt mill site contains the remains of several stone structures revetted into the slope. Some of these structures are interpreted as the remains of a series of bouse teams located either side of the main flue; they are visible as stone walls standing 5m high with buttresses forming the individual bays. Some of the other structures contained within this area include the remains of a condensing chamber and a flue opening. The flue opening consists of a stone arch 1.5m high and 1.5m wide and a tunnel which it is known runs for some distance into the slope; side flues run to the east and west of the main flue. The smelt mill originally extended over a larger area down to the river. Only that area in which significant archaeological remains are known to survive is currently included in the scheduling. A structure lying on the north side of the smelt mill complex and known as the Pattinson Building still stands. It was a 19th century silver smelter in which silver found in association with the lead was refined. The ground level flues, which run between the smelt mill furnaces and their chimneys, were constructed during the 19th century. Their prime purpose was to condense the noxious fumes produced from the furnaces; the deposits, which formed as a result of this process on the internal walls of the flues, were periodically removed and their lead and silver content retained. Before the development of ground level flues, this valuable silver deposit would not have been recoverable. The smelt mill flues also removed the noxious gases away from settlements within the valley to a more remote area. Work on the flues is thought to have taken place in at least two separate phases. The first flue, which was begun in 1808, runs from the south end of the smelt mill and follows the south side of the B6295 before turning west towards Cleugh Head where it originally terminated. On construction of the second flue in 1853 it is thought that the first was extended south and then south west to Flow Moss. The second flue emerges from the south western side of the smelt mill and crossed over the adjacent road on what is thought to have been a bridge; on the western side of the road there is a substantial flat topped platform some 6m by 18m which is interpreted as a structure associated with this bridge. The flue continued south west on a more direct route to the chimneys on Flow Moss. The remains of the flues are visible in various forms: where they survive intact they are visible as linear mounds up to 8m wide standing to a maximum height of 2m. Internally, they comprise an arched structure constructed of square masonry which also lies below ground level for some 2m to 3m. Where the roofs and upper levels of the flues are no longer intact they are visible above ground level as a ditch on average 2m wide and 0.5m deep, flanked by two parallel mounds 2.5m to 3m wide and standing up to 1m high. In some areas the upper parts of the flue walls have become spread to form an earthwork on average 10m wide. The remains of at least seven doorways giving access to the interior of the second flue are visible in its upper 2km. These entrances would have given barrow access to remove the condensed lead and silver from its walls. Each of these access points is visible as a hollow road up to 4m wide and as much as 17m long, which generally runs parallel with the flue. The road ends in a concave walling and a ramped entrance into the flue. One of the ramped access points near to Frolar Meadows is unusual in lying at right angles to the flue, which at this point is also flanked by a trackway, thought to be an original access route. Only those parts of the flues which survive well are included in the scheduling. There are two chimneys on Flow Moss. The most southerly chimney, into which both flues pass, has undergone consolidation, and is visible as a well preserved structure 7m in diameter. It is built of regular sandstone blocks and was formerly higher than its present 6m. There are two arched flue openings at its square base with the remains of mortice holes. A fragment of a central dividing wall is visible within the chimney. The most northerly chimney, approximately 40m north of the first, stands 25m high and 4m in diameter, and contains a single arched flue opening; the earliest flue bypasses this chimney in favour of the second, although traces of an earlier course to it are visible as a slight earthwork.