Summary of site:
A tombolo is a bar of sediment, usually sand or shingle, linking an island to the mainland. The only known example in Northern Ireland links the Isle of Muck, immediately east of Portmuck, with the cliff coastline of Island Magee (a peninsula) to the south west.
Muck Island is elongate in shape, half a kilometre long and about 150 m wide along most of its length and is aligned south east/north west. It is formed entirely of the Lower Basalt Formation, of Tertiary age, and at its highest point it is a little over 20 m. It is separated from the mainland cliffs by about 250 m of water.
The cliffs facing the island are around 20 m high, exposing Ulster White Limestone at the base and Lower Basalt on the upper face. There is a pronounced, vegetated step in the Ulster White Limestone just above the present high tide mark, forming a platform on which are perched several large, lichen-coated boulders. The cliff behind could be a Holocene survival (formed in the last 10,000 years) but in places it is actively undercut by modern wave action. The platform only exists in the lea of Muck Island suggesting that it was formerly more continuous and only survives here because the island protects it from the direct impact of waves.
The tombolo is immediately behind the island and takes the form of a bar linking it to the mainland coastal cliff. It appears to have a core of basalt, chalk and flint boulders, around 50 cm in diameter, thinly covered on its north eastern side by pebbles averaging 5 cm in diameter. It is steeper on its north east face which is concave in 'rial view. Although the great bulk of the rocks in the structure are local some, such as sandstones, porphyrys and granites are probably glacial erratics derived from deposits along the coast or from the bed of the North Channel.
The tombolo is not typical in a number of ways. The normal form is produced when waves are diffracted around an island from both sides and neutralise behind, dropping the sediment in a bar or a double bar with a separating lagoon. The evidence at Muck suggests that the deposit was derived from the north west by south easterly drift along the shore and became stranded in shallow water behind the island. The concave north western shore with its steep face and finer shingle contrasts strongly with the south eastern shore which is rugged, rocky and boulder strewn. This is a consequence of waves from the north west washing over the bar, confirmed by observation. Because there has been so little research and observation conducted at the site this could be an erroneous conclusion and it is possible that the process could reverse periodically but there is no immediate evidence to support the idea. This coastline does not appear to have any new sediment supply and it is believed that the tombolo is simply recycling a fairly constant volume of sediment.
The platform and degraded cliff on the mainland are believed to be raised coastal features caused by the bounce-back of land when the ice load of the last glaciation was removed. This would have lifted the coastline clear of the reach of the modern sea. They only survive here because they are protected from the destructive impact of storm waves by the island.
Access to the site is difficult and it is in pristine condition. It should be relatively easy to delimit a boundary for conservation purposes.