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Earth Science Conservation Review
| Muck Island Tombolo, Island Magee | Antrim |
| Site number: | 1136 | ||
| ESCC: | IA | ||
| Locality Type: | Cliff, Coastal section | Status: | ASSI |
| Grid Reference: | D465025 Centroid | ||
| Google maps: | 54.84925,-5.71912 | ||
| County: | Antrim | Council area: | Mid & East Antrim Borough Council |
| Period: | Quaternary | ||
| Stages: | Holocene | ||
| Site Description | |
|---|---|
| Highlights: | |
The area around Muck Island contains a well-defined shore platform and associated features cut in chalk which probably relate to higher ?Holocene sea levels but which may be periodically active under contemporary storm conditions. A contemporary gravel barrier composed mainly of chalk, flint and basalt which links the island and the mainland is active under contemporary wave conditions. The barrier appears to overwash mainly toward the south, suggesting dominance of wave attack from the north, and is submerged at all but spring low tides. | |
| Introduction: | |
A number of coastal features are present in the vicinity of Muck Island on the eastern coast of Island Magee. They include a raised rock platform and degraded cliff formed in Cretaceous chalk, and an active gravel depositional structure (tombolo) which extends from the island to the mainland. Tombolos may be defined on the basis of their morphology, but in general the term implies a particular genetic origin through deposition in a wave caustic zone, generated by incident wave refraction around an offshore obstruction and interference of the refracted waves. The tombolo at Island Magee is probably the only such feature on the open sea coast of Northern Ireland although such features are common elsewhere. | |
| Description: | |
Muck Island and the adjacent mainland of Island Magee are formed in lower basalts but immediately west of Muck Island Cretaceous chalk outcrops in a narrow band along the mainland shore. The chalk outcrop forms a terrace a few metres above normal high tide levels and it is partially vegetated on its landward margin. Landward of the terrace is a ?Holocene cliff which is degraded and vegetated in places but in other sections appears quite precipitous. The terrace is absent beyond from the shelter of Muck Island. Several large basalt boulders rest on the chalk platform north of the gravel barrier and these are rounded and covered in lichen suggesting present inactivity or infrequent motion. Discontinuous areas of interbedded sand and organic deposits also occur on the platform surface north of the tombolo. Muck Island tombolo stretches between Muck Island and the eastern shore of Island Magee. It is composed of rounded gravel and boulders of chalk, flint and basalt. Larger constituent grain sizes (average 0.5m diameter) appear to form the frame of the structure which is covered in a thin veneer of finer grains (average diameter about 0.05m), particularly on the marginal areas at both ends of the tombolo. On its northern margin the tombolo has a concave plan form developed in response to incident refracted waves. Both the east and west margins of the feature are dominated by flint pebbles with occasional basalt and fewer chalk pebbles. The basalt tends to form the largest fraction which dominates in the lower levels and may form the framework of the structure. Exotic lithologies (granite, porphyry, sandstone) also occur suggesting a glacial origin for at least part of the sediment. The central section of the feature is composed of large boulders which appear to form a framework at the margins too. The presence of flints on the Muck Island (entirely basalt) shore suggest they migrated eastward along the feature. Contemporary wave action has produced a series of berms at the north-facing marginal sections of the feature. Overwashing of the barrier occurs along much of the mid-section even at low tide. Wave approach appears to be dominantly northerly to north-easterly and no gravel beaches are developed on the rocky southern marginal areas of Muck Island or the mainland. Much of the gravel in the tombolo is imbricated through strong swash and backwash action. Active alongshore transport toward the middle of the feature through oblique swash and backwash action was observed during a site visit. Active berm construction was also observed under contemporary wave action on the north-western margin of the tombolo. Under contemporary conditions it appears that longshore transport tends to zero toward the centre of the feature where waves approach parallel to the shore. Thus a number of marginal sub-cells facilitate transport of sediment within the tombolo. This is likely to be confined mainly to the finer sediment grades. | |
| Importance: | |
The area is noteworthy for the barrier and the raised features, the latter of which have not received detailed examination. Given the likelihood of shelter afforded by Muck Island, the features have probably been preferentially preserved and thus may contain information of particular relevance for the interpretation of former sea levels. The barrier is probably not a true tombolo in the genetic sense of production by interaction of opposing wave fields induced by refraction of wave fronts around Muck Island, but is rather a coastal gravel barrier that has grounded in its present position under dominant wave approach from the north/ north-east. The feature and its associated raised platforms are of local/ national interest but have received little or no scientific study. The Muck Island tombolo is probably the only such feature on the Northern Ireland coast. | |
| Interpretation: | |
The raised platform is probably contemporary with the degraded cliff to its rear and both owe their preservation to the sheltering effect of Muck Island. The balance of evidence points to the platform being a raised feature since it is partly vegetated and the cliff is largely inactive. The possibility of infrequent contemporary storm wave activity operating on it cannot, however, be ruled out. The origin of large, lichen-covered basaltic boulders on the rock platform is uncertain. They may have originated from rock fall but there is no obvious source, or have been stranded at higher sea level or by contemporary low frequency storms, or be derived from in situ erosion of basalt intrusions within the chalk platform. The presence of interbedded sandy and organic horizons on the platform suggests high level storm sedimentation interrupting calm periods of organic accumulation. The gravel feature linking Muck Island and the mainland morphologically resembles a classic tombolo but several lines of evidence suggest it did not form in the normal manner for such features (i.e. by deposition in a wave caustic zone produced by interference of refracted waves in the lee of Muck Island). Rather, evidence points to it having originated through southward movement and eventual stranding of a gravel barrier in shallow water. Evidence for such an interpretation comes from the smooth northern plan form and overwashing of the barrier toward the south. The southern margin was, on the day of a site visit, in a wave-sheltered zone and does not display a smooth planform characteristic of swell wave action. In addition, the feature exhibits a cross-section which is steeper on the northern side than the southern, suggesting the southern shore is dominated by overwashing rather than incident wave action. Gravel is also present along the flanks of the barrier on the northern side while the southern margins are essentially rocky. Possible evidence of periodic reversals in this situation comes from the O.S. map which shows more gravel on the southern margin than the north which is the reverse of the situation during recent fieldwork. In the absence of any obvious source of fresh gravel input from the adjacent coastline it is likely that the tombolo is in an advanced stage of development in which a fixed sediment volume is redistributed within the confines of the tombolo. The only written work on the feature is a recent report in connection with the likely impacts of the Scottish gas interconnector project (Hansom, 1993). That report concluded that the feature is largely relict and is "likely to be subject to very little movement or change in the present wave climate". Such an interpretation is inconsistent with the limited field observations undertaken during this study which revealed active sediment transport on the system. The presence of flint and chalk pebbles on the Muck Island end of the feature suggests they migrated there along the face of the barrier since no local source is apparent on Muck Island itself. | |
| Conclusions: | |
In the vicinity of Muck Island, a number of diverse coastal features occur within a confined geographical area. These include degraded rock cliffs and a platform formed in chalk on which are several large boulders and sand/organic horizons. The ages of these features and levels of contemporary activity upon them are unknown. The gravel depositional feature which extends from the coast to the island is probably a grounded barrier deposited under northerly wave attack. | |
| Notes: | |
The area is accessible via private land or via a rather dangerous and precipitous path over steep rocks from Port Muck. It should This site has received little or no scientific investigation other than a recent preliminary assessment in connection with the likely impact of cable laying associated with the Scottish Interconnector Project (Hansom, 1993). Studies to determine the contemporary dynamics and origin of the gravel barrier would offer a better understanding of the sedimentary system in the area. Due to restrictions of access, it should be relatively easy to delineate for designation. This site is also important for Mineralogy and Metallogenesis. See Key Site 539 - Coast opposite Isle of Muck, Island Magee. | |
| Keywords | |
|---|---|
| Relations: | Coastal processes |
| Geomorph: | gravel barrier, raised shoreline, rock platform |
| Access | |
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| Approach: | The area is accessible via private land or via a rather dangerous and precipitous path over steep rocks from Port Muck. |
| References (2) | |||
|---|---|---|---|
| Hansom, J.D., 1993. Scottish Interconnector Project: potential impact on landforms at Portmuck south. (Unpublished Report) In Report for Turnbull Jeffrey Partnership. Coastal Research Group, University of Glasgow. | |||
| Synge, F. & Stephens, N., 1966. Late Pleistocene shorelines and drift limits in north-east Ulster. Transactions of the Institute of British Geographers, vol. 39, pp.101-125. | |||
| Map(s): | See the 1:50,000 O.S. Sheet 9 (Larne, Ballymena). | ||
| Record type: | ESCR report | ||
| Entered by: | E M Porter | ||
| Enlander, I., Dempster, M. & Doughty, P., 2026. Muck Island Tombolo, Island Magee, County Antrim, site details. [In] Earth Science Conservation Review. https://www.habitas.org.uk/escr/site.php?item=1136. Accessed on 2026-11-06. |
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