| Site Description |
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| Highlights: |
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| This site comprises basalt cliffs and shore platforms on the north facing coast between Portstewart, Co. Derry and Portrush Co. Antrim. This coastal stretch illustrates the processes of storm-wave erosion on a bedded lithology, and the processes of sedimentation on re-entrant boulder storm beaches. The platform elevations have been used, somewhat controversially, in the elucidation of relative sea level (RSL) history on the north coast of Ireland. |
| Introduction: |
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| In Ireland, as elsewhere, rock coasts are a relatively neglected coastal form. The rock platform between Portstewart and Portrush is occasionally mentioned in the context of raised beach shoreline studies. These regional RSL studies usually do not indicate whether they are describing the platform planes, or narrow terraces fronting the backshore cliffs. The only two specific references assume that the cliffs and shore platforms are raised relict features dating from the higher sea levels of the mid-Holocene or earlier, viz. Wilson and Manning (1978) and Davies and Stephens (1978). |
| Since 1980 some detailed work has been carried out on the processes of cliff and platform erosion, shore platform hydrodynamics, and the sedimentation processes of the re-entrant boulder storm beaches. Details of these studies have been reported in Carter et al. (1987), Carter and Bartlett (1988), Carter (1991), McKenna (1990) and McKenna et al. (1992). |
| Description: |
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| East of Portstewart, Co. Derry, 4km of Upper Basalt cliffs fronted by shore platforms extend to Portrush, Co. Antrim, where they abut against the West (Mill) Strand which flanks the great dolerite sill of Ramore Head. The coast has a general WSW-ENE trend and is probably fault controlled. In plan it consists of a series of shallowly indented embayments and headlands. |
| Most of the cliffs backing shore platforms are active. Heights generally increase to the east, varying from 9 to 23m O.D., averaging c. 13m O.D. Embayment cliffs are invariably degraded and vegetated, with slopes c. 30-35 degrees, although frequently displaying a remnant free-face buttress in their upper sections. In contrast headland cliffs are steep, and often vertical, with the exposed bedrock faces riven with a complex system of fractures and joints. |
| Shore platforms are found flanking the headlands, often extending well into the re-entrants on either side. The platform topography is characterised by its high relative relief and general ruggedness. This applies particularly to the cliff-base and seaward ramparts. However, most of the platforms do display a broadly planar surface for at least part of their width. It is difficult to characterise platform elevations because wave erosion has often scarped the bedded basalts, so that they possess several planar surfaces, all demonstrating differing elevations and amounts and directions of dip. Even where the surface is generally well abraded on a horizontal scale of c. 10- 20m, the finer detail relief often displays a characteristic furrow-and-cobble topography controlled by the polygonal joint pattern of the basalts. There are relatively few areas of any extent where local relief is less than 10-20m. |
| The cliff/platform junction usually coincides with a flow top, occasionally notched as on the western flank of Rinagree Point. High-tide cliff ramps are unusual, the only clear example being that on the high eastern platform at Seapark. Occasionally, as at Holywell Port, there is a narrow cliff bench between the cliff and the main platform. Several platforms have high level ramparts, rising steadily seawards from the general surface elevation. Ramparts can also be found on one side of a platform, best illustrated on the wide platform at Dhu Varren. |
| The shore platforms are broadly sub-horizontal, generally demonstrating minor overall dips of less than 3 degrees to seaward (although occasionally landward), and with extensive planar sections less than 1 degree. On their seaward margins they invariably terminate in a steep, usually vertical, low-tide cliff dropping straight down to c. 10m of water. Frequently, the final few metres of the platform surface consists of a narrow wave- quarried low-tide ramp which links the platform plane to the low- tide cliff. The platforms fit neatly into the micro/meso-tidal group in the classification of Trenhaile and Layzell (1980). |
| The plan detail also is highly indented and irregular, although the platform orientations correspond closely with the NNW trending regional tectonic joint sets described by Roberts (1976) and the mesofracture sets described by Kerr (1987). Some of the embayment platforms are found where a downwarp in the basaltic layering brings a flow top or a markedly vesicular flow unit down to the level of wave attack. |
| The platforms are traversed by many steep-sided channels. Some platforms demonstrate shore-attached and offshore sections, and the shore-attached platform can be quite restricted, e.g. the platform east of Long Port. Smaller residual fragments and reefs are widespread. Potholes are also fairly common, particularly on seaward surfaces and on quarried terraces and benches along the margins of channels. Most of these contain rounded clasts. The substrate below the low-tide cliff comprises sand littered with a considerable quantity of rounded cobbles and boulders. |
| At the rear of the platforms and in the coastal re-entrants there are pocket storm beaches of rounded boulders and large cobbles, reaching a mean elevation of c. 5.0m O.D. Several of these storm beaches back extensive shore platforms, e.g. at Holywell Port, although others such as Port Gallen give directly to the shallow water of the embayment. The major storm beaches give way landward to degraded cliffs. |
| Importance: |
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| Cliffs fronted by shore platforms are a distinctive coastal form, and are important coastal systems in their own right. The modes of energy dissipation are very different to those active on the responsive sediments of sand beaches. These coasts are ecologically and environmentally important as energy dissipators, yet the processes acting on them are poorly understood. |
| The cliff/shore platform complex between Portstewart and Portrush provides a fine example of the cliff/shore platform coastal form produced by the interaction of a high-energy marine environment and a favourable geology. There are also short stretches of contiguous Holocene shoreline which enable relationships between the contemporary and Holocene forms to be examined. This is the only stretch of basalt cliffs on the coast of Northern Ireland to have received detailed study. Data are also available on the marine environment from locally deployed tidal and wave recorders. |
| Interpretation: |
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| Shore platforms are found on this coastal stretch because of the coincidence of several favourable factors, viz. a sub- horizontally bedded lithology, a storm-wave environment, a narrow 1.5m tidal range which concentrates wave energy, and the correspondence of the direction of maximum fetch (c. 3000km to the north north-west) with that of the most severe storms and the regional joint and fracture system. Geological structure has played a primary role in optimising shore platform development. Significantly there are no shore platforms on the Ramore Head sill to the east because the dolerite does not display the characteristic bedding of the basalts which favours platform development. |
| Storm-wave quarrying of joint-bounded basalt blocks is the dominant factor in cliff and platform erosion. The ruggedness of the platform surfaces is regarded as the signature of a wave quarrying environment. The dominant storm wave mode of erosion acts at some distance above MSL, and its elevational range is controlled by fetch, cliff aspect and local bathymetry. The platforms are cut preferentially along flow tops and the less competent basalts, over a wide elevational range. As an actively eroding flow top plane rises and falls the platform elevation will show very considerable variations within a short distance alongshore. The concentration of marine erosion along the lines of weakness presented by faults and joints results in the formation of the steep-sided channels which dissect the platforms. A more advanced stage in this process, acting along the typical shore-parallel faults of the area, explains the tendency of the platforms to fragment into shore-attached and offshore sections. Dissection along smaller joints accounts for the residual fragments and reefs. |
| The low angle platform planes may be a function of the narrow tidal range, or they may simply reflect the attitude of the bedding. It is likely that both of these factors operate to reinforce each other, although horizontality may also be aided by the unidirectional, "planar swash", mode of erosion. The sloping ramparts are a function of the local landward dip of an eroding basalt unit. It could be argued that such ramparts are modified remnants of a higher platform, but the dominant bedding control typical of basalt platforms makes this explanation very unlikely. |
| Contemporary marine and sub-aerial agencies liberate joint- bounded blocks from the cliffs and shore platform surfaces. Slope processes also operate on the vegetated degraded cliffs backing the embayments. These experience occasional landslips, the largest with volumes c. 100m3. The active cliffs, debris-free platforms and boulder storm beaches of this compartmentalised coast form the elements of an integrated sediment supply- transport - deposition system in which wave action acts rapidly to sweep basalt clasts off the platform surfaces onto the storm beaches. The scarcity of visible sediment is caused by the remarkable efficiency of the transport processes, not by conditions of sediment immobility and starvation. |
| The erosional landforms of this stretch of coast have been widely used to reconstruct earlier shoreline positions. The Post-glacial shorelines are locally well-defined and are linked to a maximum RSL elevation estimated by Carter (1982) to be approximately 2-4m above present, c. 5500 to 6500 yr BP. |
| The shoreline literature identifies a number of raised Late- and Post-glacial shorelines in north-east Ireland. Various elevations are given: Synge and Stephens (1966) describe the highest Post- glacial shoreline at 9-11m O.D. in east Antrim descending to the west. A well-defined lower shoreline at 4-6m O.D. is also described. Stephens and McCabe (1977) put the highest shoreline for north Antrim at c. 7-8m O.D. Prior (1966) suggests two shorelines, the higher c. 13m and the lower c. 10m O.D., while Orme (1966) describes a Post-glacial shoreline at Portballintrae, east of Portrush, at 8-9m O.D. |
| The regional emphasis of these papers may explain their lack of precision in identifying the exact surfaces being described in most localities. The elevations quoted are well above the c. 1.0 to 1.5m O.D. elevations typical of the shore platforms which form by far the greater part of the Portstewart/Portrush foreshore. Only isolated fragments, and terraces fronting degraded cliffs, on the landward margins of the platforms fall within the higher bands. |
| There are a few publications that relate specifically to the Portstewart-Portrush shore platforms, and these assume that their present form can only be explained in the context of earlier and higher sea level stands. Wilson and Manning (1978) assert that the rock platform is raised because of its elevation above MSL; and then assign a pre- or infra-glacial age on the basis that the Post-glacial Transgression was too short to plane platforms of such width. Davies and Stephens, (1978), also assume that the platforms are elevated, and attribute a Late- or Post-glacial age, although they concede that contemporary processes have played some part in modifying the inherited morphology. |
| In practice there are many problems in using these shore platforms in palaeo-shoreline work. Shore platforms have a high survival potential, so they may have retained form through climatic and tectonic change. If this is the case it would be virtually impossible to unravel the elevation/RSL relationships as the platform morphology demonstrates elements deriving from many periods of planation, including the present. |
| Their elevation above present mean sea level (MSL) is not conclusive evidence that the shore platforms are raised surfaces eroded at a higher RSL. There is no consensus on the basic issue of the relationship between shore platform elevation and MSL. At present not enough is known about process/morphology interactions for shore platform elevations to be any more than very general guides to RSL heights. Certainly any chronological interpretations are unlikely. |
| There is particular difficulty in establishing definite elevation-process relationships on a storm dominated, high energy coastline where wave activity may extend over a vertical range of several tens of metres. It cannot be assumed that a given sea level will cut a shoreline bench approximately at MSL, and that this shoreline will be laterally consistent. Many factors including variations in lithology, structure and exposure, can cause height variations. The local relief variation on a shore platform is often much greater than any conjectured change in RSL elevation, and the perceived or measured tilts. Unfortunately, the cliff base elevation, used as the critical indicator by the shoreline workers, tends to be especially variable on basalts. |
| The elevation of the platforms at this site is readily explained by the fact that the maximum energy levels in the competent storm waves act well above MSL. Indeed the platforms show every indication that they relate to present processes. Erosion is ongoing and active, with recent erosion scars evident on many platform surfaces several metres above O.D.; while the remarkable scarcity of surficial debris below obviously active cliffs, and the rapid dispersal of rockfall blocks, indicates that high energy wave action regularly sweeps the platforms well above MSL. |
| The sea level curve redrawn from Carter (1982), illustrates that RSL on the north coast has remained in the approximate range 0 to 4m O.D. for the last 5000 years. The shore platform literature contains estimates of very rapid planation rates for platforms up to 200m wide in similar or shorter time frames. Given the known vertical range of contemporary wave processes, (Carter, 1983), it is clear that the north coast platforms have been within the zone of effective wave erosion for a sufficient time to account for their formation. In this context it should also be noted that the sub-horizontally bedded, medium hard basalts represent an ideal medium for their development. Thus it is unnecessary to have recourse to inheritance theories to account for the Portstewart- Portrush shore platforms, although this possibility is not excluded. |
| Even where there is indisputable evidence for a Post-glacial shoreline, it is virtually impossible to infer an exact RSL position. At Port Gallen east of Portstewart a contemporary platform is being cut into the seaward edges of a dissected raised platform, the latter backed by degraded raised cliffs. The base of the abandoned cliff, actually a vegetated shore platform, falls from 9.3m to 7.3m O.D. in a longshore distance of 170m. Thus the raised platforms which form the Holocene shoreline demonstrate the same structurally-controlled longshore variation as the contemporary platforms. Given the presence of such longshore tilts, it is simply not feasible to estimate a RSL elevation with any degree of precision. |
| Conclusions: |
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| The basalt cliff/shore platform complex between Portstewart, Co. Derry and Portrush, Co. Antrim illustrates the action of storm wave attack on sub-horizontally bedded, medium hard basalts. While some of the morphology may be inherited the cliffs and platforms are active at contemporary sea level. |
| The site is a good example of this particular suite of coastal landforms, and is the only one in Northern Ireland to have received any detailed study. The site is also readily accessible. |
| Notes: |
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| There is a pressing need for research on the rock coasts of Northern Ireland, both from geomorphological and ecological perspectives. At present, on a purely morphodynamic level, there are no sensible process-based guidelines for rocky coast management. |
| The rock cliff/shore platform coast east of Portstewart is characterised by complex fluid environments resulting from the interaction of lithology and waves. These processes are poorly understood and are largely underdescribed in the literature. Much more work also needs to be done on the crucial question of the relationship between mean sea level (MSL) and shore platform elevation. |
| Ecological studies of the distinctive ecological system of supralittoral coastal cliffs are similarly limited. Plant communities of particular ecological interest are cliff top maritime heath and maritime grasslands, (although on the Portstewart-Portrush coast much of this is already lost to the managed sward of two golf courses). Ledge and crevice habitats too, often support regionally rare species. Research is essential for pinpointing areas vulnerable to species loss and for predicting the impact of that loss on community structure and functioning. |
| References |
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| Carter, R. W. G. 1982: Sea-level changes in Northern Ireland. Proceedings of the Geologists Association of London, vol. 93, pp.7-23 |
| Carter, R. W. G. 1983: Raised coastal landforms as products of modern process variations, and their relevance in eustatic sea-level studies: examples from eastern Ireland. Boreas, vol. 12, pp.167-182 |
| Carter, R. W. G. 1991: Shifting Sands: A Study of the coast of Northern Ireland from Magilligan to Larne. Her Majesty's Stationery Office, Belfast |
| Carter, R. W. G. and Bartlett, D. 1988: Coast erosion and management in the Antrim Coast and Glens and the Causeway Coast Areas of Outstanding Natural Beauty. In Final Report of the Coast Erosion Survey. Department of Environment (Northern Ireland) |
| Davies, G. L. H. and Stephens, N. 1978: The Geomorphology of Ireland. Methuen, London |
| Kerr, I. D. V. 1987: Basement/cover structural relationships in the North Antrim area, Ireland (Ph.D. thesis). (Thesis) Queen's University of Belfast |
| McKenna, J. 1990: Quaternary shore platforms. In Field Guide No. 13. North Antrim and londonderry. Irish Association for Quaternary Studies, Dublin |
| McKenna, J., Carter, R. W. G. and Bartlett, D. 1992: Coast erosion in northeast Ireland: Part 2, cliffs and shore platforms. Irish Geography, vol. 25 pt. 2, pp.111-128 |
| Orme, A. R. 1966: Quaternary changes of sea-level in Ireland. Transactions of the Institute of British Geographers, vol. 39, pp.127-140 |
| Prior, D. B. 1966: Late-glacial and Post-glacial shorelines in north-east Antrim. Irish Geography, vol. 5, pp.173-187 |
| Roberts, J. C. 1976: The joint and fault patterns of the north coast of counties Antrim and Londonderry between Murlough Bay and Castlerock. Proceedings of the Royal Irish Academy, vol. 76B, pp.619-628 |
| Stephens, N. and McCabe, A. M. 1977: Late-Pleistocene ice movements and patterns of Late- and post-glacial shorelines on the coast of Ulster, Ireland. Special Issue of the Geological Journal, numb. 7 In The Quaternary History of the Irish Sea., pp.179-198 |
| Synge, F. and 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 |
| Trenhaile, A. S. and Layzell, M. G. J. 1980: Shore platform morphology and tidal-duration distributions in storm wave environments. Papers of the Geological Survey of Canada., numb. 80-10 In The Coastline of Canada., pp.207-214 |
| Wilson, H. E. and Manning, P. I. 1978: Geology of the Causeway coast. Memoir for one-inch geological sheet 7. Memoirs of the Geological Survey Northern Ireland, HMSO, Belfast |
| Map(s): | See the 1:50,000 O.S. Sheet 4 (Coleraine). |