| Site Description |
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| Highlights: |
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| Lower Basalt flows are exposed overlying Cretaceous Chalk with a complex of small explosive vents containing both basaltic and chalky agglomerate. |
| Introduction: |
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| The White Rocks locality, 3km to the east of Portrush, on the coast, shows good evidence of the explosive activity phase which was associated with the onset of Tertiary igneous activity in Antrim. This activity was probably phreatomagmatic and a consequence of magma moving into water-saturated sediments at shallow depths, probably as sill intrusions (Preston, 1982). The small scale explosive vents at the White Rocks have been described in detail by Patterson (1963) who describes a series of outcrops from Stradly Rock to Gulls Point. The irregular upper surface of the Cretaceous Chalk is overlain by lavas of the Lower Basalt Formation and the Chalk in places is significantly brecciated and both basaltic and chalky agglomerate occur. The most accessible localities are those around Stradly Rock. A general description of the geology of the area is given in Wilson and Manning, 1978. |
| Description: |
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| Patterson (1963) records and describes agglomerate occurring at nine apparently distinct localities between the shore to the west of Stradly Rock and Gulls Point to the east, (C886408) to (C900412). At Gulls Point the promontory consists of chalk-flint agglomerate which has a steep contact to the east against undisturbed chalk, with a small off-shore island composed of basaltic agglomerate. In the bay to the west of Gull Point the base of the lavas consists of coarse basaltic agglomerate up to 6m thick thinning to the west. A number of further localities occur at Sheelas Head, Portrush and Jackstone Cove showing chalky and basaltic agglomerate, containing in places large rounded blocks of basalt. At Priest's Hole there is brecciated chalk with flint fragments and pieces of rounded basalt up to 1.5m across. The agglomerate here has steep margins and grades into normally bedded chalk. |
| A further five localities showing agglomerate are recorded from Stradly Rock westwards. At Stradly Rock which is about 400m across, the Chalk has clearly been shattered, disrupted and re- cemented by explosive volcanic action which has formed agglomerate-filled vents and deposits of tuff. The best example occurs about 150m to the east of the car park (Patterson, 1963, locality 14) and appears to be roughly aligned in a NW-SE direction with near vertical contacts with the Chalk in the adjoining cliff face which shows the steep contact between dark basaltic agglomerate and shattered chalk. The NW-SE orientation and the vertical contact with the Chalk on either side suggests it may represent agglomerate preserved along the line of an eruptive fissure or vent. |
| Importance: |
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| The White Rocks vents are clear examples of the initial phase of explosive volcanic activity which preceded the eruption of the Lower Basalts and as such are important features in the overall interpretation of Tertiary igneous activity in NE Ireland. Although on a much smaller scale than the Carrickarade and Kinbane Head vents, the outcrops around Stradly Rock are clearly exposed and readily accessible and represent an important teaching locality at all levels of interest. |
| Interpretation: |
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| The initial phase of eruption in Antrim featured explosive activity, primarily seen in a series of vents exposed in north Antrim, and including the much larger occurrence at Carrickarade some 15km east of these vents. Preston (1982) has suggested that these events were probably phreatomagmatic and a consequence of magma moving into water-saturated sediments, probably as sills. These small scale vents are concentrated in a relatively narrow zone between the White Rocks and Dunluce Castle ~2km to the east. The source of the gas to provide the explosive force is likely to be a magma body moving to the surface from depth where the volume of gas was possibly augmented by steam produced by interaction between the hot magma and water contained in the sediments it was passing through. It may be significant therefore that these vents are near the centre of the roughly circular outcrop of the Portrush Sill, known to be 45m thick. This sill forms Ramore Head at Portrush, the islands of the Skerries several km offshore to the NW of the White Rocks, and also probably the offshore rocks known as the Storks (Wilson and Manning, 1978). It is known that this large body of dolerite occurs over the area at a relatively shallow depth in the crust and would have been more than capable of supplying the explosive outgassing required to produce these vents. While there is an undoubted volcanic origin for these features there is a possibility that elsewhere along the coast some apparently similar features are the result of collapse of basaltic material into solution hollows on the Chalk surface. For example features seen at the contact between Chalk and Lower Basalts at the nearby Ballymagarry Quarry could be interpreted as collapse structures. |
| Conclusions: |
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| CONCLUSION |
| The site offers clear evidence of explosive volcanism involving interaction between basaltic magma and Cretaceous Chalk country rock. |
| Notes: |
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| To maintain the integrity of the site, an area from the top of the cliff to the low water mark should be considered. |
| References |
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| Patterson, E.M. 1963: Tertiary vents in the northern part of the Antrim plateau. Quarterly Journal of the Geological Society of London, vol. 119, pp.419-443 |
| Preston, J. 1982: Eruptive Vulcanism, Chapter 28 in Igneous Rocks of the British Isles. , pp.351-368 Wiley, New York |
| 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 1:50,000 O.S. Sheet 4 (Coleraine). |