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Earth Science Conservation Review
| Portrush NNR | Antrim |
| Site number: | 60 | File Number: | C84 |
| ESCC: | EF | ||
| Locality Type: | Coastal section | Status: | ASSI |
| Grid Reference: | C857412 Centroid | ||
| Google maps: | 55.21087,-6.65372 | ||
| County: | Antrim | Council area: | Causeway Coast & Glens Borough Council |
| Period: | Tertiary, Jurassic | ||
| Stages: | Eocene, Lower Lias, Palaeocene | ||
| Lithostrat: | Lias Undivided, Lower | ||
| Site Description | |
|---|---|
| Highlights: | |
An occurrence of Tertiary dolerite intruded as a sill into Lias shales producing a fine grained dark hornfels rock containing abundant and easily recognizable fossils, particularly ammonites. | |
| Introduction: | |
The Portrush Sill is a large dolerite intrusion which forms the promontory on which Portrush is built, terminating in the cliffs of Ramore Head. It extends offshore in an arcuate chain of islands known as the Skerries and is intruded into Lias shales which have been extensively hornfelsed. The base of the intrusion is never exposed and the maximum thickness has been estimated at approximately 45 m (Hawkes and Wilson, 1975). The hornfelsed Lias shales containing abundant mainly ammonite fossils, known as "Portrush Rock", were the subject of controversy in the late 18th century between "Neptunists" and "Vulcanists". The site therefore is an important locality in the development of geology as a separate scientific discipline. A review of the early literature of the late 18th and early 19th centuries is provided by Wilson and Manning (1978) along with the petrography and field occurrence of the sill, and Hawkes and Wilson (1975) discuss detailed aspects of the petrography and mineralogy. | |
| Description: | |
The area of the sill in Portrush, covering the foreshore from the Blue Pool (near the Countryside Centre) north to near the harbour at Portandoo, was designated a National Nature Reserve by the DOE (NI). Overall the upper surface of the sill dips gently eastwards at about 5 deg. and this upper surface can be seen between the Blue Pool and Portandoo Harbour on the foreshore, overlain by extensive slabs of metamorphosed Lias forming a fine-grained dark hornfels. The contact between the sill and the Lias can be seen in a number of localities, and the dolerite in the contact zone is chilled and shows a finer grainsize than the interior of the sill. The hornfelsed Lias shales show near horizontal bedding planes and many of these planes show ammonite fossils such as Paltechioceras (Fig 4, Wilson and Manning, 1978). | |
| Importance: | |
The Portrush Sill is one of several large intrusions in north Antrim (the others are the Knocksoghy Sill and the Sheep Island Sill) and as such represents an important component in the overall Tertiary igneous province. The greater importance of this particular section of the Portrush Sill however is in its association with the over-lying hornfelsed Lias and the significance of the outcrop in the historical development of geological science from the 18th century onwards. The claim was made by the Neptunists that the fine grained dark hornfels was in fact basalt. Since it demonstrably contained fossils this was regarded as proof that basalt was formed by precipitation from sea-water. The Vulcanists who believed in an igneous origin for rocks such as basalt were soon able to show the true nature of the Lias hornfels, but not before "Portrush Rock" became known across the entire scientific world of the time. The locality is an important teaching locality for both the development of geology as a science, and also in the teaching of the concepts of magmatic intrusion and the thermal metamorphism of country rock. | |
| Interpretation: | |
The locality shows clearly the effects of intrusion of dolerite, mainly having penetrated along the bedding planes of the Lias shales. The contact metamorphism caused by the heat of the magma has produced a fine-grained dark coloured hornfels containing recognizable fossils particularly ammonites. Chilling of dolerite at the margins of the intrusion has produced a fine-grained contact zone, with the dolerite becoming progressively coarser towards the sill interior. | |
| Conclusions: | |
The site offers clear evidence of the lithological relationships involved, shows the effects of thermal metamorphism on sedimentary rocks and demonstrates chilling at the margins of intrusions. Loose material around the exposure still allows the opportunity to collect hornfels material (occasionally fossil-bearing) and dolerite, although these opportunities will clearly decline in future. | |
| Notes: | |
People examining the exposures below the HWM need to exercise extreme caution at times due to the slippery nature of the rock types, particularly when wet or covered with seaweed. The critical parts of the locality are well exposed, although many of the best examples of hornfelsed ammonites have been destroyed or removed already. The site can withstand no further sampling if it is to remain viable. | |
| Keywords | |
|---|---|
| Rocks: | Dolerite, Hornfels, Shale |
| Fossil Groups: | Ammonite |
| Structures: | sill |
| Relations: | Intrusion |
| NonGeol: | National Nature Reserve |
| Measurements | |||||
|---|---|---|---|---|---|
| Length: | ~200 m | Width: | Height: | ||
| Depth: | Area: | ||||
| Access | |
|---|---|
| Approach: | The site is foreshore between HWM and LWM, with a grassy verge between Lansdowne Road/Bath Road and the HWM. Access is open. |
| Restrictions: | The site is a no-hammer locality. |
| Management: | |
Access is open. The site must be retained as a no-hammer locality. It can withstand no further sampling if it is to remain viable. | |
| Threats: | Over-sampling |
| References (2) | |||
|---|---|---|---|
| Hawkes, J.R. & Wilson, H.E., 1975. The Portrush Sill, County Antrim, Northern Ireland. Bulletin of the Geological Survey of Northern Ireland, numb. 51. | |||
| Wilson, H.E. & 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): | For site location see the 1:50,000 O.S. Sheet 4 (Coleraine). | ||
| Record type: | Environment Serv. GIS, ESCR report, J. Preston l | ||
| Entered by: | Hamilton, M.J. | ||
| Enlander, I., Dempster, M. & Doughty, P., 2026. Portrush NNR, County Antrim, site details. [In] Earth Science Conservation Review. https://www.habitas.org.uk/escr/site.php?item=60. Accessed on 2026-11-08. |
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