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Earth Science Conservation Review
| Ballycastle – Clay-with-Flints |
| Site number: | 1307 | ||
| Locality Type: | Status: | ASSI | |
| Grid Reference: | D11884165 | ||
| Google maps: | 55.20925,-6.24375 | ||
| County: | Council area: | Causeway Coast & Glens Borough Council | |
| Period: | Cretaceous, Tertiary | ||
| Site Description | |
|---|---|
| Highlights: | |
This site extends along 300m of coastline with the two localities occurring at opposite ends of the section. The lithological contrast between the Clay-with-Flints at the two localities is remarkable and has implications for the mode of formation of the deposit. At the southeast end of the section, at Locality 1, the Clay-with-Flints consists exclusively of flint debris and is thus atypical, while at the northwest end it is clay-based and volcaniclastic and thus typical of the volcanic association identified at all other sites in the Clay-with-Flints. Given the constraints that the lithological differences place on the formation of the Clay-with-Flints, to find them at two locations in such close proximity makes the situation all the more difficult to reconcile. | |
| Introduction: | |
The site is located along the cliffs northwest of the pier at Ballycastle Harbour in northeast Co Antrim. At the site, two main localities are selected in order to demonstrate the lithological variation in the Clay-with-Flints hereabouts. Locality 1 [D1188 4165] occurs below the coastal path 260m northwest of Granny Rock at Ballycastle Harbour and is best viewed from the boulder strewn shoreline (Photograph 31). Locality 2 [D1169 4184] is situated at a height of about 15m above the end of the coastal path, some 530m northwest of Granny Rock. Accessing Locality 2 is by means of a very steep grassy bank and care must be exercised. | |
| Description: | |
Site Geology: Ulster White Limestone Formation: Chalk exposed on the shore and in the cliffs northwest of Ballycastle Harbour is representative of the Early Maastrichtian Ballycastle Chalk Member, the youngest member of the Ulster White Limestone Formation in Northern Ireland. Between Ballycastle Harbour and Castle Point, 800m to the northwest, the thickness of this member preserved beneath the basalt lavas varies considerably and the plane of unconformity representing the top of the Ulster White Limestone Formation is highly irregular (Figures 2 and 3). Clay-with-Flints: In the cliffs northwest of Ballycastle Harbour the plane of unconformity that separates chalk of the Ulster White Limestone Formation from basalt lava flows of the Antrim Lava Group is inclined southwards at about 5o. Thus, at Locality 1 at the southeast end of Site 7, the Clay-with-Flints is exposed on the shoreline just below the coastal path although it is periodically covered by boulders. It appears that the topography of the plane of unconformity etched into the Cretaceous chalk is responsible for controlling the lithological composition of the Clay-with-Flints at the two localities. To the northwest, exposures of chalk appear through the beach deposits and form extensive bedding plane surfaces below the cliff line. At Locality 2, which is coincident with the end of the narrow coastal path, chalk is exposed in the lower part of the cliffs with basalt lava flows forming their upper part. Antrim Lava Group: The cliffs framing the harbour area are composed entirely of sub-horizontal flows of basalt lava of the Lower Basalt Formation. The Clay-with-Flints detail: The lithological contrast between the Clay-with-Flints at Localities 1 and 2 is startling. At both localities, the stratigraphical position of this horizon is indisputable as its lower and upper contacts with the white Cretaceous chalk and black Palaeogene basalt lava flows respectively are exposed. Locality 1 Located at shore level at the southeast end of the coastal section, the Clay-with-Flints is sandwiched between the chalk and basalt lava and is between 0.1 and 0.4m thick (Photographs 32A and 32B). The variable thickness of the bed is dependent on irregularities in the topography of the underlying chalk surface. Locality 2 The Clay-with-Flints at this location has a maximum thickness of 1.4m and in every respect is typical of the Clay-with-Flints at all other localities around the Antrim Plateau (Photograph 33). The deposit occupies a depression in the chalk surface. On the southeast side of the exposure, bedrock is concealed by soil and vegetation, while to the northwest the margin of the depression in the chalk is evident and the Claywith-Flints is absent on top of the limestone gryke forming one edge to the depression (Photograph 34). Its overall brown to pale orange-brown colour with pinkish mottling is a reflection of the clay matrix. Small to large whole flint nodules with broken pieces of flint form a clast-supported framework in the top 0.8m of the deposit whilst in the lower part there is a greater proportion of clay matrix in which the flints are supported. In the upper part of the deposit all flints show a thick white crust which conceals a deeply brownish orange stained interior. This colour staining of the flints decreases downwards with flints in the lower levels showing medium to pale grey and buff colours typical of flints still retained in the chalk host. Petrogenesis: No samples from either locality at Ballycastle were selected for geochemical analysis. However, a sample of the clay matrix from Locality 2 was investigated petrographically and the results are described below. Petrography In thin section, the Clay-with-Flints at Locality 2 is composed of angular clasts of detrital flint, rounded highly altered lava fragments with ragged outline, angular quartz fragments and small <0.01mm equant opaque oxides in a reddish brown matrix. Quartz crystals are concentrated into bands or lenses and appear rounded, suggesting that this is sediment. The matrix is composed of a fine-grained intergrowth of quartz and reddish brown clay minerals. In detail, there is some suggestion of a primary fragmental fabric, and rounded “clasts” recognised as darker regions within the matrix, suggesting primary compositional differences. | |
| Conclusions: | |
1) In every respect, the lithology of the Clay-with-Flints at Locality 1 conforms to the theoretical composition of the flint regolith that is presumed to have collected on the land surface as a result of dissolution of the chalk bedrock. The thickest parts of the deposit here are infilling depressions in the chalk bedrock with only a veneer covering any remaining prominent chalk pillars, beneath the overlying basalt lavas. 2) In contrast to every other known outcrop of the Clay-with-Flints the complete absence of clay in the flint matrix at Locality 1 at Ballycastle is unique. In respect of the original flint regolith that collected on the land surface after dissolution of the chalk, this is hardly surprising since it has been conclusively demonstrated that the Cretaceous chalk in Northern Ireland could not have contributed a significant clay fraction prior to the volcanic eruption that led to the formation of the Clay-with-Flints. Even if there had been a contribution of clay from other, external, sources, it is believed that water, flowing through the open framework of flint nodules, which would have caused their initial release by dissolution from the chalk, would have quickly flushed any trace content of clay out of the deposit leaving behind, as found at Locality 1, a residue comprising only flint debris. At all other localities of the Clay-with-Flints the clay content may account for up to 50% of the matrix. This clay invariably originated from a volcanic source, although whether it arrived at separate parts of the Clay-with-Flints outcrop by ash-fall, or from lahars or pyroclastic flows needs to be determined at each site. 3) It is suggested that the Clay-with-Flints at Locality 2 at Ballycastle is an allochthonous deposit that formed as a direct result of a volcanic eruption, but which was confined to a pre-existing channel/depression in the chalk land surface. Complete mixing of the volcanic clay with the flint regolith, seen in pristine condition at Locality 1, occurred during movement along a linear depression in the chalk. Confinement of the clay-rich volcaniclastic Clay-with-Flints to pre-determined channels helped to isolate further any slightly more elevated parts of the chalk land surface, thus protecting those areas and their cover of flint-rich Clay-with-Flints, such as occurs at Locality 1, from mixing with volcanic material and production of the clay-rich type. | |
| Notes: | |
The full series of Clay-with-Flints records is as follows: Record no. 1300 Introduction to Clay-with-Flints of Northern Ireland Record no. 1301 Belshaw’s Quarry – Clay-with-Flints Record no. 1302 Gibson’s Quarry, Portadown – Clay-with-Flints Record no. 1303 Devlin’ Quarry, Moneymore – Clay-with-Flints Record no. 1304 Glenshane Pass – Clay-with-Flints Record no. 1305 Donald’s Hill – Clay-with-Flints Record no. 1306 Tircrevan Burn – Clay-with-Flints Record no. 1307 Ballycastle – Clay-with-Flints Record no. 1308 Demesne Quarry, Glenarm – Clay-with-Flints Record no. 1309 Magheramorne Quarry, Larne – Clay-with-Flints Record no. 1310 Bellevue, Belfast, Co Antrim – Clay-with-Flints Record no. 1311 Soldierstown Quarry, Moira – Clay-with-Flints | |
| Keywords | |
|---|---|
| Rocks: | Chalk, Clay, Flint, Basalt |
| Access | |
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| Approach: | Private land. The site comprises cliff line exposures up to 530m northwest of the harbour at Ballycastle, Co Antrim within Irish Grid Square D11 41 (Figures 22 and 23). |
| Restrictions: | Private land |
| Record type: | ESCR report | ||
| Entered by: | I J Enlander | ||
| Last updated: | 28 Mar 2019 | ||
| Enlander, I., Dempster, M. & Doughty, P., 2026. Ballycastle – Clay-with-Flints, County , site details. [In] Earth Science Conservation Review. https://www.habitas.org.uk/escr/site.php?item=1307. Accessed on 2026-11-05. |
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