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Earth Science Conservation Review
| Donald’s Hill – Clay-with-Flints |
| Site number: | 1305 | ||
| Locality Type: | Status: | ESCR | |
| Grid Reference: | C74271794 | ||
| Google maps: | 55.00348,-6.8404 | ||
| County: | Council area: | Causeway Coast & Glens Borough Council | |
| Period: | Cretaceous, Tertiary | ||
| Site Description | |
|---|---|
| Highlights: | |
In the NW sector of the Antrim Plateau, as everywhere else around its periphery, the stratigraphical hiatus between the Late Cretaceous Ulster White Limestone Formation and the Early Palaeogene Antrim Lava Group is partly represented by the Clay-with-Flints deposit. Donald’s Hill is the northern most outcrop of, and stratotype section for, the Donald’s Hill Ignimbrite Formation (DHIF). The DHIF is a bed of welded ash-flow tuff that always occurs at the top of the Clay-with-Flints, but whose outcrop is confined to the NW sector of the Antrim Plateau. What distinguishes the DHIF from the remaining, lower, part of the Clay-with-Flints in that area is that it is silicified. This distinctive and extremely hard lithology was formed by the alteration of the upper part of the Clay-with-Flints by siliceous fluids percolating downwards from the overlying basalt lava flows. Thus, at Donald’s Hill and at all other localities within the outcrop of the DHIF mentioned in the description of Site 5, the Clay-with-Flints is divided into an upper bed, the DHIF, and a lower section of unsilicified lithologies typical of the Clay-with-Flints. In the NW sector of the Antrim Plateau, apart from exposures in quarry sections, the known outcrop of the DHIF is based to a lesser extent on the distribution of natural exposures, and more on the presence of loose blocks of this highly resistant and distinctive lithology. | |
| Introduction: | |
What started as a natural outcrop of chalk on the south side of Donald’s Hill was later developed as a quarry along some 150m of hillside (Photograph 21). The chalk extracted was used in the production of agricultural lime. However, in view of the thickness of basalt lavas overlying the chalk it is unlikely that production was sustained for very long and chalk extraction now has ceased. Small, vegetated conical spoil heaps of waste material from the extraction of the chalk lie at the downslope edge of the site and partly shield the rock face from view. However, apart from the natural degradation of the rock face there has been no further despoiling of the site. | |
| Description: | |
Site Geology: The linear hillside exposure on the south side of Donald’s Hill is the stratotype section for the DHIF. Within that hillside exposure, the full thickness of the Clay-with-Flints and the most accessible section in the DHIF is located at the north end. Ulster White Limestone Formation: Lying unconformably beneath the DHIF is white chalk of the Late Cretaceous Ulster White Limestone Formation. Although the chalk section at Donald’s Hill did not form part of Fletcher’s review (1977) of the Ulster White Limestone Formation in Northern Ireland, it is located equidistant between his measured sections at Keady Mountain quarry and Curraghlane, 7km NNW and 8km SSW of Donald’s Hill respectively. At both of these sections the chalk is assigned to the Late Campanian Ballymagarry Chalk Member (Figures 2 and 3). Fletcher interpreted this as indicating that the pre-basalt erosion surface between these two sections maintained a consistent level within that particular chalk member, which would suggest that the few metres of chalk exposed at Site 5 on Donald’s Hill also belong to the Ballymagarry Chalk Member. The basal beds of the chalk are not exposed at Donald’s Hill but red-bed sedimentary rocks of the Early Triassic Sherwood Sandstone Group, that occur unconformably below the chalk, are exposed on lower slopes at Donald’s Hill, just below Site 5. Clay-with-Flints (Donald’s Hill Ignimbrite Formation): In the stratotype section on Donald’s Hill the total thickness of the Clay-with-Flints is about 1.25m of which the single bed of the DHIF is 0.9-1.1 thick (Photograph 22). Elsewhere, on the west flank of the Antrim Plateau, in quarries at Kilhoyle [C753 166] and Moneyguiggy [C757 161], 1.5km and 2.5km respectively SE of Donald’s Hill, the DHIF varies from 0.6-0.8m in thickness and overlies a section of unsilicified Clay-with-Flints between 0.5 and 1.0m thick. South of Donald’s Hill, at the eastern limit of the Sperrin Mountains, the DHIF is just over 1m thick on the NW flank of Glenshane Mountain [C748 032] where it rests on Carboniferous and Triassic sandstones. Antrim Lava Group: The upper part of the section at Site 5 on Donald’s Hill comprises basalt lava flows of the Early Palaeogene Antrim Lava Group. On the 1:250,000 Geological map of Northern Ireland (GSNI, 1997), these lavas are assigned to the Upper Basalt Formation, although it is now believed that the correct age of the basalt lavas immediately overlying the DHIF at Donald’s Hill can be confirmed at Kilhoyle Quarry [Grid Ref: C752 166], 1.5km ESE of Site 5. There, the DHIF is overlain by 75m of basalt lavas which are in turn overlain by a ‘bole’ consisting of 13m of deeply weathered, clay-rich, greenish, lateritised basalt lava, exposed near the top of the quarry, which is overlain by crudely columnar fresh basalt lava flows. Whilst it is conceivable that this ‘bole’ represents an unusually thick interbasalt flow horizon that developed during an extended period of time between successive lava flows, in the absence of any evidence to the contrary it is referred to the Interbasaltic Formation and the underlying basalt lavas belong therefore to the Lower Basalt Formation, representing the stratigraphical base of the Antrim Lava Group. The Clay-with-Flints (Donald’s Hill Ignimbrite Formation) detail: Although not at the time recognised as being of volcanic origin, the extremely hard and siliceous nature of the single bed that constitutes the DHIF was noted by Portlock (1843) and Patterson (1956), and on 1:10560 scale field maps of the Geological Survey of Northern Ireland, but its regional distribution and significance are only now recognised. Its outcrop extends from the stratotype section (Photograph 23) on the west flank of Donald’s Hill [C7385 1800] to Sounding Hill Quarry [H887 915], 30km to the SSE at Magherafelt, Co. Londonderry. The southern limit of the DHIF is not known, but the unsilicified, welded, ash-flow tuff is not present in chalk quarries at Carmean [H865 876], 4.5km SW of Sounding Hill or at any other section on the Antrim Plateau that exposes the base of the Antrim Lava Group. Meaningful calculations of the original aerial extent, volume and provenance are, however, not possible from the present-day two dimensional outcrop pattern of the DHIF. Petrogenesis: In hand specimen the rock matrix is very fine-grained, creamy white and pale grey to pinkish grey (Photograph 24). The streaky, poorly-banded matrix appears to wrap around and terminate against the clasts (Photograph 25). Clasts derived from the country rock are exclusively flint. These originally formed as bands of nodules within the Ulster White Limestone Formation, and upon release by dissolution of the limestone, in late Cretaceous and Early Palaeogene times, developed as a regolith overlying older levels of the Cretaceous chalk and the pre-Cretaceous ‘basement’. These loose flints were subsequently entrained and distributed throughout the ash flow during its passage across the exposed chalk land surface. Fragmentation of the flint may have resulted from the explosive nature of the ignimbrite-forming eruptions, indicating incorporation primarily from around the immediate vicinity of the vent, or may have occurred during accumulation on the former chalk land surface. Thus, while some of the flints are intact, with a sub-rounded outline, most are fragmented and the ash-flow is full of sharp and angular millimetre to centimetre long flakes. The colour of many of the flint clasts is radically different from their original pale greyish brown hues in the chalk. Small flints, up to 3-4cm in diameter, are a uniform dark grey with a darker outer rim, whereas larger examples, from 10-20cm long, although still retaining a peripheral pale to dark grey rim up to 4mm thick, are internally altered to a variety of shades of deep reddish purple and pink due to hematization. Petrography In thin section, the rock appears to have been a glassy, welded ash-flow tuff. A eutaxitic texture is defined in the matrix by slight variations in the optical density of individual fragments, including deformed glass shards and compacted pumice (fiammé), occasionally picked out by concentrations of opaques at their margins (Photograph 26). The matrix also contains abundant small quartz phenocrysts and crystal fragments (up to 50 vol. %) and volcanogenic lithic fragments of more basic lava up to several millimetres long. Preservation of flow banding is characteristic of parts of the creamy white to pale brown, pink or pinkish grey fine-grained matrix. Texturally, most flint clasts appear to be unaffected by their incorporation into the flow, although some show concentric discoloration and slightly lobate margins suggesting some reaction with the hot ash flow (Photograph 27). Features of ignimbrites also observed in this deposit include concentrations of discrete crystal phenocrysts in the matrix, deformation of fiammé and glass shards and rheomorphism and welding. In ash-flow tuffs of andesitic or rhyolitic composition the development of these features requires a minimum temperature in the range of 600-7500C and is dependant on volatile content and load pressure. At Donald’s Hill, the degree of silicification in the DHIF is consistent from the top down, except in the lowest few centimetres of the bed within which there is a rapid decrease in silica. In some sections, such as in the quarries at Moneyquiggy and Kilhoyle, the base of the DHIF and the change from silicified (DHIF) to unsilicified (Clay-with-Flints) lithologies is abrupt. Thus, below the gradational base of the DHIF in the stratotype section, the lower part of the deposit that rests on white chalk of the Ulster White Limestone Formation consists of about 0.3m of lithologies typical of the Clay-with-Flints. These consist of fawn brown clay usually stained deep pink to reddish purple with whole, and fragmented, uncoloured flints. In thin section, unsilicified samples of the clay matrix from below the DHIF appear to have been slightly hornfelsed. This has facilitated the preservation of primary structures which accord with the belief that the deposit originated as an ignimbrite and represents a highly altered devitrified welded ash-flow tuff. Further data, based on the chemical analysis of samples from below the base of the DHIF, add weight to the conclusion that there is nothing inconsistent with a volcanogenic origin for the Clay-with-Flints. Geochemistry Plots of immobile element ratios from the unsilicified basal part of the Clay-with-Flints at Donald’s Hill show that the volcanic material in the lower part of the deposit was andesitic in composition. Source of the pyroclastic flows: The location of the parent volcano is not known but is likely to have been located within the area of the Antrim Plateau; as such its presence is now concealed by basalt lavas of the Antrim Lava Group. In support of this conclusion is the fact that the clast content of the Clay-with-Flints consists exclusively of flint derived from the Cretaceous chalk. Therefore, it must be assumed that the volcanogenic products of the early eruption, pyroclastic flows, ash-fall and lahars, travelled only over a chalk land surface, at least within the area presently occupied by the Antrim Plateau and the outcrop of the Clay-with-Flints. At the northwest margin of the Antrim Plateau, the Clay-with-Flints is seen to rest not only on chalk but on Triassic and Carboniferous ‘basement’. Even in those areas the clast content of the Clay-with-Flints still consists only of flint. Thus, it seems undeniable that the parental acidic or intermediate volcano must have been located centrally in the area now occupied by the Antrim Plateau. This ensured that the early voluminous volcanic eruption, dispersing radially from that point source over an exclusively chalk land surface, entrained only flint debris. In the event that the parent volcano was located some distance from the present area of the Antrim Plateau, the pyroclastic flows and lahars would have been able to sample a regolith of other, exotic, lithologies, probably in greater proportion to the flints, and that is not observed in any section of the Clay-with-Flints. | |
| Conclusions: | |
1) The DHIF is a silicic welded ash-flow tuff that is rarely more than 1m thick and has a 30km long outcrop. 2) Silicification of the upper part of the original deposit of the Clay-with-Flints, by fluids percolating downwards from the basalt lavas, has produced a bed of extremely hard ash composed of 98% cryptocrystalline silica. 3) Silicification is not apparent in the clay matrix in the lower part of the Clay-with-Flints within the outcrop of the DHIF. Nevertheless, the effect of slight baking of those clays by heat from the upper part of the ignimbrite (pyroclastic flow) has preserved the same textures as are present in the DHIF. 4) The ignimbrite was generated by a voluminous volcanic eruption of andesitic composition that occurred before the extrusion of basalt lavas of the Antrim Lava Group. 5) The location of the parent volcano is not known but is likely to have been within the present area of the Antrim Plateau. | |
| 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 | |
|---|---|
| Approach: | Private land. Disused quarry [Grid Ref: C7427 1794], SW flank of Craiggore hill (Photograph 20), 335m SSW of summit of Donald’s Hill, adjacent to the boundary between Drumsurn Upper and Kilhoyle townlands, 2.42km ESE of Drumsurn, 9.5km SE of Limavady town centre, Co. Londonderry (Figures 17 and 18). |
| Restrictions: | Private land |
| Record type: | ESCR report | ||
| Entered by: | I J Enlander | ||
| Last updated: | 28 Mar 2019 | ||
| Enlander, I., Dempster, M. & Doughty, P., 2026. Donald’s Hill – Clay-with-Flints, County , site details. [In] Earth Science Conservation Review. https://www.habitas.org.uk/escr/site.php?item=1305. Accessed on 2026-11-06. |
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