Earth Science Conservation Review
| Ballycastle Pellet Chalk | Antrim |
| Site Type: | Cliff, Coastal section |
| Site Status: | ASSI |
| Grid Reference: | D118417 |
| Google maps: | 55.21014,-6.24371 |
| Rocks | |
|---|---|
| Rock Age: | Cretaceous (Campanian, Maastrichtian) |
| Rock Name: | Ballycastle Chalk Member, Ballycastle Pellet Chalk, Ulster White Limestone Formation |
| Rock Type: | Chalk |
| Interest | |
| Fossil Groups: | Foraminifera |
Summary of site:
Unlike much of the Cretaceous chalk in Britain, the Ulster White Limestone (its Irish equivalent) is a hard, well-cemented and brittle rock from which it is difficult to extract large fossils and almost impossible to recover microfossils (disappointingly, since microfossils are important aids in refining the age of the rocks). At Ballycastle there is a small deposit of a rock described as Pellet Chalk. This weakly cemented rock yields the microscopic shells of foraminifera (single celled protozoans) in great profusion and variety. The site is important because it provides this essential dating information, while the strange nature of the rock raises questions about its origins.
A walkway along the base of the cliff at Partnagree, north-west of Ballycastle harbour, passes first through basalt (lava) cliffs but after about 400m the junction between the lavas and the underlying Ulster White Limestone rises above sea level into clear view. Lying on top of the limestones at the base of the basalt cliff is a deposit of chalky material incorporating some rotten flints and angular pieces of basalt seamed by vertical and horizontal joints (natural fractures). Within this mass a soft limestone consisting of pellets, up to 3mm across, is clearly seen. A historic description of the deposit when it was less overgrown gives a length of 11m and a thickness of 4.3m and on one side the pellet rock was lying against a vertical wall of normal, hard Ulster White Limestone. Thin layers of marly (limy mudstone) pebbles divide the Pellet Chalk. A further 200m north-west, an area of the Ulster White Limestone on the shore (about 5m by 15m) is obviously disrupted, with flints and blocks of basalt up to 0.5m lying in it. Within this is a circular depression (5m across), with Pellet Chalk and flints adhering to its walls. There is an obvious volcanic vent choked with debris in the cliff behind, passing through at least 15m of earlier hardened lava flows and possibly more.
There are at least two explanations for these deposits. The first is that the material flanked a doline (a depression caused by the collapse of an unstable limestone cave) with a basalt rim and was carried into it under gravity by rain wash and the usual processes of weathering. The second is that an advancing lava flow bulldozed loose material lying on the land surface of the Ulster White Limestone into a local depression before overriding it. There are also other possibilities.
That still leaves the softness of the chalk pellets to be explained. It is obvious from jointing that the Ulster White Limestone was already hardened enough to fracture long before the earliest lava flows blanketed the area. Again there are two possible explanations. Either the chalk from which the pellets formed was never fully cemented or, if it was, then it must later have been subjected to some process of decementation. More research is needed to test these options.
The fauna of foraminifera recovered from the Ballycastle Pellet Chalk and its hollow flints includes 90 species and it is known that many more await identification. All date from the very end of the Cretaceous period, including some from the Campanian stage (around 75 million years old) and others from the succeeding Maastrichtian (between 74 and 65 million years ago). Two species are entirely restricted to the late Campanian while another defines the Maastrichtian. That they all occur together in the Ballycastle Pellet Chalk confirms that the deposit comprises mixed fragments of both stages.
The foraminiferan fauna and the chalk containing them are unique in Northern Ireland and give a glimpse of microscopic life in the late Cretaceous ocean and of the little understood processes that led to the formation of the deposit. This is a prime research site needing only the removal of obscuring vegetation to fully restore it.
A walkway along the base of the cliff at Partnagree, north-west of Ballycastle harbour, passes first through basalt (lava) cliffs but after about 400m the junction between the lavas and the underlying Ulster White Limestone rises above sea level into clear view. Lying on top of the limestones at the base of the basalt cliff is a deposit of chalky material incorporating some rotten flints and angular pieces of basalt seamed by vertical and horizontal joints (natural fractures). Within this mass a soft limestone consisting of pellets, up to 3mm across, is clearly seen. A historic description of the deposit when it was less overgrown gives a length of 11m and a thickness of 4.3m and on one side the pellet rock was lying against a vertical wall of normal, hard Ulster White Limestone. Thin layers of marly (limy mudstone) pebbles divide the Pellet Chalk. A further 200m north-west, an area of the Ulster White Limestone on the shore (about 5m by 15m) is obviously disrupted, with flints and blocks of basalt up to 0.5m lying in it. Within this is a circular depression (5m across), with Pellet Chalk and flints adhering to its walls. There is an obvious volcanic vent choked with debris in the cliff behind, passing through at least 15m of earlier hardened lava flows and possibly more.
There are at least two explanations for these deposits. The first is that the material flanked a doline (a depression caused by the collapse of an unstable limestone cave) with a basalt rim and was carried into it under gravity by rain wash and the usual processes of weathering. The second is that an advancing lava flow bulldozed loose material lying on the land surface of the Ulster White Limestone into a local depression before overriding it. There are also other possibilities.
That still leaves the softness of the chalk pellets to be explained. It is obvious from jointing that the Ulster White Limestone was already hardened enough to fracture long before the earliest lava flows blanketed the area. Again there are two possible explanations. Either the chalk from which the pellets formed was never fully cemented or, if it was, then it must later have been subjected to some process of decementation. More research is needed to test these options.
The fauna of foraminifera recovered from the Ballycastle Pellet Chalk and its hollow flints includes 90 species and it is known that many more await identification. All date from the very end of the Cretaceous period, including some from the Campanian stage (around 75 million years old) and others from the succeeding Maastrichtian (between 74 and 65 million years ago). Two species are entirely restricted to the late Campanian while another defines the Maastrichtian. That they all occur together in the Ballycastle Pellet Chalk confirms that the deposit comprises mixed fragments of both stages.
The foraminiferan fauna and the chalk containing them are unique in Northern Ireland and give a glimpse of microscopic life in the late Cretaceous ocean and of the little understood processes that led to the formation of the deposit. This is a prime research site needing only the removal of obscuring vegetation to fully restore it.
| Enlander, I., Dempster, M. & Doughty, P., 2024. Ballycastle Pellet Chalk, County Antrim, site summary. [In] Earth Science Conservation Review. https://www.habitas.org.uk/escr/summary.php?item=110. Accessed on 2024-12-26 |
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