Earth Science Conservation Review
| Church Bay, Rathlin Island | Antrim |
| Site Type: | Pit, Scarp face |
| Site Status: | ASSI |
| Council area: | Moyle District Council |
| Grid Reference: | D148506 |
| Google maps: | 55.28936,-6.19299 |
| Rocks | |
|---|---|
| Rock Age: | Quaternary (Holocene, Pleistocene) |
| Interest | |
| Other interest: | Glaciomarine deposit, Late-glacial deposit, gravel barrier, raised shoreline, The area is presently designated in terms of its a |
Summary of site:
At the height of the last glaciation of the Pleistocene ice age in Ireland (the Midlandian), continental ice sheets extended from the North Pole to the far south of Ireland and Britain. The Midlandian started around 75,000 years ago and ended about 10,000 years ago. During that time there were mild periods interspersed with times of intense cold. At the climax of the most severe cold phase, around 15,000 years ago, the ice covering what is now the northern coastal area of Ireland could have been well over a kilometre thick.
Such a thickness has two major effects. The first is a steep fall in sea level since ultimately the water needed to make the ice must come from the world's oceans. The second is the depression of the ice-loaded land which is pushed into the deep, plastic layers of rock beneath the crust. Both drastically affect sea level and, as the climate improves and the ice sheets melt, further coastal complications arise. While the addition of meltwater has an immediate effect on sea level, the unloading of the land does not result in an instant bounce-back. Equilibrium returns in a less predictable, pulsed series of adjustments. It is the interaction between these processes that has created our present complex coastline and the problems its interpretation present. The massive gravel barrier fronting Church Bay on Rathlin Island is just one example of this complexity.
The village at Church Bay is built on an extensive ridge defining the outer edge an undulating platform behind, mostly below the 10m contour. The platform ends abruptly inland at an escarpment extending from cliff lines to the north and south. The width of the platform is around 600m and its length is about 1.2km. Viewed from above, the ridge appears to run parallel with the modern coast before producing a second ridge that curves progressively inland to the east.
A gravel pit shows that the ridge consists of chalk (Ulster White Limestone) gravel, particle sizes varying between 2 and 4cm, in beds inclined at 25-28º to the east.
Sea snail shells found in the gravel have been carbon dated and found to be 12,200-12,400 years old.
Bill Carter interpreted the feature as a spit - a bar built from chalk debris driven by waves along the coast from the south. The sea extended over 600m behind the present coastline at the time. As the bar extended from the south into open water in the bay, its tip was forced east by wave action, causing it to curve. Eventually the spit extended to the far, northern side of the bay, isolating the platform behind. This would have taken place with sea levels probably 8-10m above those of the present day.
A second and earlier interpretation recognises two spits built on to a pre-existing raised beach (a beach, now stranded, cut and loaded with sediment when sea level was higher). There were isolated brackish lagoons behind them. This interpretation also recognises a second raised beach.
The internal structure of the gravels suggests initial formation on a beach front but with particle size increasing upwards it seems likely that larger waves broke over the first deposits, throwing coarser material over the top and behind them. A rising sea level would have aided this accretionary process.
Study of coastal features associated with glaciation is in its infancy here but research on the Church Bay gravel barrier is an important contribution and has given an age, some indication of sea level around 10,000 years ago and alternative explanation for the evolution of the structure.
The major threat to the site is gravel exploitation which could intensify and seriously modify or destroy the barrier in future. The present designation of the Viking cemetery on the site should be fortified by a further designation of the barrier for its geomorphological importance. These features have considerable future research potential.
Such a thickness has two major effects. The first is a steep fall in sea level since ultimately the water needed to make the ice must come from the world's oceans. The second is the depression of the ice-loaded land which is pushed into the deep, plastic layers of rock beneath the crust. Both drastically affect sea level and, as the climate improves and the ice sheets melt, further coastal complications arise. While the addition of meltwater has an immediate effect on sea level, the unloading of the land does not result in an instant bounce-back. Equilibrium returns in a less predictable, pulsed series of adjustments. It is the interaction between these processes that has created our present complex coastline and the problems its interpretation present. The massive gravel barrier fronting Church Bay on Rathlin Island is just one example of this complexity.
The village at Church Bay is built on an extensive ridge defining the outer edge an undulating platform behind, mostly below the 10m contour. The platform ends abruptly inland at an escarpment extending from cliff lines to the north and south. The width of the platform is around 600m and its length is about 1.2km. Viewed from above, the ridge appears to run parallel with the modern coast before producing a second ridge that curves progressively inland to the east.
A gravel pit shows that the ridge consists of chalk (Ulster White Limestone) gravel, particle sizes varying between 2 and 4cm, in beds inclined at 25-28º to the east.
Sea snail shells found in the gravel have been carbon dated and found to be 12,200-12,400 years old.
Bill Carter interpreted the feature as a spit - a bar built from chalk debris driven by waves along the coast from the south. The sea extended over 600m behind the present coastline at the time. As the bar extended from the south into open water in the bay, its tip was forced east by wave action, causing it to curve. Eventually the spit extended to the far, northern side of the bay, isolating the platform behind. This would have taken place with sea levels probably 8-10m above those of the present day.
A second and earlier interpretation recognises two spits built on to a pre-existing raised beach (a beach, now stranded, cut and loaded with sediment when sea level was higher). There were isolated brackish lagoons behind them. This interpretation also recognises a second raised beach.
The internal structure of the gravels suggests initial formation on a beach front but with particle size increasing upwards it seems likely that larger waves broke over the first deposits, throwing coarser material over the top and behind them. A rising sea level would have aided this accretionary process.
Study of coastal features associated with glaciation is in its infancy here but research on the Church Bay gravel barrier is an important contribution and has given an age, some indication of sea level around 10,000 years ago and alternative explanation for the evolution of the structure.
The major threat to the site is gravel exploitation which could intensify and seriously modify or destroy the barrier in future. The present designation of the Viking cemetery on the site should be fortified by a further designation of the barrier for its geomorphological importance. These features have considerable future research potential.
| Enlander, I., Dempster, M. & Doughty, P., 2024. Church Bay, Rathlin Island, County Antrim, site summary. [In] Earth Science Conservation Review. https://www.habitas.org.uk/escr/summary.php?item=1137. Accessed on 2024-12-26 |
| Previous Site | Next Site |