Earth Science Conservation Review
| Bellevue, Belfast, Co Antrim - Clay-with-Flints | |
| Site Type: | |
| Site Status: | ASSI |
| Council area: | |
| Grid Reference: | J32548110 |
| Google maps: | 54.66104,-5.94667 |
| Rocks | |
|---|---|
| Rock Age: | Cretaceous, Tertiary |
| Rock Type: | Chalk, Clay, Flint, Basalt |
| Interest | |
Summary of site:
Bellevue is important because of its earth science interest. The area provides access to an infrequently exposed deposit called the Clay-with-flints (CwF). This deposit is found between the Cretaceous white limestone below and the Palaeogene black basalt above and spans a period of time of approximately 10 million years. Because of its location beneath the basalt, the CwF is usually only exposed at the edge of the Antrim Plateau.
The Cretaceous white limestone formed on the bottom of a sea floor around 75 million years ago, during a time when the island of Ireland was completely covered by a warm, clear, shallow sea. As the sea level eventually dropped, the resulting limestone was exposed and it underwent a long period of weathering. This would have led to the development of a landscape similar to that of the Burren in Co. Clare, with abundant limestone pavement and associated hollows and caves. The Cretaceous limestone contains abundant nodules and layers of flint, a silica-rich material that does not dissolve in rainwater, unlike the limestone. As weathering continued, it left behind the more robust flints that accumulated on the limestone landscape.
At the beginning of the Palaeogene period, about 65 million years ago, volcanic activity began as a result of stretching and thinning of the crust when the North American and European tectonic plates pulled apart. The resulting lava flows covered much of Northern Ireland and preserved the top surface of the Cretaceous limestone, covering the CwF.
As its name suggests, the CwF is made up of two main components, namely flints, held together by varying quantities of clay. The colour of the clay varies greatly, as does the overall thickness of the deposit. Its position between the underlying white limestone and the overlying black basalt provides information on the environment at that time.
The origin of the CwF has been a subject of great debate amongst geologists. It was initially thought to be a result of weathering at the surface to produce the flint, combined with the residue left behind from the dissolution of the white limestone to produce the clay. Whilst this is correct in the case of the flint, the origin of the clay is more doubtful. Another theory is that the clay component came from an explosive volcanic event that would have produced abundant volcanic material including ash. This would have occurred before the main basalts started forming. The most recent theory indicates that the clay probably came from localised mudflows. These would have contained clays derived from the weathering of basalts which would have been plentiful at the beginning of the Palaeogene period.
Bellevue consists of a line of disused quarries exposing both the white limestone below and the black basalt above. The CwF is exposed both at the northern end and at the southern end of the site.
The most striking elements of the exposures at Bellevue is the uneven surface at the top of the white limestone that is clearly visible in the cliff face. This is a result of the weathering that took place after the fall in sea level at the end of the Cretaceous period. Due to this uneven surface, the thickness of the CwF varies greatly from only a few cms to over 2 metres.
There are two main divisions in the exposed section; the lower half has small to medium sized flints and a soft, pale brown clay component. The upper half has much larger flints with a compact, dark brown to red clay component. Where the white limestone has been weathered to form a hollow, the lower layer is often confined to the lower part of the hollow only. These two distinct layers represent the flints that formed in-situ as a result of weathering of the white limestone (lower layer), followed by the deposition of clay by mudflows associated with volcanic activity (upper layer).
At Bellevue there is another thin layer found at the top of the CwF consisting of dark grey to green clay that contains localised plant fragments. This would indicate that there was sufficient time after the CwF was deposited, and before the basalt lavas erupted, for soils to form.
The Cretaceous white limestone formed on the bottom of a sea floor around 75 million years ago, during a time when the island of Ireland was completely covered by a warm, clear, shallow sea. As the sea level eventually dropped, the resulting limestone was exposed and it underwent a long period of weathering. This would have led to the development of a landscape similar to that of the Burren in Co. Clare, with abundant limestone pavement and associated hollows and caves. The Cretaceous limestone contains abundant nodules and layers of flint, a silica-rich material that does not dissolve in rainwater, unlike the limestone. As weathering continued, it left behind the more robust flints that accumulated on the limestone landscape.
At the beginning of the Palaeogene period, about 65 million years ago, volcanic activity began as a result of stretching and thinning of the crust when the North American and European tectonic plates pulled apart. The resulting lava flows covered much of Northern Ireland and preserved the top surface of the Cretaceous limestone, covering the CwF.
As its name suggests, the CwF is made up of two main components, namely flints, held together by varying quantities of clay. The colour of the clay varies greatly, as does the overall thickness of the deposit. Its position between the underlying white limestone and the overlying black basalt provides information on the environment at that time.
The origin of the CwF has been a subject of great debate amongst geologists. It was initially thought to be a result of weathering at the surface to produce the flint, combined with the residue left behind from the dissolution of the white limestone to produce the clay. Whilst this is correct in the case of the flint, the origin of the clay is more doubtful. Another theory is that the clay component came from an explosive volcanic event that would have produced abundant volcanic material including ash. This would have occurred before the main basalts started forming. The most recent theory indicates that the clay probably came from localised mudflows. These would have contained clays derived from the weathering of basalts which would have been plentiful at the beginning of the Palaeogene period.
Bellevue consists of a line of disused quarries exposing both the white limestone below and the black basalt above. The CwF is exposed both at the northern end and at the southern end of the site.
The most striking elements of the exposures at Bellevue is the uneven surface at the top of the white limestone that is clearly visible in the cliff face. This is a result of the weathering that took place after the fall in sea level at the end of the Cretaceous period. Due to this uneven surface, the thickness of the CwF varies greatly from only a few cms to over 2 metres.
There are two main divisions in the exposed section; the lower half has small to medium sized flints and a soft, pale brown clay component. The upper half has much larger flints with a compact, dark brown to red clay component. Where the white limestone has been weathered to form a hollow, the lower layer is often confined to the lower part of the hollow only. These two distinct layers represent the flints that formed in-situ as a result of weathering of the white limestone (lower layer), followed by the deposition of clay by mudflows associated with volcanic activity (upper layer).
At Bellevue there is another thin layer found at the top of the CwF consisting of dark grey to green clay that contains localised plant fragments. This would indicate that there was sufficient time after the CwF was deposited, and before the basalt lavas erupted, for soils to form.
| Enlander, I., Dempster, M. & Doughty, P., 2024. Bellevue, Belfast, Co Antrim - Clay-with-Flints, County , site summary. [In] Earth Science Conservation Review. https://www.habitas.org.uk/escr/summary.php?item=1310. Accessed on 2024-12-26 |
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