GEOLOGICAL SUCCESSION AT THE GIANT’S CAUSEWAY

The Lower Basalts

The Lower Basalts are a series of up to 11 complex, olivine-rich flows that are largely medium grained in texture. Typically, flows consist of a thin basal vesicular layer, a compact middle layer and a thick vesicular top. Individual flows are on average some 7m thick and can be separated by layers of consolidated and weathered wind blown dust or by narrow layers of weathered basalt that can be topped by a thin lignite deposit. The Lower Basalts are best seen beneath the Inter-Basaltic Bed in the coastal section to the east of the Causeway at Plaiskin and Benbane heads, but these sections are generally inaccessible. However, the road from the Visitors Centre towards the Grand Causeway passes through Lower Basalts at Windy Gap (the Onion Skin Rocks) that exhibit excellent examples of spheroidal weathering. Typical Lower Basalt flow junctions are also exposed in the crags of Great Stookan just to the north of Windy Gap.

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Weathered Lower Basalts (the so called Onion Skin Rocks) at Windy Gap.


The Lower Inter-Basaltic Bed (Port na Spaniagh laterite)

The Lower Inter-Basaltic Bed was formed by the deep weathering of the uppermost flow of the Lower Basalt, possibly aided by the circulation of hot groundwater, to produce a total thickness of some 15m of weathered material that can be clearly seen in the cliffs to the east of the Causeway. As indicated in the original submission to UNESCO in 1985, the weathering involved the decomposition of aluminium silicates, the extensive leaching of silica, magnesia and calcium hydroxide and the formation of residues rich in ferric oxide, aluminium hydroxide and free silica. Complete decomposition results in a residual layer comprising iron and titanium oxides. However, along the Causeway Coast, incomplete decomposition has resulted in a mix of ferruginous, lateritic material that is predominantly red in colour and rich in iron and aluminium, and a grey/green lithomarge that is richer in silica. Within the Inter-Basaltic Bed (e.g., below Roveran Valley Head) it is possible to find in situ, rounded corestones of partially weathered basalt blocks (known locally as ‘Giant’s Eyes’). These testify to the origin of the Bed from weathering rather than through deposition.

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Partially weathered corestone in the Inter–Basaltic Bed, near the Amphitheatre.

The formation of the Inter-Basaltic Bed was accompanied by significant fluvial erosion to produce an extensive drainage network sometimes flowing in deep valleys. One such valley was formed at the site of what is now the Giant’s Causeway. Its margins are defined by the outcrops of Inter-basaltic material on the eastern side of Port Noffer and at the side of the road just below the Visitors Centre, and its form is indicated by the gradual dip of the Lower Basalts to below sea level between these two points. It was into this valley that the lava of the Middle Basalts flowed that was ultimately to be exposed millions of years later as the Giant’s Causeway.


Middle Basalts (Causeway Tholeiite Member CTM)

geologic_succession3 These are a series of thick, fine-grained, tholeiitic (olivine-poor) lavas that average c. 18m in thickness, can be as much as 30m thick, and together can exceed over 150m. However, the most striking feature of these basalts are the distinctive structural characteristics of individual flows. It was Tomkeieff (1940), who first likened the structures of the Middle Basalt lava flows to architectural elements of a classical building. By which each lava sequence is seen to comprise a ‘colonnade’ of regular vertical columns, capped by an ‘entablature’ of narrower, more irregular and often curved columns. This structure derives in part from the greater thickness of individual lava flows that were allowed to pond in river valleys that had formed on the Lower Basalt landscape, and at what is now the Giant's Causeway the first flow is almost 100m thick. The slow cooling of these thick and chemically and mineralogically homogeneous lava flows led to the development of a complex set of cooling points forming spectacular and often very regular columns. The formation of the entablature at the top of the sequence is thought to relate to the inundation of the cooling flow by water from river drainage in the area that was displaced during the eruption. The percolation of this water disrupted the cooling of the upper part of the flow to produce the narrow, irregular columnar zone of the entablature. These columns grew downwards from the flow surface and therefore take a number of inclinations and orientations related to the topography of the former landscape. In contrast, the more regular, largely vertical columns of the colonnade grew upwards from the base of the flow. The most accessible example of the colonnade/entablature junction is to be found at ‘The Organ’. The 12m columns at The Organ are the colonnade of flow 1 of the CTM. This achieves its greatest thickness at the Giant’s Causeway, where its entablature extends almost to the top of Aird Snout and where the base of the colonnade forms the columns of the Causeway itself.

The Organ, a 12 metre high basaltic ‘colonnade’ with a thick ‘entablature’ at the top.


The ‘Giant’s Causeway’

The Giant’s Causeway is described in the 1985 nomination to UNESCO as a sea level promontory carved by the sea from the lowest colonnade of the Middle Basalts between Port Ganny and Port Noffer. In reality, however, it is three promontories: the Grand, Middle and Little Causeways. Together, they are made up of some 40,000 vertical or gently inclined columns. The majority of these are seven- six- or five-sided, with a few that are four- or eight-sided. As at The Organ, the columns are divided horizontally by ball-and-socket joints so that in reality many comprise stacks of interlocking polygonal slabs. They owe their great number and regularity to the slow cooling and gradual contraction that took place at the base of a large mass of homogeneous basalt. In turn, they owe their preservation to their relative resistance to marine erosion compared to the Lower Basalts and Inter-Basaltic Bed that crop out at sea level in the two adjacent bays.

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An almost perfect penthagonal column top at Giant’s Causeway.

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