For almost a century it has been recognised that the anomalous thickness of columnar-jointed tholeiite basalts at the Giant's Causeway, Northern Ireland, was due to ponding of lava in a topographic depression. For more than 80 years this feature has been ascribed to fluvial incision, yet new field observations have found no evidence for erosion of the underlying Lower Basalt Formation anywhere in the studied area. Basalt weathering and erosion rates also suggest that there was insufficient time for fluvial incision to occur on the scale required. Local dips on the Lower Basalt Formation broadly converge about a low point centred on the Giant's Causeway itself and are discordant with the regional dip of the overlying Causeway Tholeiite Member, observations consistent with a subsidence-based model. The absence of sediment and limited development of hyaloclastite beneath the Causeway Tholeiite Member here indicates that the interval between subsidence of the land surface and emplacement of tholeiite lava was insufficient for sediment, or even a significant depth of water, to accumulate. It implies that subsidence may have been directly related to draining of a relatively shallow magma reservoir from which the tholeiite lavas were erupted. The style of faulting in the Lower Basalt Formation outcrop suggests that a brief period of inflation, perhaps associated with magma injection, may have preceded subsidence caused by magma withdrawal. This study demonstrates that even at sites as well-trodden as the Giant's Causeway, new discoveries can be made from simple observations.

近一个世纪以来，人们已经认识到北爱尔兰巨人堤道的柱状节理拉斑玄武岩的异常厚度是由于熔岩在地形洼地中堆积所致。80 多年来，这一特征一直被认为是河流切口造成的，但新的实地观察并未发现研究区域任何地方的下伏玄武岩地层受到侵蚀的证据。玄武岩风化和侵蚀率也表明，没有足够的时间在所需的规模上进行河流切口。下玄武岩地层的局部倾角大致会聚在以巨人堤本身为中心的低点，并且与上覆堤道拉斑岩段的区域倾角不一致，观察结果与基于沉降的模型一致。此处Causeway Tholeiite 段下方没有沉积物且透明碎屑岩发育有限，这表明地表下沉与拉斑岩熔岩就位之间的间隔不足以使沉积物甚至相当大的水层积聚。这意味着沉降可能与拉斑岩熔岩喷发的相对较浅的岩浆库的排水直接相关。下玄武岩组露头中的断层类型表明，可能与岩浆注入有关的短暂膨胀期可能在岩浆回撤引起下沉之前发生过。这项研究表明，即使在像巨人堤道这样人迹罕至的地方，也可以通过简单的观察获得新的发现。