Geological travels in Northern Ireland, part V:

A hike east from the Giant's Causeway on the "Shepherd's Trail" takes you along the edge of a steep escarpment, where you can look down and see all kinds of cool things.

Here, I was struck by how plainly the sequence of geologic layers was revealed. The oldest exposed layer here is the sequence of lava flows known as the "Lower basalts." (I mentioned this layer earlier, in my post about the Antrim Coast.)

Atop them is a laterite layer. Laterite is a tropical soil, red in color due to the presence of oodles of oxidized iron. Of course, basalt is a mafic rock, meaning it is very rich in iron. When that iron-rich rock is exposed to warm, wet conditions, a lateritic soil develops atop it. The laterite layer therefore represents a time of relative calm in County Antrim, a time between eruptions, when the land was in a tropical latitude & climate.

Finally, atop the laterite is another series of basalt flows. These are sometimes called the "Interbasalt" layers, or more commonly "The Causeway basalts" since they are typified by columnar jointing of the type exposed at the Giant's Causeway. Here, you can see multiple layers exhibiting strong columnar jointing. (The stratum directly above the laterite layer is the one that filled the paleo-valley that is exposed today as the Giant's Causeway itself.) The Causeway basalts have been dated to about 60 million years ago, in the early Paleogene (about 5 million years after the extinction of the dinosaurs). Their tectonic cause was the rifting of Laurentia, separating Greenland from Europe. These basalts are part of a larger basaltic province, the Thulean Plateau, which can also been found in Scotland, the Faroe Islands, Iceland, and parts of Norway, as well as the eponymous area of Thule, Greenland.

Labels: antrim coast, basalt, geology, giants causeway, laterite, northern ireland