Sunday, September 25, 2016
Portrush is built on a rocky finger that juts out from the north Antrim coast. Although built mainly on bedrock, it is a beach town - sort of classic beach town, with an amusement park, a beach promenade, and big crowds in the summertime.
Beaches have formed on both sides of Portrush. The beach on the east is much larger and includes a large foreland and dune system (and therefore another golf course). The landform is actually convex seaward, probably reflecting the abundant sediment and the role of the small islands offshore as a breakwater. The beach on the west is more of a classic swash-aligned, crescent-shaped beach, backed by a seawall, and if there ever were any dunes, they were long ago sacrificed for park space, the railroad, the promenade, and homes.
Geologically, what makes Portrush important is its bedrock. I walked the peninsula (to Ramore Head) early one morning, but didn't really get a chance to examine the rocks carefully. But apparently, if I had, I would have found baked ammonites. Which in the late 18th century, presented a problem for Neptunists, who wanted to precipitate all rocks out of seawater and Plutonists, who wanted to generate everything igneously. A number of famous geologists argued about the significance of what appeared to be fossils in volcanic rock, but eventually others showed that a large basaltic (dolerite) sill had been intruded among the Jurassic shales, baking the sediments and their fossils. The igneous rock was injected about 60 million years ago as the modern Atlantic began to open. We'll see a lot more of these basaltic rocks in a subsequent post.
*12 October. Callan Bentley at AGU just posted about Portrush and its geologic significance. Check out his blog: AGU Mountain Beltway.