Geological travels in Northern Ireland, part III:

After a brunch in the village of Moira with my old friend Andrew and his newly pregnant wife Nadine, Casey and I drove up the coast of County Antrim. Her friend Jodie had loaned us her Audi and arranged for us to stay at a condo in Port Rush. Road trip!

This is the view south from an area called Garron Point.




I stopped and poked around amongst the boulders on the shore. Note the boulders are two colors: black basalt and the white chalk.








Here's Casey staring out across the North Channel at the Mull of Kintyre (Scotland), only 12 miles distant at the closest point.









Awesome, awesome, awesome. There's so much going on in this picture, I don't know where to start! Very prominent (and annotated with a dotted line) is the contact between the light-colored chalk and the overlying dark-colored basalt. This chalk layer is really a white limestone at this locality. Unlike the same layer where it famously outcrops at Dover (England), here the chalk has been compressed by heavy overlying lava flows. These basalt layers are called "lower" because they are the bottom of a three-part stack of igneous eruptions. The layers are all tilted here at Garron Point because they have slumped: large blocks of strata have slipped downward and outward, sliding along an underlying clay layer, the Lias. Conveniently, the Lias is Triassic in age, the overlying chalk is Cretaceous, and the basalts here are Paleogene: one formation per period. It's worth noting that the word "Cretaceous" itself comes from the Latin word creta, or "chalk." The entire Cretaceous period is named for this brilliant white layer of rock, which also extends across southern Britain and into France. This chalk is made up of gazillions of little coccolithophores, like I mentioned in an earlier post about ocean acidification.

Here's an image from a tourist sign at Garron Point which may make the geology a bit clearer. Note the sketch in the upper right of the slumped blocks.





Large grey nodules of flint that are present in the chalk exposed at Garron Point. These nodules probably form diagenetically -- after the sediment is deposited and the component bits were organizing themselves into rock. Smaller bits of silica (possibly from siliceous sponge spicules) dissolved and reprecipitated in these concentric nodules. Flint breaks conchoidally, like glass, and so these nodules were a terrific local source of arrowhead & axe tools for Stone Age peoples in Ireland. Pound coin for scale.

Lastly, here's a shot of sunset from the Torr Road, which is a crazy twisty little road that runs along the northeastern Northern Irish coast.

Labels: antrim coast, basalt, chalk, northern ireland, travel