Monday, September 26, 2016

Magilligan Point


Magilligan Point is best appreciated from the air (see link below). It is a large cuspate foreland with an amazing series of beach ridges. Parallel ridges document the building of the feature; truncation of the ridges, particularly near the tip, document erosion. These patterns tell the story of changes in sea level, changes in sediment supply, and simply the accumulation and rearrangement of material over thousand of years. It's among the larger depositional coastal features in Ireland and also has a large dune system.

AERIAL VIEW


The landform defines the entrance to Lough Foyle, which is the estuary of the River Foyle (which flows through Derry).

We drove across it, but my only exposure to the beach itself was at the very tip of the spit, a short crossing from Donegal's Inishowen Peninsula (at least it would have been a short crossing, had the ferry not broken down, leaving us to drive around).


White Park Bay



White Park Bay caught my eye when first scoping out our trip on Google Earth, subsequent homework confirmed it, and once we got there, even the interpretive signs described it. The Antrim Coast hosts many large landslides, (landslips, in Irish) where the basalt or other formations overlay older sedimentary layers that are often mushier. This is just a really elegant example - the aerial on google tells paints a pretty clear picture.

AERIAL VIEW

In this case, I don't see the basalt, although I think it forms the cap farther inland. The slide is occurring on Cretaceous chalk deposits and Jurassic marine clays. I would have loved to walk down to the beach and spend some time exploring, but one thing always leads to another, and we only had three weeks! So we'll settle for some photos from the top edge.


Giant's Causeway

About 60 million years ago, late in the splitting up of Pangaea, the North Atlantic Ocean began to open. The rifting was accompanied by extensive vulcanism -- the evidence of which can be found in basaltic rocks from Greenland to Northern Ireland. These basaltic flows are exposed in multiple layers along the eroding edge of the Antrim Coast, nowhere as well as at the Giant's Causeway.

AERIAL VIEW

The slowly cooling basalts form the familiar polygonal columns (rarely regular polygons and not always hexagonal). Their broken tops form the shore platform and the causeway itself. The nature of these broken surfaces is to bow either either up or down - the ones that curve down form puddles.



The Causeway is mainly a story about platforms and sea cliffs and igneous geology. Not about beaches. But, in the midst of all the columns and the tourists, I think there really is a beach. This big pile of broken and rounded columns forms a distinct berm at the head of a trough in the platform. I think this is a beautiful example of boulder berm (try to ignore those huge dinosaur-size boulders - not sure what erosional process forms those). We'll see other examples later on this trip, but for another example (not basalt) from the other side of the Atlantic, see August 2010: Schoodic Peninsula.










Carrick-a-Rede

The next two posts visit a couple of the most popular tourist spots on the north Antrim Coast - the Carrick-a-Rede (Carrickarade) Rope Bridge and the Giant's Causeway. I'll say a little more about each on the other blog, too, maybe with more emphasis on the tourists than the geology (hshipman).

Carrick-a-Rede is a small rocky island connected to a larger small rocky island (Ireland) by a wire rope bridge. Originally, the bridge (earlier versions) allowed fishermen to access their boats which they pulled up on one side of the island. Now the main function of the bridge is to allow something like a couple hundred thousand tourists to go back and forth every year and to raise money for the National Trust (a good thing). According to Wikipedia, the salmon fisherman don't use the bridge anymore because there aren't many salmon left - sounds like a familiar problem.

The small island is a volcanic plug - more Paleogene volcanics as the modern Atlantic began to open. But it's emplaced amidst older sediments - chalks and limestone, seen in the nearby cliffs. The cliff exposures were heavily shaded, but apparently these rocks are combined with ash and capture the more violent eruptive activity associated with this period (I read this stuff, I don't necessarily figure it out for myself).

For a nice, introduction to the geology of this and other sites along the Antrim Coast, check out:









Sunday, September 25, 2016

Portrush



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.

AERIAL VIEW


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.



Ballycastle



This beach is also tucked into a bay between headlands, but unlike Ballygalley, in which the small pocket beach was backed by high ground, here a more robust barrier has formed across the valley of the Glenshesk River. The beach is sandy, with dunes (and therefore a golf course).

AERIAL VIEW

We arrived in Ballycastle during the waning stages of Lammas Fair, so the park at this west end of the beach was packed with carnival rides and food booths (hshipman).


Thursday, September 22, 2016

Ballygalley



Ballygalley is just around the corner from Drains Bay in the previous post, separated by a rocky promontory. There is an extensive boulder lag (basalt, I think) between the slipway (boat ramp) at the east end of the bay and the beach in Ballygalley itself.

AERIAL VIEW

The beach at Ballygalley is a broad pocket of sand and gravel, maintained by the orientation of the coastline, and backed by a combination of cobble berms and seawalls.

Given my short visit, my limited observations, and my lack of thorough homework, most of my geological interpretations are little more than speculation. The Antrim Plateau, which reaches the coast itself in places, is capped with basalt, dating to extensive volcanism 60-80 million years ago. It is underlain by a wide variety of Paleozoic and Mesozoic sedimentary and metamorphic rocks, some of which - chalks and clays, for example - are prone to landslides that can bring down the basaltic cap rock. The Antrim coast is marked by many large slides that extend from divots in the edge of the plateau down to the coast. These can be seen in the aerial imagery and, at least to my eye, looks like what may have formed this boulder-strewn shore.


Drains Bay


Drains Bay lies between Larne and Ballygalley in Antrim (the northeast of Northern Ireland). It was the beginning of our trip along the Antrim Coast along A2, which follows right along the shore most of the way.

The fact that the road follows the coast is significant. The first paper I ever read about this coast spoke about the potential role of the road in reducing sediment supplies to Antrim Coast beaches. My brief tour didn't give me much insight into this, although I suspect this impact might vary greatly from one location to another. The geology of the eroding coastline rangers from bedrock promontories to large landslides of softer sediments. The beaches include tiny pockets between seacliffs, larger barriers across stream mouths, and broad strands complete with dunes and golf courses.

AERIAL VIEW

This beach reflects a common theme of most of the beaches we saw on the trip - a sandy beach with steep gravel or cobble storm berm. But that doesn't mean it looked like the others -- since the geographic setting, the wave exposure, the composition, the geology, and the simple scale of the beaches were all over the map. So was the weather.




Sunday, September 18, 2016

Portmarnock Beach


I guess this post is a preface. Over the next few weeks, as evenings and spare time allows, I'll try to capture three weeks of Irish shorelines, from the Antrim Coast to the Cliffs of Moher to the Bray Strand. I suspect I added 30 or so beaches to my collection this trip. Or at least 30 that merit display -- there were dozens more that were less distinguished, where I took few photos, or where the weather didn't lend itself to nice specimens.

This was the first beach I saw on this side of the Atlantic, just before landing at DUB on the morning of August 26th. Portnmarnock is a barrier beach, one of many on this coastline separated by rocky headlands and small river mouth estuaries. There's a golf course built in the dunes - a very common theme on Irish beaches ("links" comes from an Old English term that basically means coastal dunes).

The posts that follow will trace a generally anti-clockwise path around Ireland's coastline, although a bulk will be from the north and the west. Like the U.S., but for somewhat different geologic reasons, Ireland's east coast differs geographically from its west coast. The eastern side (Irish Sea) is fairly tame and subdued, at least compared to the rugged peninsulas of the western side (Atlantic Ocean).


Sunday, August 21, 2016

Jackson Rock


Fulford Harbor, like pretty much everything else on Saltspring Island, is oriented NW-SE, reflecting the underlying geologic structure of the islands sedimentary rocks. Last week, during our annual trip to the island, I paddled from the head of the harbor out to Wen,Na,Nec (August 2013) and Jackson Rock, then back on the other side.

AERIAL VIEW

The northeast side of the harbor (the Fulford/Reginald Hill side) is steep and rocky and there are few beaches. The southwest side (from Drummond Park out toward Isabella Point) is a little more gradual and includes both more erodible glacial? sediments and a stream mouth or two, so the beaches are a little better developed.

Here on Jackson Rock, the only material available to build the tiny beaches is broken shell. And the orientation of the beaches and bars is a messy function of the islets' irregular configuration and bathymetry.