Ballybunnion

Ballybunnion

Ballybunnion is nestled within Ireland’s spectacular southwestern coastline. Known for its golfing and sunbathing, it is, in my opinion, one of the most striking beaches in the country. As you make your way down to the sand, you’re greeted by the Ballybunnion castle ruins, which invite you to take a closer look.

Ballybunnion Castle

The castle offers a great vantage point for appreciating the scenery; from the balcony you can look across the low lying beach to the steep, cavernous cliffs that are over 100 meters tall. It was an incredibly hot and sunny day in June when I visited Ballybunnion, and within no time the beach was crowded with swimmers and sunbathers.

The Legend of the Seven Sisters (1)

This legend begins at the striking castle that sits above Ballybunnion beach. It was here that a great chieftain lived with his seven daughters. From this spot, he could guard the coast from pirate invasions day and night. One day an approaching pirate standard was seen approaching from the horizon, but it was only a matter of time until it was captured by Irish ships.

The chieftain ordered the crew to be slaughtered and thrown into the sea; the captain and his six brothers were captured and reserved for a more painful death. The seven brothers were kept in the castle, allowed to roam free about fortress, and before long they fell in love with the chieftains seven daughters.

The prisoners convinced the Irish women to run away with them to Denmark an hatched a plan to sneak away in the dead of the night. They chose a cold and stormy winter’s night and there wasn’t a star in the sky. When the group of lovers attempted to escape from the castle down a ladder, they were met by armed soldiers on the ground who brought straight to the chieftain.

The furious chieftain ordered a cruel punishment; the lovers were torn from each other’s arms and marched to the edge of the precipitous cliffs. Here lay a giant chasm from which the sea could be seen churning in the winter storm below. The daughters were hurled into the cavern, one by one, their screams consumed by the roaring ocean below. What became of the seven brothers, no one knows, but the cave is still there and is known as the “Cave of the Seven Sisters.”

The Geology

Ballybunnion cliffs and sea caves.

The chasm that the daughters were thrown into is known colloquially as a “blowhole.” It forms when a conduit or cave propagates from the top or bottom of a rock and creates a continuous opening between the sea to the land above. These features propagate through the rock by physical and chemical erosion; the rough seas carve away at the base of a rock which rainwater penetrates the top. The image below provides a cross sectional view of a blowhole. There is a great picture of the Ballybunnion blowhole here.

Photo Credit:Wikipedia

Ireland’s sea cliffs in Co. Clare and Co. Kerry, near the Shannon estuary, contain excellent rock exposures that help geologists to understand what the environment was like during the Carboniferous Period (359 – 299 Ma). The sea cliffs are part of a geologically significant area – the Clare Basin.

The Clare Basin is just one example of a basin; in geology a basin is any area where sediments are accumulating, usually a depression in the land or seafloor. The cliffs at Ballybunnion contain a record of what the basin and nearby land were doing during the early Carboniferous. The Clare Basin formed as a result of extending continental crust and was separated from the sea. Marine deposits are rare but cyclical, an indication that periodic changes in climate and sea level could have flooded the basin and supplied it with seawater. The composition of basin deposits shows us that the adjacent continent, where sediments were sourced, was tectonically inactive at the time of deposition(2).

Ballybunion beach

The base of the cliffs contains dark shale’s which are part of the Clare Shales formation. The depositional setting, a geologic term to describe the environment in which a rock was created, of this group of shales has been interpreted as a deep water basin.

More importantly, Ballybunnion contains a complete record of the Ross Formation (3). This formation is a lower (older) member of the Clare Basin. What makes is geologically significant is that it is dominated by turbidite deposits.

What is a turbidite deposit?

A turbidite is the result of a turbidity flow. This is a process that occurs in a deep water environment, such as the deep Clare Basin, and usually on a slope. Overtime suspended sediments, small particles of sand, mud, silt, etc. that are basically “floating” in the water at the seafloor, build up until the slope cannot support the sediments and fails. This slope failure causes a huge, turbulent, underwater avalanche of sediments. When the sediments slow down and settle, they form what is known as a turbidite.

Turbidites can help us figure out what direction sediments were flowing and where the material came from in the past. The shape of a flute cast, a sedimentary structure that forms in turbidites, shows us which way the sediments moved, what geologists call a paleocurrent. During a turbidity flow, sediments scour the ocean floor and create elongated incisions that look like someone scooped out a part of the bedrock. If you are at the beach and dig your heel into the sand and drag it along for a few feet, you will create a flute cast in the sand. The picture below shows the underside of a flute cast and the red arrow indicates the direction of flow.

Flute Casts in County Down, Northern Ireland. Photo Credit (without arrow): University of Otago, Department of Geology. Photo Contest winner Alan Cooper.

The turbidites in the Lower Ross Formation have been interpreted differently by geologists, a controversial issue in geologic research. One interpretation of the northeast flow is that the sediments came from the west or northwest and flowed southeast. Another interpretation is that continental sediments entered the basin from the south and flowed northeast (4).

Ammonite fossil at Ballybunnion beach

Underneath the Ross formation and Clare Shale lies older Carboniferous limestone which can also be seen at Ballybunnion beach. This limestone contains well preserved marine fossils such as the spectacular ammonite above.

(1) Project Gutenberg EBook of Notes and Queries, Number 238, May 20, 1854, by Various

(2) Clare Basin Upper Carboniferous Deepwater Sediments. (2012). Retrieved from http://www.sepmstrata.org/page.aspx?pageid=154

(3) Martinsen OJ, Lien T, Walker RG. 2000. Upper carboniferous deep water sediments, western ireland; analogues for passive margin turbidite plays. Program and Abstracts – Society of Economic Paleontologists.Gulf Coast Section.Research Conference 20:533-55.

(4) Pointon MA, Cliff RA, Chew DM. 2012. The provenance of western irish namurian basin sedimentary strata inferred using detrital zircon U-pb LA-ICP-MS geochronology. Geol J 47(1):77-98.

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