Coastal Geology of Iveragh

The Iveragh Peninsula has a remarkable stretch of coastline that is well known for its spectacular scenery and dramatic contrasts. Iveragh is surrounded by ocean on three sides, bound to the north by Dingle Bay and to the south by Kenmare Bay. Around the coast, the setting varies considerably from ravaged and wave-beaten rocky shores, long sandy beaches, sheltered coves, and steep precipitous cliffs.

Coastlines are highly dynamic regions that are eroded and built up by the force of the ocean to form an enormous range of impressive and inspiring landforms. Winds, currents, weather, vegetation, gravity, and human influences all shape the land along the coast, however, the most powerful agent of change along the Iveragh coastline is undoubtedly the daily battering of the sea. Waves have an unparalleled capacity to erode, transport, and deposit large volumes of rock and sediment.

*Beaches such as Derrynane are constantly being eroded and re-built by the ocean dumping sand in calm conditions and dragging it away during storms.*

Waves form as the wind blows across the sea and form a swell. Wave strength and height depend on the wave fetch, which is the distance that the wind blows across the water, the wind speed, and wind duration. As waves approach the land, they break. When a wave breaks, the water that rushes up the shore is known as swash, and the water that flows back towards the sea is called backwash. Waves can either be constructive or destructive and this is largely determined by the relative strength of the swash and backwash. Destructive waves have stronger backwash than swash so they pull material away from the shoreline, resulting in erosion. Destructive waves generally have large fetches, high energy, and are big and steep. Constructive waves have stronger swash than backwash so they deposit material on the shoreline. Constructive waves are less powerful and smaller than destructive waves.

Coastal Erosion

Iveragh has a dominantly rocky coastline, which is characteristic of areas that face persistent erosion from high energy waves and gale-force winds. For thousands of years following the last glacial period, the Atlantic Ocean has worked against the rocks of Iveragh’s coast, exploiting variations in rock hardness, orientation and seeking out zones of weaknesses created by fractures and faults. The elevation of Iveragh’s coastline fluctuates greatly, from the towering Kerry Cliffs to the many low-lying bays and strands. This extreme variation partially depends on the strength of the rock, as more resistant rocks tend to create steep gradients. However, on the Iveragh Peninsula, the coastal profile is largely an extension of the overall topography and the positions of the mountains and valleys that punctuate the region.

Rocky coastlines host many spectacular erosive features and while these features may seem unmoving and permanent, they are fleeting configurations that will eventually disappear as the relentless cycle of erosion continues. Although the landforms are very young, geological speaking, to truly appreciate these landmarks it is important to understand that the coastline of today began its evolution over 380 million years ago.

Bays and Headlands

The western coast of Iveragh is heavily serrated by inlets, bays, and headlands. Bays and headlands form because the sea erodes areas of rock at different speeds. This variation is usually due to the presence of different rock types, for example, igneous rocks tend to be harder and more resistant to weathering anderosion, while sedimentary rocks are usually softer and so weather and erode more readily.

Slight variations within rock types also result in variable erosion, such as those found within the sedimentary Old Red Sandstones that dominate Iveragh. Structural weaknesses within the rock, such as faults, fractures, and cracks within the rock itself can also cause the formation of bays and headlands. When waves attack a section of coastline, the softer or weaker areas of rock are eroded, carving out bays while the harder and more resistant rocks protrude out as headlands.

Rocky shores

Across Iveragh, many of the shorelines are composed of rocky outcrops that lack significant beach deposits. Rocky shorelines are highly irregular with jagged rock platforms, wave flung boulders, rock pools, and much coastal erosion and weathering.

Explore any of Iveragh’s rocky shores and you are likely to be drawn towards the unusual and mesmerising patterns created by weathering and erosion. The rocks of the shore often show substantial variation across a small area. One moment the rocks may appear colourful and smooth, the next, blackened and heavily corroded.

*Rocks smoothed by abrasion at White Strand near Cahersiveen.*

Erosion along the shoreline is largely driven by wave action, but currents and tides also play a significant role. A particularly common feature is areas of rock that appear smooth and undulating. This type of rock surface is formed by abrasion. Abrasion occurs when pebbles and sand carried by the waves and tides grind over the rock surface like sandpaper, gradually smoothing the rock. When pebbles enter a small depression in the rock, the water may flow in a swirling pattern, carving out smoothed hollows.

*Heavily weathered rocks at St Finan’s Bay*

Weathering occurs when the rock structure is weakened by the elements and is broken down into smaller pieces. Weathering can be mechanical, chemical, or biological and involves processes such as alternate wetting and drying, frost action, and salt crystallisation. Chemical weathering occurs as the structure of the rock is altered by interaction with the rainwater or seawater that gets into cracks or holes in the rocks. As this water can be weakly acidic, over time the rocks can dissolve. In many areas, the rocks appear heavily corroded or punctuated with numerous holes due to this chemical weathering.

*A rockpool full of life and colour at Glanleam, Valentia Island.*

Rockpools form when the areas between high and low tide are alternately submerged and exposed to the air throughout the day. Natural depressions fill with seawater and these rocky intertidal pools create a unique, rich, and biodiverse marine environment.

Landforms of Coastal Erosion

Below some of the most distinctive landforms of erosion found on the Iveragh Peninsula are outlined. Many of these features are best seen from the water and there are a host of boat tour providers that explore Iveragh’s coast. You can explore some of Kerry’s boat tour providers here. However, here we have focused on the best locations to appreciate the coastline from the land.

Sea Cliffs

Sea cliffs are steep faces of rock that form through the repeated action of waves crashing against the coastline. Cliffs form as the base of coastal rocks are attacked by waves, carving a wave-cut notch between the high and low watermark. As this notch grows larger, the overlying rock becomes destabilised, resulting in slope failure and collapse. As this process continues cliffs retreat further, leaving behind wave-cut platforms that may extend for hundreds of metres in front of the cliff.

Iveragh possesses spectacular coastal cliff scenery across a range of scales. The vertical cliffs of Iveragh are striking for their combination of coloured layers of Old Red Sandstones, which range from grey-green to purple and red. These layers provide a record of millions of years of Earth’s environmental and climatic history. Imagine the vast amount of time required to build these formidable rock layers. These cliffs often expose classic deformation structures, such as folds, that formed as the rocks crumbled under the immense mountain building forces.
Iveragh’s highest cliffs, the Kerry Cliffs, rise a vertiginous 305m above sea level. At Valentia Island, the Fogher Cliffs plunge around 180m into the Atlantic from the northern face of Geokaun Mountain. The more moderately sized Abbey Island cliffs in Derrynane are equally worth a visit.

Sea Caves

Sea caves are large holes or tunnels found at the foot of a cliff. These again form through the action of waves hurling against the rock and exploiting weak spots in the cliff. Over repeated cycles of wave pounding, these eroded hollows expand, becoming dark and alluring caverns. Caves are common on the sides of headlands, where erosion tends to be focused due to wave refraction around the headlands.

Coonanna Cave — Coonana Cave, Over the Water

Among the best place to see caves are Cooncrome (Irish: Cuas Crom) and Coonanna Harbour, both located ‘over the water’ from Cahersiveen. The numerous small caves and inlet found at these coves, were undoubtedly influential in the naming of the site, with ‘cuas’, translating as cave in Irish

Arches

Sea arches are natural archways in rocky headlands. These form when a cave erodes all the way through a headland or when two caves erode through a headland on either side.

Sea arches can be quite difficult to see from the land but on the road leading from Cuas Crom to Kimego, an isolated sea stack protrudes out into the sea, surrounded by the evidence of previous rock collapses, in the form of large boulders scattered across the shoreline. A sea arch can also be seen towards the southern end of Bray Head. If the angle and the light are right, you might also be able to see a sea arch on the northern end of Scariff Island.

Sea Stacks

Sea stacks are pillars of rock that stick out of the sea and are often found near headlands and islands. Sea stacks form when the roof of a sea arch collapses, following years of ongoing weathering and erosion and the outer wall is isolated as a steep column of rock.

A prominent sea stack can be seen in the distance from the Canglass Blowhole, near Kimego. From the beach at Inny Strand, a series of sea stacks and stumps at the northern end of Scariff Island are visible.

Sea Stumps

Sea stacks are attacked further by the weathering and erosion and will eventually collapse as waves continue to attack the base. All that remains of many former sea arches and stacks are low projections of rocks called sea stumps.

Sea stumps are found in profusion at the base of the towering Kerry Cliffs and at Bray Head, where they serve as a reminder of the mutability of the coastline.

Blowholes

A blowhole is a hole or tunnel that joins the roof of a cave with the land surface above. These features initially form as sea caves. When sea caves grow upwards and landwards to form vertical shafts, they may eventually become exposed at the surface when the overlying rock roof collapse. Normally this collapse happens progressively from the cliff face, but blowholes form on the rarer occasions where the roof falls further into the cave.

There is a large blowhole at Canglas near Kimego. Within the exposed walls of the pit here, the original depositional layers of the Old Red Sandstones are still visible. Eventually, the arch forming the coastal limit will collapse, leaving behind an inlet.

Landforms of coastal deposition

Coastal deposition occurs when waves slow down and lose energy, causing the sea to drop or deposit the sediments it had been transporting. The major landforms of coastal deposition are beaches, spits, bars, and tombolos.

Beaches

Beaches are amongst the best-known coastal features and show considerable variation in form, depending on the shape of the coastline, the local geology, and the prevailing weather conditions. A beach is an accumulation of materials such as sand, gravel, and pebbles on the seashore between high tide and low tide levels.

Ballinskelligs Beach — Sandy Beach at Ballinskelligs

Most sandy beaches are formed in shallow waters on gently sloping shores, where incoming waves have low energies. These waves are generally constructive, with powerful swashes that push the beach material up the wide shore. As the waves spread out over the shore, much of their water soaks into the sandy or pebbly ground. This results in weak backwashes, with sediments deposited rather than removed from the shore. Some of Iveragh’s sandy beaches are found at Ballinskelligs Strand and Reenroe and Inny.

Contrastingly, pebble beaches tend to form where cliffs are actively eroded, and wave energy is high. Across Iveragh many of the beaches have large pebble size material at the top of the beach. This larger material was thrown up the beach during high-energy storm events. The beach at Waterville is a superb example of a storm beach and is almost completely covered by large pebble to boulder-sized material and has a very steep gradient.

Many of the beaches around Iveragh are pocket beaches that occupy small bays, nestled between rocky outcrops. Once formed, headlands shelter bays and themselves become more vulnerable to erosion. The limited energy waves entering these bays cause beach material to be deposited within these pockets forming curved sandy beaches.

Pocket beaches are found at Glanleam, St. Finian’s Bay, Kells, and Glanbeg.

Sand spits

A sand spit is a long narrow ridge of sand or shingle extending out from the coast, where one end is joined to the land and the other terminates in the sea. Sand spits form through longshore drift. In longshore drift, the sediment is pushed onto the beaches at an angle of about 45 degrees by the swash. The backwash then drags the material straight back towards the sea at right angles to the coast.

Through this process, the material is transported in a zig-zag pattern across the shore. When the coast changes direction, this process continues, and material is deposited as a thin strip extending out from the coastline. Sand spits are often also found near estuaries, where deposition can be brought about earlier because the flow of water from rivers into the sea is stronger than the drift, resulting in sediment deposition and accumulation which can gradually form a spit.

Rossbeigh Beach is an excellent example of a sand spit on the Iveragh Peninsula and projects outwards from Glenbeigh. The presence of the sandspit at Rossbeigh is closely related to the influx of water from the River Caragh and River Behy, as they drain into Dingle Bay. The Rossbeigh sandspit and its dune system act as an important protective barrier for the Castlemaine Harbour area. This is a Special Protection Area and Special Area of Conservation with a range of critically important habitats including tidal mudflats, sandflats, and dunes, and hosts a large population of protected bird species.

Sand Bars

A sand bar or bay mouth bar is a narrow ridge of sand or shingle that extends partially or entirely across the mouth of a bay. It usually formed when a spit extends across a bay or an estuary and cuts it off from the sea.

A sand bar is found at Derrynane, which almost fully encloses the bay to form a lagoon. The Derrynane Seashore Nature Trail, created by environmentalist and multimedia artist Vincent Hyland, explores the dune system and the area behind the sand bar at Derrynane.

Tombolos

A tombolo is a ridge of sand or shingle that joins a former offshore island to the mainland or another island. A tombolo is formed when a spit grows in length until its seaward side reaches a nearby island. Tombolos tend to form because the sheltering presence of the island creates an area of slow-moving, low-energy water behind the island which favours sediment deposition.

Derrynane Harbour and Abbey Island Tombolo

A short tombolo connects Abbey Island to the mainland at Derrynane and another tombolo connects Beginish Island to the smaller Church Island to the east.

Coastal Erosion

Landforms of Coastal Erosion

Landforms of coastal deposition

Ten reasons to get outdoors on Iveragh in Spring

The Geological History of the Iveragh Peninsula