The Kerry Cliffs are savage and wild, offering not only a dramatic setting and spectacular views of the Skellig Islands, Puffin Island, and the Atlantic Ocean, but also exposing some fascinating geology. These cliffs stretch for over 5km and rise sheer over 300m above the Atlantic. Walking along the path at the Kerry Cliffs is a walk on the edge of a landscape that was sculpted by millions of years of geological activity, weathering, and erosion.

The rocks of the cliffs formed more than 385 million years ago when layers of sediment gradually built up over millions of years. Each of these layers acts as a physical reminder of how this rock was laid down and can be linked to a specific depositional event, such as a flash flood or the existence of a river channel. The layers vary in thickness and accumulated at a rate of about one-tenth of a millimetre per year, shedding some light on the immense time taken to create these formidable cliffs. Preserved within these layers are millions of years of Earth’s history during a period of considerable change. Throughout this period, Earth saw an increase in plant and animal diversity on land, the evolution of many fish, as well as the appearance of the first amphibians.

This cliff section is remarkably rugged with a highly serrated appearance and many promontories extending out into the ocean. This rugged form developed through the constant assault from the elements over thousands of years. The Atlantic incessantly froths and crashes against the base of the cliff, providing an opportunity to see nature at its most powerful. Scanning the cliffs for evidence of coastal erosion, you will see sea caves etched into the base of the cliff and places where large areas of the cliff face have collapsed, leaving large slabs of rock slumping into the sea. These cliffs are dynamic, and erosion may expose new features of geological interest over time.

At one section of the cliffs, you may notice a clear disruption in the layers, where one section of the rock layer seems to have buckled. This formed when a fault developed within the layers during the mountain building, splitting the rock and allowing this section to be pushed above the others.

Wildlife is also a draw and many bird species including puffins, gannets, and razorbill frequent the cliffs, swirling overhead and plunging into the sea below.

Aside from being an outstanding picture point, Com an Easpaig allows you to review at a glance many of the geologic processes of Iveragh’s past and present. These processes involve the building of mountains and valleys, the formation of geologic faults, and the subsequent erosion and destruction of these features.

From Com an Easpaig we see the impact that the Variscan mountain building event had on the landscape. The mountain ridge that extends eastwards from Com an Easpaig lies on the major Portmagee anticline which has its corresponding syncline in the Ferta valley. Anticlines are n-shaped folds that usually form mountain peaks, while synclines are u-shaped folds that usually form valleys. These folds were created by compression during continental a collision.

To the north, we see the impact that faulting had on the landscape. Faults are large fractures and fissures that allow blocks of rock to move relative to each other. The straight course of the River Ferta as it flows past Cahersiveen and into the Portmagee Channel, moves along the line of a major geologic fault, known as the Coomnacronia Fault. This fault is believed to have been an important northern margin of the Munster Basin (Parker, 2017). The presence of this fault created a zone of weakness in the rock and allowed for the formation of an eroded rock hollow that acted as a natural drainage route for the river.

Geology can also be seen in action in the present day. Evidence of erosion can be clearly seen looking back at the Kerry Cliffs, which are actively being eroded by the power of the Atlantic Ocean. The erosion of these cliffs and the wearing down of the mountains by river channels and streams will ultimately be followed by rebuilding and new rock formation, as the geologic cycle continues.

St. Finian’s Bay is an area of geologic significance for several reasons. Firstly, although the area is predominantly underlain by the Old Red Sandstone Formation, the area to the north of the cove contains two types of volcanic rock. At St. Finian’s Bay, some of the magma formed during the development of the Munster Basin remained below the surface where it intruded into joints and cracks of the already formed Old Red Sandstones and cooled and solidified to form a rock called dolerite.

The second volcanic rock is tuff. The Keel tuff has a pale colour and is dominated by ash and pumice clasts. Pumice is formed when a volcano erupts explosively, and it is very light because it contains many gas bubbles. Laminations or layers can be seen within this deposit as it grades into the overlying sediments. This tuff deposit was important for assigning a date to the rocks in the area and the Valentia tetrapod trackways (Williams et al., 2000).

The area around St. Finian’s Bay also contains some ancient and fascinating fossils of now extinct fish. The Devonian saw a huge increase in the diversity of fish life, which has led to the period being referred to as the Age of the Fishes. However, many of the creatures that dominated the Devonian seas would be very unfamiliar to us today. During the Devonian, armoured fish that were encased in bony plated armour became common. To the north of the bay within the older Valentia Slate Formation, fossils belonging to a species called Bothriolepis are found (Russel, 1978). Bothriolepis were widespread and abundant during the Devonian. These were jawless fish that had a front end that was heavily encased in bony plated armour. The back end of the body had no bony plates and likely lacked even scales.

To the south of the bay within the younger St. Finian’s Sandstone Formation, fossils belonging to a species called Sauripterus have been found (Russel, 1978). Sauripterus were predatory fish that ranged from 1-3m to up to 7m and lived in freshwater ecosystems from the Middle Devonian until the Early Carboniferous.