A mystery of Irish landscape history

Mesozoic and Tertiary rocks distribution

A map of Ireland showing the distribution of Mesozoic and Tertiary rocks in Ireland.

Courtesy of the Royal Dublin Society.

Mesozoic and Tertiary rocks distribution
Courtesy of the Royal Dublin Society.

Mesozoic and Tertiary rocks distribution

A map of Ireland showing the distribution of Mesozoic and Tertiary rocks in Ireland.

Courtesy of the Royal Dublin Society.

Enlarge image

One of the striking features of Irish geology is that, across large parts of the island, the rocks are over 250 million years old. In other words, most Irish rocks date from either the Palaeozoic or Pre-Cambrian Eras. Mesozoic Era rocks, including New Red Sandstones from the Triassic, limestones and clays from the Jurassic, and chalks from the Cretaceous, are only extensively represented in the north-east where they have been preserved under, and close to, the basalts of Antrim-Derry.

Another small area of Triassic rocks can be found at Kingscourt, Co. Cavan, and is an outlier of the north-eastern Mesozoic that has been preserved through faulting. Tertiary Era rocks are also only widespread in the north-east where they are represented in the basalts, and in the clays and lignites that now partly fill the subsiding area around Lough Neagh.

The widespread absence of post-Palaeozoic rocks outside the north-east has given rise to a prolonged, and still not clearly resolved, academic debate about the history of the Irish landscape. What has happened to large parts of Ireland over the last 250 million years? In some areas, for example south Munster, the Carboniferous limestones are found in lowland valleys but they are no longer found on the ridges where the older Old Red Sandstones are exposed.

Limestone

An illustration of the sedimentary rock limestone.

Copyright Geological Survey of Ireland 2006.

Limestone
Copyright Geological Survey of Ireland 2006.

Limestone

An illustration of the sedimentary rock limestone.

Copyright Geological Survey of Ireland 2006.

Enlarge image

But when were the limestones removed, and were younger rocks laid down and then eroded away? Was Ireland covered by a Cretaceous Sea in which chalk formed a hundred million years ago, and did the forces of erosion and weathering (including chemical solution) later remove the chalk cover from those parts where it was not preserved by a basalt overlay? Or was most of Ireland outside the zone of active rock formation during a very long period of geological time? Precisely what was the balance between denudation and deposition?

The Bend on the Blackwater

For most of its 167 km journey to the sea, the direction taken by the River Blackwater is influenced by geological structures that encourage its flow from west to east. These geological structures are linked to a great mountain folding some 300 million years ago and have produced a series of ridges and valleys that follow west-east axes. The Lee (89km) and Bandon (72 km) rivers also follow these geological ‘gutters’ for much of their course to the sea. Looking at the geology and at the physical features, it seems as if the Blackwater should continue eastwards and flow out to sea at Dungarvan Harbour. Instead, this river turns abruptly south at Cappoquin and forces its way through several ridges to reach the sea at Youghal. The Lee and Bandon make similar, if rather less abrupt, turns south. The reasons for this unusual behaviour have been extensively debated by geographers and geologists. One suggestion is that the north-south sections of the rivers of south Munster originated tens of millions of years ago when this part of Ireland had a chalk (Cretaceous) cover that has now disappeared. With many, many millions of years of erosion, hundreds, perhaps thousands, of metres of younger rock were removed, and the west-east ‘gutters’ in the older folded rocks were exposed and became major drainage channels. However, the lower sections of some of the well-established north-south rivers had sufficient energy to continue to dominate. The result is that in their lower reaches the great west-east flowing rivers divert along the long-established routes of older north-south flowing rivers. Ideas like this are of interest because they help us appreciate that some parts of the landscape may have great age. But they are also very puzzling and sometimes seem to raise more questions than they answer. Many of us may be satisfied just to gaze at that great river bend at Cappoquin and to wonder at the wooded beauty of the lower Blackwater.

Map drawn by Stephen Hannon.

The Bend on the Blackwater
Map drawn by Stephen Hannon.

The Bend on the Blackwater

For most of its 167 km journey to the sea, the direction taken by the River Blackwater is influenced by geological structures that encourage its flow from west to east. These geological structures are linked to a great mountain folding some 300 million years ago and have produced a series of ridges and valleys that follow west-east axes. The Lee (89km) and Bandon (72 km) rivers also follow these geological ‘gutters’ for much of their course to the sea. Looking at the geology and at the physical features, it seems as if the Blackwater should continue eastwards and flow out to sea at Dungarvan Harbour. Instead, this river turns abruptly south at Cappoquin and forces its way through several ridges to reach the sea at Youghal. The Lee and Bandon make similar, if rather less abrupt, turns south. The reasons for this unusual behaviour have been extensively debated by geographers and geologists. One suggestion is that the north-south sections of the rivers of south Munster originated tens of millions of years ago when this part of Ireland had a chalk (Cretaceous) cover that has now disappeared. With many, many millions of years of erosion, hundreds, perhaps thousands, of metres of younger rock were removed, and the west-east ‘gutters’ in the older folded rocks were exposed and became major drainage channels. However, the lower sections of some of the well-established north-south rivers had sufficient energy to continue to dominate. The result is that in their lower reaches the great west-east flowing rivers divert along the long-established routes of older north-south flowing rivers. Ideas like this are of interest because they help us appreciate that some parts of the landscape may have great age. But they are also very puzzling and sometimes seem to raise more questions than they answer. Many of us may be satisfied just to gaze at that great river bend at Cappoquin and to wonder at the wooded beauty of the lower Blackwater.

Map drawn by Stephen Hannon.

Enlarge image

These topics have recently been ably reviewed and summarised by Professor Frank Mitchell (1997) and by Professor Adrian Phillips (2001). They note that as early as 1862 the geologist J.B. Jukes speculated that much of Ireland had once been covered by rocks much younger then the Carboniferous limestones. This, he felt, might explain the curious drainage pattern in south Munster where the rivers, most notably the Blackwater, follow a west-east route for much of their length, but then turn abruptly south for their final journey to the sea. While Jukes's ideas remain controversial, over a dozen sites have now been found, in areas outside the north-east, at which deposits can be dated to various times in the Mesozoic and Tertiary Eras.

Particularly important is a site at Ballydeenlea near Farranfore, Co. Kerry. Here Cretaceous chalk has been found, and may have been preserved when the limestone on which it rested collapsed. This site, so far from the main body of preserved chalk, provides substance for the argument that chalk was once more widespread in Ireland but that it has been removed during a very long period of denudation, perhaps covering much of the Tertiary. However, there is also evidence for deposition and the formation of tropical or sub-tropical soils, and many aspects of Ireland's Tertiary Era history remain to be clarified.

The extent of tectonic change - uplifts and downwards brought about by earth movement - also remains unclear, particularly in relation to the later Tertiary period. Professor Frank Mitchell has argued persuasively that there was more such change than geologists sometimes accept. But the debate remains open: a challenge for ongoing geological research.