Nov 30, 2015

ICE AGE | Through a Glass, Darkly:

Our Incomplete View of Ice Age Ireland

Trying to view Ireland's Ice Age is like attempting to peer through a frozen lake

An unfortunate and inescapable fact about Ice Age Ireland is that much of this time remains a mystery to us due to the relentlessly destructive power of the ice sheets. Numerous times over the course of the Pleistocene (c. 2.6 million years ago–Present), these ice sheets are thought to have extended fully over the surface of Ireland, in the process erasing much of the evidence from former ages. This has left Ireland with only a very patchy record of the Ice Age, a situation which is in marked contrast to Ireland's eastern neighbour Britain, which has a far more comprehensive Ice Age record as southern parts of this land remained free of ice throughout the Pleistocene. Ice Age Ireland, then, is to us mainly a hidden land, and though we can catch glimpses of its true nature here and there, in essence we can only view it as through a glass, darkly.

To begin with, much of the evidence for Ireland's Ice Age animals comes from very late in the Ice Age, with the vast majority of it belonging to the last 30,000 years or so of the Pleistocene, from perhaps 45,000 years ago up to the epoch's end 11,700 years ago. For the remaining two-and-a-half million years of the Pleistocene, our understanding of the nature of Ireland comes almost entirely from the remains of Ireland's Ice Age plants, in the form of fossilised pollen.

This record starts out in the best manner possible as we have a pollen sequence from one site in Ireland which is thought to span the latest Pliocene (c. 5.3–2.6 million years ago) to the earliest Pleistocene, documenting the changes wrought by the descent into the Ice Age.

The pollen sequence from this site, Pollnahallia, which is around 20 kilometres north of Galway City, shows a warm, woody vegetation gradually giving way to more hardy plants like heather and juniper. Similarly, the sediments change as you move upwards through the sequence, becoming a mixture of clay and silt which then becomes increasingly sandy until only pure silica sand is left.

The upsurge of sand in the sequence is thought to be the result of the retreat of the forests and the ever colder conditions, which exposed more rock that was then deeply weathered, the resulting sands being carried by the wind from the west and deposited at Pollnahallia. The presence of a thin coat of glacial till at the very top of the sequence signifies that the Ice Age had begun.

The hardy plant heather held on longer than some other plants in Ireland as the Ice Age descended

So, Ireland's fossil pollen record starts out very promisingly, detailing the land's surrender to cold and ice. However, following this, our knowledge of Ice Age Ireland drops to almost zero, with a gap in the record of around two million years – accounting for around 80 per cent of the Ice Age's entire length. It is only from around 500,000 years ago onwards that we again get a glimpse of Ice Age Ireland, in the form of a few different pollen sequences that are known to belong to interglacials.

The oldest of these pollen sequences is from Ballyline in Co. Kilkenny, and it is the nature of the pollen which demonstrates conclusively that it is from an interglacial, with a variety of trees represented such as oak, alder, elm, and hornbeam, as well as walnut trees, yews, firs, and spruce, not to mention smaller plants like heather and a variety of herbs. It is impossible, however, to say just which interglacial these plants belong to, except that most experts hold that it is one which occurred over around 430,000 years ago.

A younger pollen sequence is that which has been ascribed to the so-called Gortian Interglacial. This interglacial gets its name from pollen remains found near Gort in Co. Galway, in mud and peat deposits capped by glacial material in the banks of the Boleyneendorrish River. Gort is not the only site where pollen thought to belong to this interglacial has been found, though, with thirteen other sites from across Ireland, including ones in counties Limerick, Cork, Waterford, Mayo, and Tyrone, having pollen with Gortian affinities.

Locations of some of the major sites which have yielded important Ice Age pollen sequences

The pollen from these various sites is very interesting as it gives us a clear picture of the changes in vegetation which take place from the beginning of an interglacial to its end.

For instance, at the beginning of the Gortian Interglacial you see the invasion of pioneering plants such as willow, juniper, herbs, as well as deciduous shrubs. Next, these plants are crowded out as warmer climate trees such as birch, pine, oak, elm, and hazel push forth from their exile in glacial refugia. As the climate eventually begins to deteriorate once more, though, some of these trees disappear and birch and pine are joined by firs, spruce, alder, and yews to dominate the landscape. Then, even, the birch and pines fall away and plants like heather join the remaining trees as the most common elements of the vegetation, demonstrating that the climate has become wetter and that bogs have begun to grow. Finally, even this hardier assemblage is seen to yield to the might of a greater advancing force as the land descends into another glacial.

As comprehensive a view as this pollen collection gives us of the evolution of an interglacial, though, it is, just as with the Ballyline pollen, not possible to say exactly which interglacial it belongs to. Some argue that the Gortian Interglacial corresponds to an interglacial which began around 425,000 years ago, others that it aligns most closely with one that began about 335,000 years ago. Still others contend that it is younger again, corresponding with an interglacial that started around 245,000 years ago. Thus, despite the detailed nature of the pollen record from the Gortian Interglacial, there is much we do not yet know.

With Ireland's Ice Age interglacials only grudgingly yielding any information, then, it is only natural that we might turn to their colder counterparts for answers. However, when it comes to Ireland's glacials, the record is, if anything, even more restricted, at least in terms of time span covered. There is a physical record of only two glacial periods from Ice Age Ireland: the older one termed the 'Munsterian', after glacial deposits found in Ireland's southern province Munster, while the younger one is called the 'Midlandian' after deposits in the Midlands.

In the coldest depths of glacials during the Ice Age, Ireland would have been completely covered in ice

The Midlandian is known to correspond to the last glacial of the Ice Age which began around 120,000 years ago and lasted until the end of the epoch 11,700 years ago, but the dating of the older Munsterian is far less sure. Usually, it is thought to correspond to a glacial that began around 300,000 years ago or one that started 190,000 years ago, but there is still a debate as to its true dating, with some experts even suggesting recently that these Munsterian deposits actually belong to the Midlandian.

Ultimately, it is only this last glacial, the Midlandian, which offers us a detailed view of the animals, plants, and landscape at a particular time in Ice Age Ireland, and even then this is mostly restricted to the later parts of the glacial.

In the end, trying to look back into Ireland's Ice Age is like standing on the surface of a frozen lake and striving to peer into the waters below. Here and there we can catch a glimpse of movement, of form, but we are always just short of a certain view. It is not our willingness to see that is at fault, then, but the lens through which we observe, which is by its very nature opaque. Strain as we might, we can never form a true picture of this unknown world, our view ever obscured by ice.