PAT JESS: Beautiful, absolutely wonderful.
DARRYL GRIMASON: Fantastic.
PAT JESS: And you really see the flat floor, even though that’s covered with the debris, but the flat floor that the glacier scooped out and running up the steep sides up to the hills. Standing here, you can just imagine it. Ice is inevitable in its flow. It is absolutely merciless. It’s going down the valley. It’s taking in front of it everything but the very hardest of rock and, looking down here, it doesn’t look as if there was much that stood in its way.
DARRYL GRIMASON: The classic U-shape of Glenariff Glen sets it apart as a relic from the Ice Age, but there are other features in our landscape that give us major clues to our icy past, if you know what to look out for.
As the ice melted after the big freeze, it dumped that bank of earth into this breath taking valley. What we’re looking at is an Ice Age terrace or delta.
That terrace is in the upper reaches of the Glenelly Valley in Tyrone. As the ice melted and retreated, it created a lake here. The melt waters were held in on three sides by the hills and on one side by the retreating glaciers. Our delta was the high water mark. Stephen McCarron can read this landscape like a book.
This place wouldn’t have looked like this at the end of the last Ice Age, would it?
STEPHEN MCCARRON: No. It would have been full of water up as far as halfway up the side of those hills there on the far side of the valley, and it would have stretched that water but inland lake or sea would have stretched about ten miles in that direction.
You would have the rising Sperrin Mountains, which formed a natural rock barrier to the escape of the water. So that would have been the equivalent of the age of a bath, say. Water was constantly being added to, but it was filled right up to the rim, and there’d be water trickling off and into the Moyola Valley and down that direction.
DARRYL GRIMASON: And on this side, what’s the side of the bath here?
STEPHEN MCCARRON: Well, on this side, there’s nothing to stop it going out in the present day. So there must have been a large barrier, presumably of ice, coming from the direction of Donegal. What happened, we think, was that the Donegal ice, as it melted back and retreated in that direction, exposed a lower level in the hills, and suddenly that lower level was available for the water in the bath, so to speak, and the water would have rushed out of it. Catastrophically, there would have been huge amounts of water rushing out very suddenly, cutting into the hillside and going northwards towards Dunnamanagh.
DARRYL GRIMASON: If we were to cut into that flat top terrace today, what would we find in there?
STEPHEN MCCARRON: Well, these are composed of layers of sand and pebbles and salt. All that material came from the glacier and got dumped in the lake.
PAM FOGG: Well, this is an erratic, a glacial erratic boulder, and it’s called an erratic because it shouldn’t really be here. It’s on a different sort of rock. This is a limestone rock beneath us. And so it must have come from the top of the mountain because that mountain is sandstone, the same rock as this. So somehow or other it’s been brought from the top of the mountain down to here, and the only way it could have been brought, obviously, is in ice or on ice. So the ice has moved down from the mountain. Maybe there’s been a rock fall. The rock’s fallen onto the ice, transported down, the ice has melted and it’s just dropped here, and in fact the whole field is full of these erratic boulders.
Video with transcript
In the early part of this video, Pat Jess describes Glenariff, one of the 9 Glens of Antrim (For more information visit causewaycoastandglens.com) She paints a picture of a glacier-filled valley being eroded by the river of ice. The video contains many pieces of evidence of our icy past. As the video progresses, see how many of them you can list. The sand and gravel deposits associated with the ice dammed lake and other features make Co Tyrone the biggest producer of aggregates in Northern Ireland - about 55%. This is of significant economic importance in a largely agricultural county.
As the video perspective shifts to look down the valley, look out for the Fairy Thorn left standing in the middle of the field – many still believe it is very unlucky to cut or damage one – many others just wouldn’t chance it! For some of the myths and associations, go to remarkabletrees.org.
Other evidence of ice movement is shown by the erratic boulders below Cuilcagh Mountain – the ice was powerful enough to move them many miles from their geological origins. One distinctive rock from Ailsa Craig in the Firth of Clyde shows which way the ice moved. The map shows where the microgranite pebbles have been found. This same microgranite is used to make curling stones.
Did you note all the evidence of the Ice Age? Some are very large – U shaped valleys with steep sides and flat floors. Others are less distinct – hummocks or low hills of material deposited by a glacier (moraine). Even smaller are the erratic boulders. Have a look at the Nant Ffrancon example in our Geology Toolkit.