The mystery of the Fan Hir glacier
This green smudge doesn't look like much, but it's an example of many similar sites in a certain part of south Wales that have had geologists and geographers arguing for years.
What you're looking down on is the eastern flank of Fan Hir, a sandstone lump in the Brecon Beacons (map). The Google marker labeled "A" is the steep talus slope of the mountain side; the smaller label to the east (on the right) is a lower ground feature, an elongated hill running parallel with Fan Hir's eastern flank for about a kilometre.
The geographical argument has been centered on how this elongated echo got there.
It's a relatively recent addition to the landscape, laid down during the last real cold snap our planet suffered before the relative warmth of the Holocene. This cold snap wasn't cold enough to bring an ice sheet clattering over the UK as previous ones had, but it was cold enough to result in localised glaciers at higher altitudes. The higher parts of the Beacons were just high enough, and cold enough, to play host to dozens of little glaciers, most of them perched on the side of the a slope like this.
So what is this long, thin, landform? Is is a glacial moraine (a mound of debris left behind by a moving glacier)? Or is is the result of rock debris falling over a massive pile of snow and accumulating at its base?
For years, the argument went one way or the other. Dr Richard Shakesby of Swansea University has settled it once and for all (he says) by studying the site and pointing out the following:
- clasts (stones) in the landform are heavily striated; striations can only be caused by the stones being dragged along the ground by ice. So it wasn't a massive snowdrift.
- if it was debris that had bounced down a snowdrift, where did the debris come from? The summit of Fan Hir hasn't been eroded enough for this theory to make sense.
But there was still another problem with the glacier theory. This glacier was on an east-facing slope, moving eastwards and southwards. This goes against all theory for UK glaciers; no matter how awful you might think our weather is, we get enough sunshine to force most glaciers to face the north, so they can keep out of the sun as much as possible. If the Fan Hir glacier was moving south, how come it didn't get melted into oblivion by the sun?
The answer Shakesby gives is amusingly simple: it kept getting topped up with fresh snow. Snow that had fallen on the flat sandstone top of Fan Hir would get wafted eastwards by the prevailing westerly winds. The fresh fluffy flakes would fall off the the eastern edge of Fan Hir, straight down on our plucky glacial friend. Yes, the sun had a melting effect; but the wind fought back and supplied the glacier with enough raw material for it to expand and grow.
And so it slid its way southwards along Fan Hir, grinding the rocks below as it went, and leaving behind this elongated moraine - small compared to Fan Hir itself, but huge on a human scale.
Why am I telling this story? Because geography is what we build nations, cities and cultures upon. Because geography is tangible, landscapes matter, they are historical evidence you can clamber over and dig your fingers into. (Lift your mud-caked fingernails to your face, and smell the geology.) Because I adore exploring places and trying to understand their history, how they came to be what they are now, and what they were before. Because climbing mountains, and understanding what made them the shape they are, is fun.
I've just added "walk to fan hir" to my todo list. Fancy coming along?
Sunday, October 05, 2008
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