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Sunday, 6 July 2014

Glen Valtos -- erratic meltwater flow?

On the Isle of Lewis, not far from Callanish, there is the most spectacular meltwater channel on the island -- called Glen Valtos.  It's about 2.5 km long, and flows from west to east, and while it is quite shallow and insignificant at its intake end, it is about 45m deep at the eastern end, where it decants into Loch Miavaig.

So far so good.  The trouble is that the orientation of this channel is quite at odds with the other evidence from the neighbourhood which shows Devensian ice movement broadly from south towards north.  The orientation of the channel is about 90 degrees out.  This is shown on the map below, from a paper by DG Sutherland:

The channel is shown in the top right corner of the map.  The only way to make sense of this map is to have a large glacier mass offshore, to the west -- that would be no problem during a major glaciation, since the coastline of the time would have been far to the west.  In general, subglacial meltwater flow (which this must have been) accords with ice movement as recorded through other features, and with the overall ice gradient.  But nonetheless, I am somewhat perplexed as to why Sutherland, Peacock and other authors have insisted that this channel is of Late Devensian age -- causing them all to get into a bit of a tangle in trying to make sense of the field evidence relating to moraines and fluvio-glacial and other features.

To me, it would make much more sense to argue that the channel is a very old feature,  dating from an earlier glaciation, and maybe used again and freshened up during the Devensian.   It doesn;t make a great deal of sense to me to assume that nearly all of the glacial landforms in the UK are only about 20,000 years old, when we know that there have been several earlier glacial episodes which must have resulted in great landscape modification.

Talking of meltwater channels going the wrong way, this reminds us of the Gwaun-Jordanston meltwater channel system in Pembrokeshire, which has an overall orientation which is very difficult to explain by reference to the last movement of the Irish Sea Glacier, which was broadly NW towards SE.  The channels, in contrast, run east - west and then NE towards SW.  They MUST be very old...... and formed at a time when ice from the Welsh Ice Cap must have been in the ascendancy.

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