Wednesday 21 September 2016

Snowball Earth rAmble at Port Askaig and Bunnahabhain

Another rainy geological walk. Five hardy souls braved the weather to join Dave to look at Islay's classic 'snowball earth' deposits. First up was the world-famous Port Askaig Tillite - a sequence of silty rocks with a large number of varying sized 'stones' - mainly of granite - embedded in it. Dave explained the two competing theories about its origin: either it was deposited on land as a melt-out of an ice sheet (or a series of ice sheets) or it was deposited in the sea from dirty floating ice. We looked at some alleged 'dropstones' near the lifeboat station - it could be that these did in fact drop off the base of the floating ice and penetrate into the sediment - or that the laminations wrapping around the stones are diffracted cleavage planes (i.e a later metamorphic event).

The jury is still out, but nevertheless there is a considerable thickness of undoubted glacial deposits which are about 650 million years old and it is possible that they correlate with similar deposits in other parts of the world and - if they are of the same age - they could be evidence that the Earth was indeed entirely ice-covered. Again the jury is still out because we can't date these tillites here very accurately but we do think that Islay was probably in the tropics at this time so so much ice deposit would be unusual!
Then onto Bunnahabhain to look at the Bonahaven Dolomite - a sequence of carbonates that overly the Tillites and are purported to be a 'cap carbonate' which is quite typical of these glacial deposits in other parts of the world and are believed to be evidence of dramatic changes in ocean and atmospheric chemistry as the world recovered from the snowball event. The rocks contain stromatolites - evidence of bacterial activity in (probably) warm water - ie the usual conditions you'd expect in the tropics - they were the coral reefs of their day (before corals were invented!).
Then back to Dave and Isobel's van for lashings of tea, ginger cake and whisky to warm us up!

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