Tag Archives: ice retreat

Minching about on a sunny Isle of Lewis

By Rich Chiverrell

Port Skigersta delta

Port Skigersta delta

One of the smaller ice-masses draining the former British-Irish Ice Sheet, the Minch palaeo ice stream drained much of the NW sector of the British–Irish ice sheet (∼15,000 km2) feeding sediments to the large Sula Sgeir fan fronting the continental shelf. But if this is small, standing on the east coast of Lewis (Outer Hebrides) looking across the sunlit, blue seas and skies east to the feeder fjords and mountains of the Summer Isles and Wester Ross helps one visualise how large this former ice sheet really was. Our aim for this ongoing Briticechrono Transect 8 fieldwork was to secure a series of targets for Optically Stimulated Luminescence (OSL) dating from outwash sands from Lewis, one of the outermost land-masses on the western flank of the Minch Ice Stream. This work will support previous sampling efforts targeting boulders on the Scottish Mainland and the Hebrides for cosmogenic nuclide dating. Previous OSL sampling had targeted the inner sector of the Minch on Skye and north of Ullapool. The team (Rich Chiverrell, Matt Burke, OSL Postdocs Rachel Smedley and Alicia Medialdea) set off first thing on Tuesday morning to join Transect Leader (Tom Bradwell) on Lewis. Departing a cloudy Manchester via Glasgow Airport we landed before lunchtime to blue skies, sunshine and searing temperatures at Stornoway Airport.

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Inspired by our surroundings, the weather and the prospect of excellent sediments the four newcomers sped off to meet up with advance team Tom Bradwell, Adrian Hall and Maarten Krabbendam (from the Netherlands) at Port Skigersta in the far north of Lewis. The site an embayment on the western flank of the Minch Ice Stream gave stunning views across the water back to the ice source areas in western Scotland and beautiful turquoise seas. The sediments were very impressive with the sequence a stacked delta sequence with steeply dipping fore-set sands capped by top-set horizontally stratified gravel. Intriguingly the basal delta is buried by laminated bottom-set muds, in turn buried by a second delta fore-set and top-set couplet. The repeating delta suggests changes in water level probably lake level, dammed between the ice stream and bedrock rise into Lewis. We sampled both deltas close to the fore-set – top-set contact. And then for some geological tourism, the raised beach at Galsom guided by Adrian Hall, stunning and confusing sediments, all contributing to produce a plethora of hypotheses. Difficult to address under the banner of Briticechrono, the beach deposits (guess the isotope stage) appear altered by over-ride by ice, locally there is a surface diamicton and the beach pea/rounded gravels are probably thrust or stacked. We have targeted an outwash (ish!) deposit above a glacial diamicton, fingers-crossed for contributing to the debate. Excellent food followed at the Cabarfeidh Hotel our home for the next few days (well some of us!).

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Day 2, still warm, still still (no winds) and the sunshine popped in later! After an epic breakfast (Lewis did us proud) back to the north and just south of the Butt of Lewis the west coast Swainbost Sands offered much promise. The sections were epic more glaciotectonics, tills, shells than you can shake a stick at, and the beach!!!! One of the best beaches I have seen in the British – Irish Isles…. Selecting targets was challenging, much of the outwash deposit was rich with shells, thrust, tectonised and not where it was deposited! How? Well by marginal movements and override by ice and at a substantial scale. Three sample locations were found and in the back, along with crucial in gamma detector comparisons, duplication with different detectors at some of the samples. The sampling completed our targets after ~36 hours on the islands, and so we racked our brains for other targets. After a quick visit ~5/6km south down the west coast of Lewis where we encountered convincing striae in steeply foliated Lewisian gneiss, where the glacial lineation trends cross obliquely the metamorphic structure heading northwest. We also prospected for sites further up-ice around Stornoway; another fine meal at the hotel and some gin-assisted colour-by-numbers approaches to former ice geometry and let’s see what tomorrow brings for our last 3-4 hours on this eye-opening island (hopefully a final sample)….

Luminescence dating sampling at Cherry Orchard Farm, near Delamere

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By Richard Chiverrell

Another day and another quarry, but this time the BriticeChrono Terrestrial team Rich Chiverrell and Matt Burke met up with some friends, with luminescence dating team Geoff Duller and Holly Wynne from Aberystwyth and stratigraphic geru Geoff Thomas to tackle the delights of rural Cheshire, Transect 3. Breaking all the rules for BriticeChrono quarry investigations the sun was out and not a snowflake in sight or site for that matter. Cherry Orchard Farm is one of a series of sand and gravel quarries to the east of the mid-Cheshire Sandstone Ridge, recently sampled for cosmogenic nuclide (CN) dating.

The site (location 17 on the map) makes an intriging pair with cosmogenic nuclide location ‘Urchin’s Kitchen’ (location 16), a deeply incised bedrock channel eroded subglacially. We hope to compare the performance of luminescence and CN dating techniques with pairings like this. The setting contains numerous the active and former sand and gravel extraction sites around Delamere Forest, and is located on an extensive (8x5km) gently undulating triangular terrace or bench raised >10m above the floodplains of the Weaver Basin. The terrace is fed by channels flowing from the Sandstone Ridge and presumably a former ice margin on the southern edge of zone 5 (see the map).

Quarry operator (Richard Wilding) was fanastically co-operative and allowed us full access to the sections which reveal shallow water sandur and fine-grained glaciolacustrine sands. The sands were a dream to sample, well sorted, stratified, the right grain size for luminescence dating and with excellent exposure throughout the section. Four samples were taken arrayed vertically through the sequence, though probably almost identical in age given the depositional environment, we sampled different lithofacies or depositional environments. The lengthly process was completed in 5 hours, it takes 60 minutes to record the gamma dosimetry (with a field gamma spectrometer) for each sample, which gave plenty of time for discussion, strategy and logging. Then for some differing journeys home, it can’t take that long to drive to Aberystwyth can it?! Can’t wait for the dates and the next phase of sampling on transect 3…..