Tag Archives: Hebrides

Gripped in the jaws of the Minch

By Richard Chiverrell and Tom Bradwell (photography by Alex Ingle)

1292 km, 51 cores, 177m of sediment, not a bad haul

1292 km, 51 cores, 177m of sediment, not a bad haul


We, the science crew of RRS James Cook Cruise JC123, sailed from Southampton Friday 3rd July bound for the last three transects of the NERC funded Consortium Britice-Chrono, our aim is to work out the timing of the last deglaciation of Britain and Ireland. After a quick stop outside the Solent to test the BGS vibrocore we made hast (10 knots) northwards through the North Sea running geophysical surveys for the North Sea sector (Transect 2) as we went, and in the early hours of Monday 6th July we rounded the northern tip of Scotland on schedule for our speedy (19 knots) tide-assisted passage through Pentland Firth between Orkney and the Scottish Mainland onwards to Transect 8 and the delights of the Minch palaeo-Ice Stream extending north from Skye between the Scottish Mainland and the outer Hebrides towards the edge of the continental shelf and the North Atlantic.

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The Minch seaway ~28-25,000 years ago received ice flow from the major fjords in NW Scotland feeding palaeo-ice stream, which extended north and northwest across the continental shelf. This ice stream dominated the northwestern sector of the British Ice Sheet (BIS). The land- and sea-scape probably developed over multiple glacial episodes, but the sea floor landforms and uppermost geology reflect the most recent deglaciation after 25,000 years ago. The aim of Britice-Chrono is to work out the timescale for this deglaciation, and this has involved fieldwork on land, dating outwash deposits on the Isles of Lewis, Skye and on mainland, and glacially eroded bedrock and boulders across the region. The offshore phase of this research has occupied us, so far, for the last seven days and nights, and involved surveying the sea floor for the morphology and the sediments using acoustic sounding techniques, but critically sampling the sea floor sediments. We have two coring systems on board, a percussive vibrating corer that can sample down to 6 m below the sea bed penetrating the tough materials laid down beneath and in front of former glaciers, and a gravity powered piston corer capable of sampling up to 18m in softer sediments. Our aim is to find shells in these glacial sediments to radiocarbon date and work out the timing of deglaciation.

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The success of the efforts for both our cruises relies on the excellent 24 hour work ethic, diligence and company of the cruise team, science crew, BGS and NOC Piston coring teams and the RRS James Cook captain and crew, all of whom do everything they can to help us. The middle 2-3 days of T8 were particularly nerve-wracking as the BGS crew toiled night and day to fix a particularly truculent problem with the vibrocorer, part of the solution to which lay in finding and removing an electrical break in the 1500 metres of cable that winches the BGS vibrocore to and from the sea bed. Thankfully time was spent obtaining important piston cores in the inner Minch and collecting valuable geophysical data, as the BGS team worked around the clock. As ever in Britice chrono’s experience, the BGS team, had everything needed on board to solve the problem, and cheers greeted the announcement of ready to go, and there followed ‘an in at the deep end’ test of the repair in 500-600 metres of water off the continental shelf fronting the Minch ice stream. Success, with 4.14 metres of glacimarine muds recovered, and on leaving the waters of T8 a further 11 vibrocores were recovered containing the key shell-rich glacimarine and subglacial muds our project requires.
Calm seas, epic sunsets

Calm seas, epic sunsets


Looking back on the Minch experience, it is certainly one of the prettiest (former) ice streams we have worked on during the Britice-Chrono cruises, with land in view and visiting the Inner Hebrides passing the Isles of Skye, Lewis/Harris and Raasay amongst others. The leg has been a considerable success, we have collected 1292.6 km of geophysical data (multi-beam and sub bottom profiler), 51 sediment cores (39 vibrocores, 12 piston cores) and 177.2m in vertical sediment profile; who said the Minch was a small ice stream? Our travels have taken us from Raasay Sound in the south over the edge of the continental shelf at 59° 15’ N, and into near shore waters fronting Cape Wrath and the Summer Isles. The answers to the Britice-Chrono geochronological questions must wait on many months of laboratory analysis, but we leave the Minch with all teething troubles behind us, and a growing bounty of cores in the locker. We are ready for the delights of Shetland…!
A 12m piston core (Tom Bradwell for scale ~ 1.74m)

A 12m piston core (Tom Bradwell for scale ~ 1.74m)

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)….