Main image on top: Shelly till at Achastle

Significance: a deposit containing many shell fragments derived in part from the bed of the Moray Firth. It was the subject of intense debate in the mid-19th century – was it the product product of glacier ice or icebergs?

Miller gave his grand vision of the origins of the shelly till:

“The northern current would be deflected by the more powerful Gulf Stream into an easterly course, and would go sweeping over the submerged land in the direction indicated by the grooves and scratches, bearing with it every spring its many thousand gigantic icebergs, and its fields of sheet ice many hundreds of square miles in extent.”

The shelly till of Caithness today receives little attention, but this was a deposit that was known to geologists worldwide in the mid-19th century.

Hugh Miller around 1855 described the boulder clay and the underlying scratched and polished pavements. He pronounced:

“The agent which produced such effects could not have been simply water, whether impelled by currents or by waves. No force of water could have scarred such distinct, well-marked lines on such small stones. The blacksmith, let him use what strength of arm he may, cannot bring his file to bear upon a minute pin or nail, until he has first locked it fast in his vice… the smaller stones must have been fastened ere they could have been scratched.”

But was this true? The debate was already underway on the glacial theory in Scotland. Louis Agassiz had declared “This is the work of ice!” in 1844 but the shelly till was a crucial test. To the devout, the shells in the boulder clay were undoubted products of the biblical flood. Robert Dick of Thurso was first to recognise that the shells, boulder clay and scratched rock surfaces might have a glacial origin. The careful collecting of shells from the boulder clay by Charles Peach led him not only to produce remarkably detailed faunal lists but also to conclude by 1868 that the boulder clay was a deposit of land ice moving out of the Moray Firth. T. F. Jamieson, an outstanding Ice Age geologist of the day, preferred, like Miller, an origin from sea ice, with debris raining down from ice bergs and transport from the NW. This is a rare example of an erroneous interpretation by Jamieson. First, James Croll (1870) and then Peach and Horne (1881) were able to provide a series of arguments that the shelly till was indeed deposit by an ice sheet filling the Moray Firth and moving onto the plain of Caithness.

The most obvious clue to the passage of ice across the former seabed is the presence of shell debris. This is often fragmented and abraded – Jamieson observed as early as 1866 that where shell fragments are elongate, the glacial striae run lengthways along them. Even minute foraminifera when viewed under the microscope have a rubbed and worn appearance. Occasionally, whole, or nearly whole valves may be found, especially in sections along the Moray Firth coast. For example, the shelly till at Lybster Harbour contains masses of rounded gravel with abundant curly wurly Turritella. The greater distance of glacial transport at sites on the north coast means that shell here has been more thoroughly ground down and it is usually present only as small chips. Shell is also absent from the top 2 m of exposures due to postglacial weathering and associated decalcification. The shells comprise mainly cold and relatively deep water species and include Arctica islandica, Astarte sp, Mya truncata and mussels. The shelly till also contains erratics of Jurassic and Cretaceous rocks and fossils eroded from the floor of the Moray Firth. And even some fragments of Jurassic fossil wood.

Shelly tills occur in only a few parts of Scotland, including northern Lewis, Kintyre, Berwickshire, Buchan, Caithness, and Orkney. In each case, the last ice sheet flowed across marine sediments before moving on to the current land area. In Caithness and Orkney, ice moved out of what is now the Moray Firth and carried with it large amounts of debris derived from erosion of pre-existing marine sediments. In northeast Scotland some of this material was transported as huge rafts and so retains much of its original bedding. Caithness has a massive raft of glacially transported Lower Cretaceous sandstone at Leavad. In Orkney however the original sediment was much more thoroughly mixed to form part of the matrix of lodgement till.

The minimum age of a shelly till is provided by the shells within it. In Caithness and Orkney, amino acid ratios indicate that the molluscs lived and died between Oxygen Isotope Stages 9-3 from 280-30 ka (Bowen and Sykes, 1988). The youngest shells provide a limiting age on the enclosing shelly till that indicates deposition by the last ice sheet. This makes sense because we now know that Caithness was ice-free at around 30 ka, so marine molluscs may have lived and died then on the floor of the Moray. We also know that after deposition of the shelly till the Moray Firth ice sheet finally disappeared from Caithness around 16 thousand years ago.

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