Arctica islandica Habitat
Mya truncata Habitat
Spisula solida Habitat
Turritella communis Habitat
Tridonta elliptica Habitat
Shelly tills occur in only a few parts of Scotland, including northern Lewis, Kintyre, Berwickshire, Buchan, Caithness and Orkney, where 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.
Shelly till at Scrabster
Significance: a deposit containing many shell fragments derived in part from the bed of the North Sea, the subject of intense debate in the mid-19th century over whether it was a product of glacier ice or icebergs.
The shelly till of Caithness today receives rather too little attention for 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."
Miller proclaimed 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 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 perhaps to first to recognise that the shells, boulder clay and scratched rock surfaces might have a glacial origin. The careful collecting of 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 transport from the NW. This is a rare example of an erroneous interpretation by Jamieson as first 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 moving onto the plain of Caithness from the Moray Firth.
The most obvious clue to the passage of ice across the former sea bed 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 Turritella. The greater distance of glacial transport at sites on the north coast means that shell here has been more thoroughly ground down and 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. No study has been made of the pollen content of the shelly till - it is possible that its western boundary could lie further west than currently mapped. Studies in Buchan by Rodger Connell and others have shown that tills without any apparent erratic content from offshore may yet contain pollen from this source.
Shelly till bluffs at Keiss Harbour