Significance: a glacial deposit containing many shell fragments derived in part from the bed of the North Sea
In a few parts of Scotland, including northern Lewis, Berwickshire, Buchan, Caithness and Orkney, 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. In Orkney however the original sediment was much more thoroughly mixed to form part of the matrix of lodgement till.
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 but occasionally whole or nearly whole valves may be found, especially in sections in the eastern parts of the islands. The greater distance of glacial transport at sites in the western and northern areas means that shell here has been ground up and is usually present only a tiny flakes and chips. Shell is also absent from the top 2 m of exposures due to postglacial weathering and associated decalcification. The shells comprise mainly cold water species and include Arctica islandica, Astarte sp, Mya truncata and mussels.