Glacial history before the last ice sheet

Evidence of events before the last ice sheet is almost absent from Orkney. The sediments of the outer Moray Firth basin and from the continental shelf west of the islands do, however, provide a long record of the Quaternary history of Orkney. The presence of ice-rafted material in North Atlantic sediments suggests extensive northern hemisphere glaciation as long ago as 2.4 Ma (Shackleton, Backman et al. 1984) but there is no direct evidence of glaciation in or around Scotland until around 1 million years ago.

In the early Pleistocene, temperate marine conditions prevailed in the northern North Sea and the shoreline lay considerably farther east than today (Andrews, Long et al. 1990). The first establishment of glacial or periglacial conditions on Orkney may be recorded by glaciomarine sediments dating from around 850 ka from a borehole 125 km E of the Orkney coast (Sejrup, Aarseth et al. 1987). The first presence of small glaciers on Hoy may date from this cold interval. The Brunhes-Matuyama palaeomagnetic boundary at about 730 ka coincides with a switch to more extreme climatic oscillations, with the first of several extensive glaciations of the outer Moray Firth by Scottish ice (Stoker and Bent 1985; Bent 1986).

This early Cromerian glaciation may represent the first time that the Scottish ice sheet reached Orkney. Orkney was almost certainly covered by ice sheets during the Anglian or Elsterian glaciation, commencing around 440 ka, although no glacial sediments of this age are known from Orkney. Anglian ice sheets reached the edge of the continental shelf west of the Outer Hebrides and Shetland (Stoker, Hitchen et al. 1993). In the northern North Sea, the Scottish and Scandinavian ice sheets may have been confluent. Here there was extensive erosion of early Pleistocene sediments, with the cutting of deep channels by meltwater (Cameron, Stoker et al. 1987). In the outer Moray Firth, the Elsterian channels are filled by sediments of the Ling Bank Formation, which locally include a temperate marine fauna indicating the penetration of North Atlantic Drift water into the northern North Sea and the establishment of interglacial conditions (Andrews et al 1990). In the early Saalian, the apparent presence of a tidewater ice front north and east of the Witch Ground Basin (Bent, 1986) would imply, if correct, that Orkney was covered by ice at this time. The diamicts of the Fisher Formation, deposited beneath or close to the margin of ice sheets, occur well to the east of Orkney (Andrews et al 1990) and imply glaciation of the islands in the late Saalian. Sediments of the Coal Pit Formation occur beneath the bed of the North Sea some 75 km E of Orkney and correlate with the lower part of the Ferder Formation off the east coast of Scotland (Johnson et al., 1993). The Coal Pit Formation includes a temperate marine fauna and a reversed palaeo-magnetic section correlated with the Blake Event of 105-115 ka (Sejrup et al 1987). The Coal Pit Formation is therefore thought to represent the last, Eemian interglacial in the northern North Sea (Andrews et al .1990). The upper part of the Ferder Formation consists of structureless diamicts of probable subglacial or glaciomarine origin. An Early Weichselian ice cover on Shetland and in the inner Moray Firth can be inferred (Johnson et al., 1993) but it is uncertain how much of Orkney was ice-covered at this time. The overlying Cape Shore Formation comprises marine sands and muds and includes shells which have given radiocarbon dates of around 30 ka and which, together with other data, place the Cape Shore Formation in the mid-Weichselian (Johnson et al., 1993). The outer Moray Firth was therefore free of glacier ice for at least part of the mid-Weichselian prior to the advance of the Late Weichselian ice sheet after 30 ka. No organic interglacial or interstadial deposits have been discovered on Orkney. A raised cobble beach is exposed in the north of Hoy at 6-12 m above present sea level and rests on the inner margin of a marine abrasion ramp. The beach gravels lack erratic material and are overlain by head and till. The age of the beach is unknown.