Shelly till at Bomasty
Striae at the southern end of the exposure
Detail of rhomb porphyry erratic
Mill Bay, Stronsay
Significance: this coastal section provides perhaps the best available exposure of the shelly till which is characteristic of the eastern part of Orkney. The nearby section at Bomasty (illustrated above) displays similar material.
The section at Mill Bay
(HY 665256) forms a continuous cliff 6-10 m
high over a distance of nearly 1 km (Gordon in Hall, 1996).
Where did the material in the till come from?
The till mostly includes material from adjacent Devonian flagstones and siltstones, but a range of exotic rock types was also recorded, including igneous and metamorphic lithologies, fossiliferous limestone, chalk, flints and fossil wood. A wider range of glacial erratics has recently been collected from Stronsay by Brown and Hall. The assemblage is illustrated separately . The stones include at least one rhomb porphyry from the Oslo region, together with gneiss, granite, quartzite, porphyry and other rocks of Highland or Norwegian origin, plus limestone and fossil wood of probably Carboniferous age from the central lowlands of Scotland and Mesozoic chalk, flint, conglomerate, shale and glauconitic sandstone from the inner Moray Firth. Peach and Horne (1880) inferred that the erratics were derived from the Scottish mainland, including eastern and NE Scotland, and the floor of the Moray Firth and North Sea. To this can now be added a component of Norwegian origin. The mixed assemblage is entirely consistent with ice moving across the North Sea bed (Bradwell et al, 2008) and picking up a range of material deposited there by earlier movements of ice and icebergs. The ultimate source of the ice that deposited the shelly till at Mill Bay thus remains uncertain.
Numerous fragments of shells, including Arctica islandica, Mytilus and Mya truncata, are present in the till, generally broken, smoothed and striated. Striations on bedrock at Mill Bay are aligned W15-35˚N (Gordon, 1993).
The lithological composition and erratic and shell content of the Mill Bay till clearly indicate that ice moved onshore from the east (Peach and Horne, 1880; Rae, 1976) in accordance with the general pattern established for the Orkney Islands.
How old is the till?
Rae (1976) obtained an infinite radiocarbon
date of >44,300 BP (T-1152) from shell fragments in the till. Although the radiocarbon
date is inconclusive, Rae argued that on the basis of probability it suggested
that the last ice sheet had not deposited the till. Amino
acid epimerization analyses suggest that shells in the till are no younger than
the last interglacial, which has also been taken to support the hypothesis that
the maximum age of the till is Early Devensian, with ice cover from 80-60 ka (