Shelly till at Bomasty

Shelly till at Bomasty

glacial erratics on Stronsay

shelly till

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Mill Bay striae 

Striae at the southern end of the exposure 

Rhomb porporhy detail

Detail of rhomb porphyry erratic 

Mill Bay, Stronsay

Bomasty 

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). The famous geologists Ben Peach and John Horne visited in 1879, just prior to the start of their work in the NW Highlands. Peach and Horne recorded for the first time a reddish-brown, gritty clay containing striated stones and broken shells and the site was of key importance to their 1880 paper on the glaciation of Orkney.

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 (Bowen and Sykes, 1988; Bowen, 1989; Bowen, 1991). As this is the youngest till on Stronsay and as the Orkney is now known to have been glaciated in the Late Devensian in the period from 32-16 ka (Phillips et al, 2008), these interpretations are no longer tenable. The till dates from the last glaciation yet includes a suite of older shells.