Chalk and flint erratics

Significance: the presence of chalk and flint in the tills of Orkney provides important evidence of the direction of ice flow across the bed of the outer Moray Firth.

For well over a century fragments of chalk and flint have been recovered from the shelly tills of Orkney and Caithness. Rae (1976) plotted the known distribution in Orkney, demonstrating a marked concentration on the eastern side of the islands. The till at Banks of Runabout, Shapinsay, also contains chalk. In Caithness, chalk erratics occur from Wick northwards (Crampton and Carruthers, 1914).

The outcrop of the chalk on the floor of the Moray Firth has been mapped by the British Geological Survey. Whilst it is possible that unknown small outliers exist, the known outcrop is quite restricted in area and confined to a N-S oriented linear zone some 10-20km across. To carry erratics directly to eastern Orkney, the flow of ice must have moved between SSE-NNW to ESE to WNW. To carry erratics to northern Caithness the flow is from the E or SE. Against a background of multiple ice advances and of ice margins across the Moray Firth, more complex flow patterns are possible, with pick up and carry of erratics by successive glaciers. Transport by icebergs from calving glacier margins may also have occurred.

shelly till        glacial erratics