Shells have been found in the boulder clay in Eday, Shapinsay and Stronsay. Some shells pushed up from the sea bed have been useful in dating the clay deposit. Westray, being further from the source, tends to have only very small unidentifiable fragments. Some of the most striking erratics are the lumps of flint and pebbles of chalk that you might rarely find on a pebbly shore. The flint nodules, which originally occur as knobbly lumps within the chalk, strongly suggest the presence of an outcrop of Chalk under the sea not far away. The nearest outcrop is in the Moray Firth to about 25km east of Wick. The Chalk is of Cretaceous age, about 100 million years old. Some good flint nodules have been found on Papay.
Occasional erratics of crystalline rocks of granite, gabbro, quartzite or gneiss have turned up on Westray, both as pebbles and as larger rounded boulders, and may well have originated from either Scandinavia or Scotland. The geographical origin of some erratics can be ascertained in the laboratory, many granites for example, while others such as rhomb porphyry or laurvikite are easily recognisable.
A major dilemma about where the ice came from is that one ice stream can transport rocks, which might be picked up later by another. Even when you have checked the geography of your erratic, a Scandinavian rock may have been carried part way across the North Sea then later picked up by Scottish ice. The same can be said about Scottish rocks pushed up from the seabed by an ice stream from Norway.