Aspects of glaciation in Orkney

The earliest ice flow for which there is evidence originated from between south and south-south-east. Following this ice flowed from the south-east and then from between east-south-east and east. These flows occurred within a single ice sheet. There is no evidence that the ice had its origins in Scandinavia. Throughout Orkney much of the material eroded by the ice sheet was finally deposited only a short distance down-ice$ Approximately three-quarters of the erratics (>25mm.) and of the gravel fragments (2.8 - 19mm.) has probably been transported less than 10 km. Variations in the grain size composition of the till and in its colour can also be systematically related to the nature of the bedrock lithologies in the path of the ice flow after allowance has been made for similarly linlited down-ice transport. A number of sequences of till units were discovered. These built up at sites in areas containing outcrops of contrasting lithologies where, as the ice flow direction changed through time, the nature of the' lithologies traversed by the later flow differed substantially from that of the lithologies traversed during the earlier flow and where conditions were favourable for the preservation of the deposits of both flows. It is proposed that the characteristics (lithological composition of the gravel fraction, grain size composition, colour) of the till at any point in Orkney in either the horizontal or vertical plane can b.e considered as a function of ice flow direction during the period of genesis of the till and of the bedrock lithologies occurring within 10 km. in an up-ice direction. Aa the ice thinned deflections of the flow occurred due to the interaction of the underlying relief with the ice surface slope. Finally hummocky drift features formed beneath stagnant ice and from debris accumulated on the ice surface So its margin. Subsequent to the retreat of the ice sheet, ice reoccupied parts of Hoy. In northern Hoy an end moraine and a kame terrace have been identified. A radiocarbon date of )44,300 B.Po obtained from shell material in the till l.aid down by the main ice sheet ia used to suggest that this may have last crossed Orkney prior to the late Devensian glaciation ( 25 - 13,000 B.P.)