The glacial history of the upper Mersey Valley

In the upper Mersey Valley there have been three and possibly four glaciations during the Pleistocene. From youngest to oldest they are named the Rowallan Glaciation, the Arm Glaciation, and the Croesus Glaciation. In addition sediment from a Tertiary glaciation and a Lower Palaeozoic glaciation have been recognised. The Rowallan Glaciation covered the smallest area of the three Pleistocene glaciations. The glacier formed a reticulate system with the main source an icecap on the Central Plateau, and occupied an area of approximately 282 sq km. Major valleys were almost completely filled with ice and the glacier terminated in at least six ice lobes. In the region the Rowallan Glaciation covered an area of approximately 282 sq km. The lowest point reached was 390 m above sea level in the Mersey Valley and the equilibrium line altitude for the system was 1050 m, which indicates a snowline some 930 m below present and a drop in the average annual temperature of 6.2°C. The Rowallan Glaciation attained its maximum before or about 13,400 ± 600 yr BP and deglaciation was well advanced or complete by 9760 ± 720 yr BP. The Arm Glacial system was larger in extent than the Rowallan Glaciation and at its maximum occupied an area of approximately 500 sq km. The , lowest point reached was in the Mersey Valley at 330 m and it had an equilibrium line altitude of 950 m. This suggests a snowline 1030 m below present and a drop in the average annual temperature of 6.7°C. The age of the Arm Glaciation is not known but estimated to be in excess of 100,000 years BP. The Croesus Glaciation is the oldest glaciation and had a very large extent, completely blanketing the upper Mersey Valley. It terminated far beyond the area of this study. The lowest altitude reached, the equilibrium line altitude and the estimated temperature drop during the glacial maximum are unknown. Some sediments of Croesus age show a reversed remanent detrital magnetism and thus have a minimum age of 730,000 years BP. The Tertiary and upper Palaeozoic glaciations are postulated on the basis of scattered outcrops of lithified till in the region. The Rowallan, Arm and Croesus glaciations of the upper Mersey Valley are tentatively correlated with other glaciations in Tasmania, namely the Margaret, Henty and Linda glaciations of the West Coast Ranges. The Rowallan Glaciation is also tentatively correlated with the glaciation of the Snowy Ranges on the mainland of Australia and with radiocarbon dated glaciations of New Zealand and Chile.