The glacial sequence in part of the Rakaia Valley, Canterbury, New Zealand

Abstract

From examination of the glacial deposits and landforms of the Rakaia Valley, four major ice advances have been recognised. In chronological order, these are the Woodlands, Tui Creek, Bayfield, and Acheron Advances. The Bayfield Advance was double, and the Acheron triple. All are regarded as of late Pleistocene age, the Waimaunga Glaciation being represented by the Woodlands Advance, and the Otira Glaciation by the three later advances. Differences in the height of moraines and outwash surfaces allow the first three advances to be clearly differentiated; the Woodlands Advance is further distinguished by the degree of weathering of its deposits. The Acheron Advance occurred after the initiation of the present Rakaia Gorge, and outwash surfaces comparable to those of the earlier advances are lacking in the main valley, although present elsewhere. Morainic deposits of this advance show that two distinct glaciers derived from different sources were present in the area, one occupying the main Rakaia Valley and the other the Lake Coleridge trough. Meltwater drainage systems were well developed during the Otiran advances, large volumes of water finding an outlet into the Selwyn Valley. Lake deposits, abundant in the Rakaia Valley, are regarded as evidence of the former presence of several small ice-marginal lakes, rather than of one large body of water.