Glacial geology of the Cobb valley, northwest Nelson

Investigations of the Cobb valley in northwest Nelson, New Zealand, yield e vidence of at least two and possibly as many as four phases of glaciation i n the valley and adjacent areas. (1) The oldest of these advances is undate d, but is probably mid Quaternary, as it is preserved on old valley floor r emnants c. 100 m above the modern valley. Before the next recorded glacial advance, periglacial conditions occurred at least briefly in the valley. (2 ) The valley was then overrun by a glacier that deposited two basal tills a nd probably extended into the modern fluvial gorge. The extensive roche mou tonnee fields through much of the valley and the basal tills near the Cobb Dam may relate to this advance. (3) The moraine underlying the Cobb Dam is inferred to be an end moraine of Last Glacial Maximum age (LGM). A suite of recessional moraines extends upvalley from the head of the Cobb Reservoir. The second of these has a minimum age of 17 000 radiocarbon years and is c onsistent with an LGM interpretation for the Cobb Dam moraine. It is possib le that the LGM and deglacial sequence outlined in (3) is the retreat phase of advance (2). (4) Deglaciation was rapid, but there is circumstantial ev idence for a temporary readvance from some cirques during the deglaciation. There are no known Holocene glacial deposits.