Evidence for a rock-avalanche origin for ‘The Hillocks’ “moraine”, Otago, New Zealand

A landform in Otago, New Zealand, previously interpreted as a glacial deposit, has been investigated, described and reinterpreted as a rock avalanche deposit. ‘The Hillocks’ is a conspicuous cluster of small conical hills on the Dart River floodplain. The landform is protected under a local bylaw because of its identification as an outstanding example of a glacial kame deposit. However, the geological and geomorphological setting, and the deposit morphology, sedimentology and lithology, suggest that it was formed by a large (c. 22.5 × 106 m3) rock avalanche subsequent to glacial retreat, and that the deposit temporarily dammed the Dart River valley. Relative age dating evidence suggests that it is at least several hundred years old but younger than ca 7500 B.P. This work highlights the problem of paleoclimatic reconstructions using ‘moraines’ as indicators of regional climate events. Despite similarities in form and, in some cases, sedimentology, by applying an understanding of landslide initiation, runout and depositional process, we demonstrate that it is possible to distinguish the deposits produced by landslides from those produced by glacial deposition.