Loess in the Manawatu district, New Zealand

Abstract

Loess covers most of the terrace and rolling lands of the Manawatu district. A volcanic ash, the Aokautere Ash, interbedded in the loess provides a datum plane that allows measurement of thickness of the loess that accumulated after the ash was erupted. This increases from about 2 ft at a distance from the two main rivers to about 10 ft on their south-eastern banks. The loess coarsens with increasing thickness and most of it is thought to have been blown from the river beds by the prevailing north-westerly winds. It was probably accumulating rapidly when the ash was erupted, and continued accumulating until shortly after the formation of the Ohakea Terrace. The width of bare river beds would have been at a maximum during the aggradational phase that culminated in the formation of the Ohakea Terrace, and the bulk of the loess probably accumulated then. This phase started about 20,000 years ago and ended about 10,000 years ago. The differences in soils on the loess in areas of similar climate are attributed to differences in grain size of the loess and to soil development that took place while the loess was accumulating; this would be more effective where the loess accumulated slowly than where it accumulated rapidly. To the north of the Manawatu district, differences in the soils are due to an increasing proportion of fine volcanic ash in the loess as well as to an increase in rainfall.