A correlation of rock types with soils, topography, and Erosion in the Gisborne-East Cape region

Abstract

The rock types of the Gisborne — East Cape region have been arranged in groups in order of increasing severity of erosion. The recent alluvium of flats and terraces in Group I is almost free from erosion. In the next three groups the rocks are stable, but the severity of slip and sheet erosion in the soil increases from the limestone in Group II and the sandstones of Group III to the massive siltstone, Taitai sandstone, and basalt of Group IV. Soil erosion is still dominant over rock erosion in the banded mudstone and Clarence greywacke of Group V where incipient gullying has begun, but the unconsolidated sandstone and close-jointed mudstone of Group VI are prone to moderate slumping and gullying in the bedrock. Severe and very severe gullying with subsidiary slump and flow erosion occurs in the shattered Cretaceous shales, argillites, and erodible greywackes of Group VII. The incohesive bentonitic mudstone and crushed mudstone of Group VIII have been subject to extreme flow erosion with associated slumping and gullying.