The Kaikoura Period of Mountain Building

TOWARDS the end of the Tertiary Era the New Zealand area was subjected to intense pressure which was relieved by the breaking of the earth’s crust along more or less vertical fractures, or ‘faults.’ The present  relief of New  Zealand is most entirely due to movements which occurred in this period of mountain-building, or ‘ orogeny’ as it is termed by the geologist; and, as its most notable result was the Kaikoura Mountains, it is called the ‘ Kaikoura Orogenic Period.’

Professor Cotton has aptly described New Zealand ‘as a concourse of earth-blocks of varying size and shape.’ These earth-blocks were raised during   this   period   of   mountain building.  The edges of each block were denned by great faults—lines of fracture in the earth’s crust — along which movement took place, some blocks rising great heights, while others were pressed down failed to rise. The highest blocks form the mountainous back-bone, so typical of New Zealand, and, generally speaking, the present coastal areas and foothills are the marginal low-lying blocks. Perhaps the most spectacular movements of this period were those which brought the Kaikoura Mountains into existence, for in their formation upward movements of 10,000 feet occurred. The structure of the blocks is two-fold. First there is an ‘undermass’ of rocks formed by the sediments of Mesozoic times, but now folded and altered. Above this lie horizontal beds of Upper Cretaceous and Tertiary age, these forming the ‘covering strata.’ The subsequent geological history of New Zealand is the story of the wearing down of this concourse of earth blocks by agents of erosion. Rain, running water, and wind have played their part. Changes in temperature and the freezing of water have also assisted in wearing down the rocks. But perhaps the most remarkable changes were brought about by the action of moving ice during New Zealand’s great Ice Age. Unstable conditions naturally resulted from these movements of mountain-building. The force of gravity acting downwards tended — and still tends—to cause readjustment and this action is increased by erosion which removes material from the higher blocks and deposits it on lower country. Stresses are set up between adjacent blocks, and these, if sufficiently great, are relieved from time to time by renewed movement along the old faults. This movement sets up a vibration which travels outwards in all directions as an earth wave or earth­quake. New Zealand’s liability to earthquakes is, therefore, an aftermath of the Kaikoura period of mountain-building.