WE have seen that New Zealand took form as a result of the Kaikoura movements of mountain-building. This left the land a mosaic of earth blocks, each composed of two units — first an ancient ‘undermass,’ then above that horizontal younger rocks. Our geological history since that time has been the story of the wearing-down of those earth blocks by all the agents of erosion. Of these the most powerful, under normal circum­stances, are rain, running water, and physical processes like changes in temperature and the freezing of water. Waves play their part on the coasts; glaciers are immensely powerful agents in certain limited areas; and under dry conditions wind can also be effective in wearing down the rocks.

These eroding agents quickly attacked the surface of the higher blocks, and in the course of time the less resistant upper layers were stripped off to re-expose the more ancient rocks beneath. At the present time the mountains of the Southern Alps are carved in the older greywackes of the undermass,   while the once continuous cover has, with rare exceptions, been completely removed. On the lower blocks which form the coastal lands on either side of the mountain ranges, however, the upper layers are only partly stripped.

The covering-beds have still survived in a few places on the surfaces of inland blocks which failed to rise with their neighbours. The edges of such blocks are faults, forming the boundaries of the higher surrounding blocks whose cover has been removed by erosion.  Such ‘ intermontane basins,’ as they are termed, are well developed in Central Otago and in Canterbury.

The Otago Central Railway follows a chain of lowlands which are the depressions in a broken plateau of block mountains.  Here part of the cover of younger rocks is still preserved in the basins, but the uplands are of schist or greywacke, are re-exposed ancient rocks of the undermass. Even   more   striking   are   the   inland   basins   of Canterbury. The Trelissick or Castle Hill Basin, with its weathered limestone’s of Tertiary age, is an enclosed space some five miles long by three broad, almost surrounded by greywacke mountains 6,000 to 7,000 feet in height. Of similar formation are the Waiau-Hurunui and the Hanmer Basins.