In Otago the mountains have flat tops because they used to be a plain
Range after range, the summit line of Central Otago is horizontal. They are fragments of a plain that wore down over sixty million years and was then broken and lifted in pieces.
Mountains are supposed to have a point. It is so automatic that a child draws them as triangles and nobody corrects the drawing. Central Otago, in the interior of New Zealand's South Island, contradicts that shape with a persistence that draws attention: range after range, the summit line is horizontal.
The Rock and Pillar Range, the Dunstan Mountains, the Raggedy Range, the Old Man Range. Seen from the flat they look like long walls with their tops trimmed level. This is no geographical coincidence or trick of distance: it is the clue that these mountains were not raised as mountains.
01 Summits that look like tables
The region is large: 31,186 square kilometres, New Zealand's second by area, with 253,900 inhabitants in mid-2025. Almost all that population lives on the coastal strip, in and around Dunedin, and the interior is left as a mosaic of ranges and basins with small towns.
The first surprise of that interior is its repetition. A flat-topped range rises, a flat basin drops, another flat-topped range rises, another basin drops. The pattern is called, in geology, basin-and-range terrain, and in Otago it shows with unusual clarity.
The explanation has two acts, and they need telling in order. First a plain was worn down. Then it was broken.
02 Sixty million years of wear
The starting material is schist, a metamorphic rock of tight laminations that here forms a block of continental dimensions. It belongs to Zealandia, the continental mass of which New Zealand is the emerged part, around 85 million years old.
For some sixty million years that block did not rise: it wore away. Erosion lowered relief until it left a surface of very little slope, almost a plain, which geologists call the Otago peneplain. Layers of younger sediment were later laid on top of it.
That is the detail that changes everything. The flat surface did not form on a mountain top: it formed at the level of a plain and sat there, undisturbed, for tens of millions of years. What you see today at a thousand metres is a piece of ancient ground.
03 Then it was lifted in pieces
The uplift came late, in the last two million years, and it was not the process that built the Southern Alps. There the collision of plates piled up rock and produced sharp peaks. Here the push was gentler and what it did was reactivate old faults that already crossed the block of schist.
Along those faults, whole strips of the block went up and others stayed down. Those that rose became ranges and kept the old flat surface on top; those that did not stayed as basins. The phenomenon belongs to the Kaikōura Orogeny, between the late Tertiary and the Pleistocene.
That is why the summits are level and why they are all more or less the same height: they are fragments of the same original plain, raised to different heights and separated by the faults that split them. Central Otago is not a mountain chain. It is a jigsaw made from a single broken piece.
04 The stones that stayed standing
The second feature of the landscape appears when you climb onto those tablelands. Above the tussock, the yellow clumped grass that covers the interior, blocks of schist rise in strange shapes: towers, tables, fingers. They are called tors.
Their origin is tied to that of the flat summits. When erosion stripped away the Tertiary sediment layers covering the peneplain and left the schist exposed, not all the schist resisted equally: the more fractured zones broke down and the more compact ones stayed standing, protruding from the ground.
The consequence for the landscape is that Otago's heights look nothing like a normal mountain's. There are no ridges or avalanche chutes: there is a broad, gently rolling surface, strewn with loose rocks. You walk along the top as you would across a moor.
05 And the basins stayed flat
The other side of the mechanism is the basins. The blocks that did not rise spent millennia receiving the sediment washed down from the neighbouring ranges, so their floors filled in and ended up even flatter than they were.
Those filled basins have names of their own and they are the skeleton of the interior: the Maniototo plain, the Manuherikia valley, the Ida Valley, the Strath Taieri. They sit low for how mountainous everything looks: Alexandra, in the Clutha basin, is 150 metres above sea level.
Through them runs the Clutha, New Zealand's largest river by discharge, which crosses the mosaic cutting gorges where it had to breach a raised block. It is the thread that stitches the basins together.
06 How to travel it
The first practical consequence is that here the roads run along the basins and cross the ranges at specific passes. Driving through Central Otago is a succession of broad flats and short climbs, not a continuous mountain zigzag. Distances pass quickly and the landscape changes abruptly at each threshold.
The second is that the high ground is walked, not climbed. Getting onto the Rock and Pillar or the Old Man Range asks for no technique: it asks that you withstand wind and cold on an exposed tableland. It feels closer to crossing a raised steppe than to summiting.
And the third is the one that reorders the trip. If in each flat summit you see a piece of one ancient plain and in each basin the gap left by a block that did not rise, the interior of Otago stops being a collection of ranges with different names. It becomes a single surface, broken and set at different heights, that can still be read whole.
