The Cobb Valley, Mt Arthur and the Tablelands
Westwards from Nelson city in Kahurangi National Park, lies the Arthur Range and its culminating peak, Mount Arthur. Further to the west is a great uplifted plateau – the Mount Arthur Tablelands, and the Cobb valley.
This region is one of the most interesting in the country for trampers and naturalists with its impressive mountain and bush scenery, remarkable botany and geology and interesting human story.
Rocks and landscapes
The Mt Arthur/Cobb area has many special geological features. Nowhere else in New Zealand has such a complex series of ancient rocks been eroded into such distinctively different landscapes.
Mount Arthur is made of hard, crystalline marble, transformed (hardened) from limestone, originally laid down under the sea some 450 million years ago. Below ground are some of the deepest shafts and most intricate cave systems in the country and exploration of these is far from finished.
During the ice ages small glaciers carved smooth basins called cirques high on Mt Arthur, polishing andscraping the tough marble. The floors of the cirques are studded with sinkholes where surface water in summer is taken underground into extensive cave systems.
The gently-rolling Tablelands are a remnant of a once-extensive sea-level plain which over 45 million years ago stretched across New Zealand. As the land sank below sea-level, thick quartz gravels and then limestones were deposited on the ancient plain. In the last 14 million years the plain has been uplifted, mostly buckled and folded into mountains, its limestones and quartz gravels eroded off, but here and there remnants have survived, as with the Tablelands.
The Cobb valley is marvellously different again. Rivers have cut down into the rising landscape from nearAorere Peak; northwards the Burgoo, eastwards the Waingaro, westwards the Roaring Lion and southwards the Cobb. With the onset of the ice ages, these valleys filled with glaciers, the largest was the Cobb. It carved a classic U-shaped, straight valley, polishing and smoothing bedrock and dumping ridges of moraine as it went.
Today s empty valley cuts through a wide range of very old rocks, some volcanic in origin, some metamorphosed through time. These include: sandstones, schists, undersea fan deposits, shales and quartzites. In one or two special localities, fossils from these dim ages lie preserved; trilobites and graptolites, the advanced life-forms of those archaic seas.