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The Cobb Valley, Mt Arthur and the Tablelands
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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.
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Rocks
and landscapes
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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.
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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.
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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.
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