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The eastern ranges give breadth to the Southern
Alps, from the main divide east and south towards the Canterbury
Plains. From North Canterbury to Lake Hawea they include such
mountains as the Puketeraki, Torlesse, Mount Hutt, Old Man,
Two Thumb, Ben Ohau and Barrier Ranges.
The rock type is predominantly greywacke (Torlesse
Supergroup). The altitudes are typically within the range
1800-2300 m (5905-7546'). Both rates of uplift (0.5-3 mm/year)
and of erosion (0.1-0.5 mm/year) are moderate to low.
The ranges are separated by basins (e.g. Mackenzie Country)
and wide braided river valleys (e.g. Rangitata, Rakaia, Waimakariri
Rivers), with lakes (e.g. Lakes Pukaki, Ohau, Tekapo) and extensive
moraines and terraces.
There are variations within this unit, with the
most eastern parts being less steep, more rounded, more dissected,
and retaining little glacial imprint.
Precipitation decreases markedly from the main
divide, to perhaps 1500 mm (59 in.) to 600 mm (24 in.) as one
moves east.
Extensive bare ground and debris mantle, as well as the quasi-absence
of forest south of the Rakaia River, contribute to a dry and
eroded appearance.
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