The West Coast has been described as a region of mountains, rainfall and rivers. These features combined with the processes of uplift and erosion - have resulted in a landscape of unique character, two thirds of which is mountainous.
The Alpine Fault runs most of the length of the region. East of the fault are deeply dissected mountain ranges. To the west, rivers and streams are steeply graded - the distance from source to sea seldom exceeding 50 km. Towards the coast alluvial and beach deposits occupy a 10-15 km wide strip which extends inland along river systems. Plains areas are, with some exceptions, generally localised and composed of outwash silts and gravels. They are subject to frequent flooding.
With most of the region being mountainous or hilly and forested, soils are generally leached podsolised yellow brown earths or gley podsols which are shallow in depth and low in fertility. The combination of steep slopes, high rainfall and seismic activity commonly result in high erosion rates.
At high altitudes the crests of the ranges are frequently bare.
In the valley floors the soils are recent gley or organic soils. On higher terraces these are the poorly drained and badly leached pakihi soils.
On lower lying sites closer to valley floors, the soils are more freely drained and have a higher natural fertility. These soils are more productive and form an essential element in the region’s agricultural economy.