It forms the northern side of
and is the longest sandspit in New Zealand, stretching for about 26 km above sea level and another 6 km underwater. The spit runs in from west to east, and is made from fine golden sand - as
to the west of the spit is mostly composed of late
, i.e. silica but with traces of other heavy minerals,
. The erosion of the cliffs into fine sand carried on the sea currents creates Farewell spit further east.
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The continual movement of the currents piles up the sand in a curved hook shape. Predictions state that the spit will grow almost 2 km in the next 5 years alone.
The northern side of the dunes are steeper and unstable being constantly exposed to the prevailing winds which average over 25 km/h. The southern side, that which faces Golden Bay is more stable and largely covered with vegetation. The tide here can recede as much as seven kilometres exposing some 80 square kilometres of mud flats; a rich feeding ground for the many sea birds in the area but also a death trap to the frequently stranded whales.