The beach and quayside are protected by a long sand spit which allows access to the Ocean at the northern end. At this end is Sentinel Rock which forms a natural rocky link to the mainland. As recently as 1000 years ago the sandspit was almost certainly covered in forest as there is lots of evidence of burnt tree stumps present today. The sandspit was formed by the combined action of wind and waves which blew the sand down the coast from the north until it reached this spot where it began to form a beach.The sand can contain such other minerals as ilmenite, iron and very rarely gold.
In the early 1860's local authorities helped by the military built a groyne from the mainland at Mangawhai Heads to Sentinel Rock to protect the northern channel and prevent it from siltation. The southern channel has always been too shallow for access by boats.
The picture shows the harbour today. In the centre right is the sand spit and behind it is the forested northern side of the harbour entrance which provides further evidence that the sand spit was probably a forest in the past. When you stand at GZ and face the ocean look to your right. In the distance you will see a sand mountain. The height of this mountain is exactly what you would have seen directly ahead of you about 100 years ago. Sand mining over many decades reduced the height of the sand spit to its present level. An important geological lesson has to be learned from this experience. Whilst sand mining is important it has to be controlled in order prevent over mining and the threat of natural disasters. The sand mined from Mangawhai was of good quality and used in the construction industry.It will be seen below that such over mining was the cause of serious problems at Mangawhai Harbour.
In 1978 a severe storm breached the sand spit and very slowly the previously accessible northern channel began to silt up.
The problem was further worsened by Cyclone Bola in 1988 leaving the northern channel silted up completely. The river at Picnic Bay that flows into the Harbour polluted as a result of siltation and the water became putrid and discoloured as it no longer had a natural outlet. Apart from the obvious health hazard it left no protection for endangered species on the sand bank sanctuary.
The local public authorities declined to deal with the problem and local people formed a working group to deal with it themselves.
On 11th February 1991 they began the "Big Dig" to free up the northern channel, rebuild the groyne and fill in the breached sand spit.
This task took five years of sheer hard work by the local people.
When completed in 1996 they then began a programme of planting on the sand spit. Initially they used 1500 plants at the western end and then started a lengthy process of planting 25000 per year in order to stabilize the sand spit and dunes.
In order to claim this Earthcache you must:
Email me with the answer to these questions -
1. what plants were used to stabilize the sand spit and dunes? (The answer can be found on the internet!)
2. by taking a sample of sand at GZ and examining it closely (preferably under a magnifying glass) tell me how many different colours of sand you see in a sample of 20 grains and describe the texture, i.e fine, medium or coarse.
It would also be appreciated if you took a photograph of the Harbour from GZ, possibly including your GPSR.