The Boulder Bank

The Boulder Bank is a natural spit made up of boulders, cobbles and pebbles. It separates Nelson Haven and Harbour from The Tasman Bay. It is 13.5 kms long and (except at the Glen) is about 1.5 kms offshore. At high tide it is about 55 meters wide and 6 meters above sea level. As a natural barrier providing a safe harbour it provided the incentive for the New Zealand Company to chose Nelson as the site for a city in 1841. Other places such as New Plymouth, Napier and Timaru had to create artificial rock barriers to protect their harbours from storms. When the first settlers arrived, the bank consisted of several shallow areas that formed islands at high tide. The most southern island was Haulashore. The bank is dynamically growing, and all of the islands except Haulashore Island have merged to form the Boulder Bank. In 1906 to ensure a safe and permanent entrance to Nelson Harbour the shallow neck from Haulashore to the main bank was removed to create a reliable cut for shipping. The cut has since been enlarged and is kept navigable by constant dredging. Before the cut was made, access to the harbour was via a narrow channel 2 meters deep and 90 meters wide between Haulashore Island and Fifeshire Rock, also known as Arrow Rock. Both names for this prominent rock honour the ships that encountered it. Without man’s intervention, it is thought the boulder bank would eventually reach Tahunanui Beach. At the head of the Haven where streams enter this tidal estuary, natural infilling has occured. The present Haven is about 1,400 hectares, being 20% smaller than its original size. The infilling process has accelerated since 1841 with the clearing of bush and natural vegetation. About 100 hectares have been reclaimed over the last 40 years and now contains the marina, boat harbour, fish factories and other industries. Scientist view of the geology The Flaxmore Fault runs south from near the mouth of Cable Bay, forming scarp-faced* cliffs at places such as Mackay’s Bluff. The fault then follows the direction of the road to the Glen and along the base of the Grampians. The uplift of the crust to the east of this fault has formed the mountains, whereas the land depression to the west has caused Tasman Bay. The rocks at MacKay’s Bluff are metamorphic (volcanic in origin) intrusive, and consequently often hard and crystalline in character. This had considerable influence on the formation of the Boulderbank, since the rocks have not easily eroded and remain large and rounded. Their shape allows them to be moved by the natural currents and tides. * scarp = a line of cliffs formed by the faulting or fracturing of the earths crust.

The Boulder Bank A simple sunset from the front deck Tasman sea, bay from the garden lawn