Nelson Rock and Mineral Club
The Boulder Bank is a 13 km barrier that runs from MacKay Bluff in the east to The Cut, opposite Magazine Point in west Nelson (Figure 1), Away from MacKay Bluff, it lies 1-2 km offshore, and contains a tidal water area known as Nelson Haven. In plan, the bank is curvilinear, and runs more-or-less parallel to the foot of the Richmond Hills. In cross-section, it is seen to consist of two units: a lower platform that rises out of the mud ca. 300-600 m offshore, and rises to just below mean tide level, and an upper ridge that rises to about 5 metres above high tide (Figure 2). Both units are composed predominantly of granodiorite coasts, similar in lithology to the rocks of MacKay Bluff - the Cable Granodiorite. In the ridge, the material is well-sorted, and tends to become finer and more rounded with distance, westwards from MacKay Bluff (Figure 3). Clasts in the platform deposits are less well-sorted and sub-rounded to sub-angular, and size reduction to the south-west is less distinct.
Initial interpretations of the Boulder Bank suggested that it had been formed by longshore drift of material from MacKay Bluff. This is constant both with the lithology of the deposits and the size and shape sorting from north-east to south-west. If this interpretation is true, the Bank is a notable example of a spit, though not particularly remarkable.
For various reasons, however, doubts about this explanation have arisen, mainly due to the large size of the clasts. Whilst the finer sediments are well within the range of material that can be moved as bedload by present-day tides, the coarsest boulders (up to 2 meters in diameter) seem to be too large to be transported any distance. In this case, it has to be assumed that the Boulder Bank is, at least in part, a fossil feature, and that the coarser material was laid down in a more energetic sedimentary environment.
Considerable debate has therefore flourished about the origins of the Boulder Bank (and likewise of the similar feature in Cable Bay). This debate continues .Two main theories have been proposed:
What to see on the Boulder Bank
The Boulder Bank shows a range of interesting features characteristic of marine beaches of this sort. These include:
Other places to see boulder banks
Cable Bay: a similar feature, forming a barrier across a bay, that is subject to the same processes.
Dickinson, W., Hartstein, N., Warren, K. 2011. Geology and History of the Nelson Boulder Bank. In: Lee, J.M. (ed). Field Trip Guides, Geosciences 2011 Conference, Nelson, New Zealand. Geoscience Society of New Zealand Miscellaneous Publication 130B. 18 pp
Hartstein, N.D. and Dickinson, W.W. 2006 Wave energy and coast transport in eastern Tasmania Bay, New Zealand. Earth Surface Processes and Landforms 31, 703-714.
Johnston, M.R. 2001 Nelson boulder bank, New Zealand. New Zealand Journal of Geology and Geophysics 44, 79-88.