Hmm...
Some sketchy info here:
ttrl.co.nz
Taranaki is also a source. I don't know what the relative contributions of Taranaki and Taupo are to the Taharoa deposit. The transport isn't always northward, since the waves do not always come from the south - possibly some Waikato sand also went south to Taharoa. But large as that deposit is, I think it is small compared with the northern total. This little map is clearly deficient because it doesn't indicate the vast amount of ancient ironsand in the Kaipara heads. This fact doesn't seem to be widely recognized, although I found this:
stuff.co.nz
Clearly this isn't a new phenomenon. I became aware of the ironsand in the Kaipara when I took a walk down the track to the beach at South Head and discovered that the soil was red. It was when I mentioned this to the university geologist that I learned about the formation of the Kaipara heads. Whatever the flow of ironsand down the west coast rivers is now, it must be miniscule compared with what it was at various times in the past.
The way sand travels upcoast isn't by a smooth steady flow. I believe the flow is very lumpy, with a variability that I believe is created by changes in the channel and sandbar configurations of the harbour entrances. As sand piles up in the bars, the channel is pushed northward, until some event, maybe a conjunction of a big tide with a big storm, causes the flow to break through at a different point. A big blob of sand then migrates to the northern side and starts to make an impact on the beaches to the north. This is my theory anyway, and it's consistent with the behaviour of sand spits at the mouths of rivers. Something of this kind must have happened at Whatipu some time in the past 10 years or so, and the northward march of a big plug of sand has reached Karekare, expanding the beach significantly and completely covering the rocks at the southern end. It is possible now to walk across to the offshore rock at low tide. I did this and discovered that seals live on it.
It is interesting to note that waves were beating against the cliffs between Karekare and Whatipu not so very long ago. Apparently six square kilometres have been added to the beach a Whatipu since the 40s, and more strikingly, I'm sure I've seen an old photo showing Cutter Rock surrounded by sea (can't recall where, unfortunately).
(It might have been here: www3.interscience.wiley.com
In other words the current Whatipu swamp is a very recent phenomenon. By the way, in case you haven't been out there for a while, a big chunk fell off Cutter Rock, which is now about 2/3 of its previous size. Needless to say DOC have posted warning signs all over the place about the danger of falling rocks, when in fact the dangerous time was before (when there were no signs), and now that they have fallen the danger has passed.
flickr.com
I think it should be renamed Old Silverback.
As you can see I'm quite interested in the West Coast. ;-)
P.S. Here's a fine painting:
teara.govt.nz
and a nice pic:
trekearth.com |
