Kia ora from New Zealand. In fact we're in Wellington, Victoria University and what I'd
like to do today is follow up with a whole bunch of questions that people came up with
about the the mystery of the boulder bank in Nelson. And so I've come here to Wellington
today to speak with Warren Dickinson who has for many years researched the Nelson
boulder bank and many other similar features in coastlines in New Zealand and elsewhere. So
Warren, thanks heaps for your time right now! Yeah I'm looking forward to you know finding out
what you think about some of the suggestions people have made with this video, and we'll
go through them if you like one by one - some of the different points people have suggested. Sounds
good! - and then you see what you think. So it's a really fantastic opportunity to have the expert
giving some some comments and feedback about this mystery - because number one - it's still a bit
of a mystery isn't it? It is! and and I think i'm only an expert because I've looked at it
probably more than other people have, and you make observations along the way, and I've been working
on it for perhaps 30 years and so I have some ideas about it. Brilliant! Okay here's number one:
So a lot of people have made this point which is that the boulder bank looks man-made. How
do we know that it isn't? or is it in fact man-made? When I first saw it I looked across
at it from the road and I thought "oh that's a man-made structure!" and I actually went out and
looked at it and as soon as you get out there and look at it you realize it's not man-made,
but I think the best observation here is to look at a sketch from Charles Heaphy in 1841 showing
that the boulder bank was alive and well in 1840, before the pakeha really came and I don't think
the Māori had really the technology at that time to build some structure like that, so we can say
that it was here before 1841, it's not man-made. So there's been a couple of suggestions that it
might be related to earthquakes and tsunamis. Is there some possibility that that's the case?
Yes well earthquakes and tsunamis are kind of one-off events and
if they do occur it's possible to build up some deposits. You can see the helter skelter and the
chaos of the deposit left behind. We don't really find this in Nelson boulder bank the boulders and
pebbles on the boulder bank are all very nicely sorted, they're all about the same size and we
don't find all this debris. Plus we don't find any evidence for debris behind the boulder bank and I
think it's pretty safe to say that we probably didn't make these by earthquakes or tsunamis.
It's not the debris from some sort of earthquake or tsunami okay. Is it possibly a glacial
moraine? So all the boulders on the boulder bank are the Cable Bay granodiorite. If you had either
a glacier or rivers you'd expect to have deposits, or you expect to have a variety of different types
of rock, not just from one type. Plus the fact that there's no evidence for any glaciation
in the recent past in the Nelson area. Okay certainly not at sea level anyway. Not at sea
level correct. There are glaciers higher up in the mountains but they never made it down even close
to sea level. So that's a bit of a unfortunate because I quite like that one! Now those are three
easy questions. Right should we go to something a little bit more complex? We can indeed. We
had a couple of questions from someone saying it looks very similar to Chesil Beach which is a long
beach / sort of spit in the UK. Do you know about Chesil Beach? Yes I had the good fortune to visit
Chesil Beach a couple years ago. It looks like the boulder bank and you can see that from the air
you say "oh that looks much like the boulder bank!" - it's a long spit. In fact Chesil Beach is
about 13 kilometers long. But when you look at it up close you can see that it's made up of a lot of
smaller pebbles but nothing anywhere near the size of the boulders on the boulder bank in Nelson.
And these are all easily moved by waves. Here we can see the boulder bank at Nelson and you
can see that to the left there are cobbles and boulders and going up the shore here we
can see that it graduates into pebbles of about the same size as the ones on Chesil Beach. If
you happen to see the Nelson boulder bank at high tide you would only see the pebble ridge.
If you see the Nelson boulder bank at low tide you would see the pebble ridge
plus the big boulders on the boulder platform down below mean tide level. Okay and then I have
another question which is how far out do these boulders go, as far as we know, out to sea?
Well they go out at least a kilometer where they become buried by Tasman Bay muds. So it could even
be further than that. It could be in fact they probably are underneath the muds. Right okay
as many of you know the boulder bank has always been thought to have formed by longshore drift.
The way longshore drift works - if the waves hit the the coastline at an oblique angle
they will pick a particle up and move it in that direction as well sort of down
drift and they'll toss it up on top of the ridge. And then what happens
in order to get that particle back down into the swash zone you need to have backwash of the waves.
And what generally happens in gravel beaches is that the water percolates down into the ridge
very quickly and it loses all its energy. So there's very little energy to pull that
particle back down into the wave zone once it's back down in the wave zone then it can move it
back up in the direction of the long shore drift. And there's no question that the waves can wash
large boulders and cobbles up on top of the pebble ridge, but it's getting them back down
back into the wave zone that is the problem. In other words the backswash is very low energy. One
possible idea that's been put forward is that once these boulders sit on top of the gravel ridge,
that they are on loose pebbles and loose gravels that the boulder might just slide back down by
gravity. In actual fact it might be a very minor process but it's never really been documented,
so making this process continuous is probably not enough to move the big boulders laterally
alongshore the drift. Now what more can you tell us about these boulders? Is there anything
else that we need to know? This picture shows essentially what's below the large boulders
at Nelson. We have a very poorly sorted pile of rounded rocks below
the large boulders on the surface. And in that poorly sorted material are there boulders also?
Yes there are, and we found very large boulders such as this two meter roller which was somewhere
in this big pile of poorly sorted debris. So you can see that the boulders are actually sort
of a lag on the surface. In other words they're left behind as the waves are eroding this whole
lower feature. Some of the larger boulders come to the surface and they provide if you will kind
of an armour. Okay and so that armour of boulders is what's left behind. The smaller pebbles have
been picked up thrown up on the boulder bank and are behaving a bit like Chesil beach,
but the other stuff left behind still has had to have got there somehow and
that's the mystery is that correct? I think you said it better than I can!
And it's this boulder bank deposit which gives rise to the boulder bank which I find is the
big mystery. And how that deposit got there we really don't understand that well. So one of our
questions was I thought a very good one, which was: is it a possibility, even hypothetically,
that there's an outcrop of granodiorite parallel to the boulder bank and that by perhaps a fault
line has down thrown the outcrop? It's now been eroded away which was the source of all those
boulders extending along the length? Yes that's an incredibly interesting observation and a very good
question. And so if there is bedrock it's quite deep. I'd have to say in my way of thinking that
would be my answer to the origin of the boulder bank, but again we're lacking concrete evidence
on this. What I can say though is that all the sedimentological evidence points to something like
that. Is there a way you could use some technique to dig deeper underneath the platform of boulders
to find out what's there? Yes what you could do is put a little drill rig offshore perhaps
500 meters and start drilling and you would be looking for granodiorite.
You could actually drill a series of holes starting sort of at the top of the boulder bank
and then make a proper cross-section through the boulder bank offshore, But drilling is really
expensive and I'm not sure the good citizens of Nelson would like to see a drill rig out there.
Don't you think we should crowdsource a drill rig? [laughs] only a few million bucks! That would be
very interesting, I don't think it would cost several million, it might just cost a million,
uh nothing for crowdsourcing though! [laughs] That's right! If you're out there everybody,
you know maybe put a suggestion to think whether it be worth the effort! Sometimes a mystery is
almost more interesting than the actual answer, that you know it kind of when it finally gets
revealed it sort of kills the journey. It does kill the journey. The processes that have made
the boulder bank deposit are very difficult to envision and difficult to find around the world.