 |
Serpentine at Dolar Point
Day 1 |
I woke up. Had breakfast, packed my
last bits of clobber – bag with geo hammer, various chisels, hand
lens, compass-clinometer, square protractor, surveyors note book, as
well as other toys – cameras, astrotrac (incase the clouds clear),
ukulele, clothes, fruity malt loaf. All the essentials. Set up
various lights on timers so it looked like I was at home randomly
wasting electricity. Gave the neighbours the spare key.
The sun was shining, what could
possibly go wrong? Four and a half hours later, my car started to
make a most distressing squealing noise which stopped for a while
when I tapped the brakes and then started up again ten seconds later.
Not good. I did the last 20 miles expecting a wheel to fall off.
Coverack was the last word in quaint.
It also has zero mobile phone reception. Some people – even me
some of the time – would call this a blessing, but right there and
then, I needed to call the RAC. Lucky me – Hostels International
put payphones in their establishments. Half an hour later the man
from the RAC was jacking my car up. He couldn't see a problem with
the brakes, or anything else that should make a squeal, so he took it
for a spin. Three miles later, he pronounced that there is nothing
wrong with it. It's like going to the bloody doctors. Well I didn't
imagine the squeal!!
 |
| Moho under water |
Ah well, I'm was here at least – The
centre of the earth. Well not quite. Coverack has the unique
distinction of having a sequence of rocks exposed that represent the
transition from the earths mantle to the earths crust (known in geoscience as the Mohorovicic discontinuity or Moho for short), and as a
geoscientist I was here to see it. A problem immediately became
apparent. It was at that moment, under water, and the tide would not
be all the way out for another 5 hours. Bugger! Scrambling over the
rocks behind the the lifeboat station (now a restaurant) at Dolar
Point enabled me to get up close and personal with the mantle, and
despite the dire predictions of a salty old sea dog who was fixing
his foc'sle or something at the slipway, the tide had turned and
after an hour or so there was a widening strip of pebble strewn beach
to examine, but as the light was failing, I thought it might be more
productive to visit in daylight.
Day 2.
 |
| Serpentine Pebbles |
Breakfast at the hostel with a nice
family from Wales was followed by a short walk down to the beach. It
rained briefly. At last luck was with me – the tide was out and the
sun started shining. I started my traverse from the harbour north of
Dolar Point. What is immediately apparent is that the simple layer
cake sequence I had imagined Peridotite/serpentine –
Troctolite-gabbro- basalt, would not be immediately apparent.
Hundreds of years of sub-aerial exposure followed by exposure to sea
water had ensured that the shore line rocks possessed a routine
brown-black colour – that's all of them. This was of course not
strictly true. On closer inspection of both the exposures themselves
and the adjacent pebbles it was possible to track the differences.
The mind blowing realization that I was walking across the Moho (!!!)
was tempered by the obvious fact that hundreds, thousands, maybe
millions of people had already done so at Coverack over the years –
it is a small seaside town – there is a beach – the equation is
simple enough... (Sloshing seawater) x (sand and pebbles) =
(screaming children)/(dog walkers)... But enough ruining my Moho
Mojo! I've crossed the Moho and I knew I was doing it – so there!
 |
| Bastite Serpentine with Chrysotile (white asbestos) veins |
To walk at low tide from the
picturesque little harbour to the northern end of the beach takes you
from the depths were the mantle peridotite reacted with water and
heat to form the more easily flowing serpentine; across the
Moho, represented at Coverack by the unfamiliar
mottled red cumulate rock troctolite, the boundary between the mantle and the
crust; and into the course grained mafic lower crust represented by
gabbro and basalt dykes. All of it helpfully squeezed up 10km from
the earths depths and laid end on at Coverack beach for those in the
know to walk over and wonder at. The reality is not as clear cut.
The exact points were mantle ends – Moho begins – Moho ends –
crust begins are blurred by erosion, cross cutting intrusions this
way and that, and a couple of geological imposters (several huge
lumps of gabbro and basalt that appear were no gabbro or basalt
should be – the remains of a former coastal defensive structure).
 |
| Gabbro Veins |
 |
| Basalt dyke |
Moving swiftly on. A short car journey
(in the car with nothing wrong with it!!), in a howling gale and
hideous rain shower, took me to the basement, the rocks onto
which this slab of ophiolite was thrust at the closure of the Rheatic
ocean. This is a sequence of what were once volcanics. The heat and
pressure created by the collision of two continent sized sections of
crust re-assembled the minerals into forms that were stable in such
extreme conditions. The result is seen at Lizard Point itself –
Man o war gneiss. This rock has been through a lot, and one
look at the painful contortions that can be seen in the rocks at
Lizard is enough to tell a double story – First of all the heat
and pressure that caused the minerals to reconstitute themselves into
gneiss in the first place, and then a pressure so great that
it can cause solid rock to bend! The tide was coming in rapidly, and
the swell was big, and the shower had started again, so finding a
piece of Man o war gneiss on the tiny swatch of beach below the old
lifeboat station was exciting, but I got a little bit with a nice
s-bend in it to illustrate the meaning of pressure.
 |
| Contorted Man o war gneiss at Lizard |
During yet another shower, food with
the nice family from Wales who seemed to have adopted me by this time
was eaten in a cafe at the top of the path. This was followed by a
trip to Mullion Cove were exists the geological frontier between the
materials from the mantle and ocean floor, and the sediment into
which they smashed. The resulting rock is hornblend schist, and the
cliffs on the north side of the cove are made of it, whereas those on
the south shore are serpentine. By now the tide was 95% in, which
left a strip of harbour beach about yeah wide on which to find a
sample of hornblend schist, and it was getting more exciting by the
second as huge waves raced to meet me and the frequent showers (did I
mention the frequent showers?) turned to hail stones. Another
bracing British seaside day. After several advances into the fray,
and a corresponding number of retreats, I had a worthwhile sample of the hornblend schist and the serpentine, oh and
wet feet too. The storm was getting worse, and I thought that discretion
was probably the better part of valour. An almost endless phalanx of
rain heavy clouds marched in from the Atlantic like an invading army,
I was soaked to the skin, and I thought my day was about over.
 |
| Serpentine from Mullion Cove |
Day 3
I got up. It was raining heavily. I
went home.
.JPG)
