This Earthcache is located in the
Marlborough Sounds at French Pass. The French Pass Earthcache
is intended to inform people of the specific coastal, seabed and
tidal conditions which result in the fastest tidal flows around the
New Zealand coast line as well as some of the unique geological
aspect of the region.
Cache location
This Earth Cache is located at French
Pass. There is a turn off from State Highway 6 near Rai Valley
which will take you to French Pass and many other nearby bays. Note
that the road is unsealed beyond Elaine Bay but is in generally
good condition all the way to French Pass.
Geology of the Marlborough
Sounds
The Marlborough Sounds is a unique region
in New Zealand as it is the only land area sinking into the sea.
The Sounds are a continuation of the Richmond Range which extend to
the south and west, which has tipped into the sea. The bedrock they
consist of formed 280 million years ago and has moved 53km since
the Pliocene 7.7 million years ago. The geology of the
Marlborough Sounds is predominantly schist bedrock, which in some
cases has been highly fractured. This is overlain, on predominantly
steep slopes, by a thin layer of clay
soils.
Sedimentary,
volcanic, and mineral belts are found in close proximity.
Concentrated deposits of nickel, chromium, cobalt, molybdenum and
manganese are found in the mineral belts, and cause stunting of
vegetation at low altitudes.
This land mass
continues to move northwards at a rate of 6.6mm per year. This
movement is a result of the Sounds being on the boundary of the
Pacific and Indo-Australian plates. This active geology has led to
dramatic ridges rising out of the sea, and a sheltered marine
environment. It has also resulted in an area with frequent
earthquakes, including a magnitude 7.8 earthquake recorded on 16
October 1848 which caused extensive damage in the area and was felt
strongly as far away as East Cape, northern Taranaki and Banks
Peninsula.
The configuration of the
area and its effects on tidal
currents
French Pass is the narrow stretch of water
between the mainland of the New Zealand South Island and D'Urville
Island. This stretch of water has the fastest tidal flows in New
Zealand. Although the pass is 500m across, the main navigable
channel is only about 130m across and 21.5m deep at low water. Most
of the rest of the pass is made up of broken reefs. It is an
extremely hazardous stretch of water for seafarers especially since
most of the hazards are generally invisible. Strong flows, water
level variations, turbulence cause by the unevenness of the sea
floor, tidal rips and back eddys all make this pass a treacherous
waterway.
On one side is Cook Strait and Pelorus
Sound with a tidal range of up to 2m while on the other side of the
pass is Tasman Bay with a tidal range of up to 4m. The resulting
substantial pressure gradients are complicated by a time difference
of about 25 minutes between the high tides on either side. The tide
flows through the pass at up to 7 knots ( 13km/h) under normal
conditions but has been observed to flow even faster under some
conditions.
When the tides change the currents are
sometimes strong enough to stun fish.
The pictures above and below were taken
seconds apart. The launch was holding steady in the stream and the
two kayaks went past like they were running river
rapids.
History
The first recorded European navigation of
French Pass was in 1827. Admiral Jules Dumont d'Urville navigated
the pass during his second voyage to New Zealand in the French Navy
corvette Astrolabe. Approaching the narrowest part of the pass, the
vessel swung sideways and did not respond to steerage. Although the
vessel struck rocks twice it was then washed over the reef into
Admiralty Bay. Men on the corvette said, "It was a most impressive
sight to see the "Astrolabe" at one moment lying on her beam ends
as if about to be swallowed up by the whirlpools that surrounded
her, lift herself by a graceful movement, advance majestically
through the midst of the waters from which the fury had departed".
Admiral d'Urville was then of the opinion that no one should
attempt to navigate the French Pass except in extreme
emergency.
In pre-European times the local Maori traversed the pass in
their canoes or 'waka'.
Logging
Requirements
In order to log this Earth Cache as found
you must do the following:
1. Take a photo of yourself and / or your
group at or near the posted coordinates with your GPS clearly
visible and a view of the Pass in the background. A bonus would be
to have the beacon that marks the main channel in the view as well.
Post this photo with your log. Please note time of day picture
taken as well as date when logging. Please note that this
requirement is optional as per current Earthcache
guidelines.
2. Go to the second waypoint at S 40
55.517 E 173 50.079. Here you will find another lookout over the
pass with an information board which will provide the answers to
the following questions:
- The currents in French Pass cause many
whirlpools and back eddys. What is the largest of these
named?
- What speed was the current through the
pass measured by a lighthouse keeper in the 1950's and under what
conditions?
- What type of land animal has been
observed to swim across the pass?
3. Given the geology of the region, is
French Pass likely to become deeper or shallower over time and
why?
Note: Do not log your find until we
confirm your answers are correct.
Thanks for visiting our Earth Cache, we
hope you enjoyed the drive, walk, the views and found this an
interesting and informative area.
For more information on Earth Caches go
to http://www.earthcache.org