Rocks
Sedimentary rocks are one of three classes of
rock. Sedimentary rock is broken into three sub classes below:
- Clastic- These rocks comprise of
existing rock compressed under immense pressure.
- Biogenic- These rocks are formed from
the remains of living things.
- Evaportic-These rocks are made out of
dissolved minerals in water which have been precipitated out of
solution.
The
Nanaimo Group
The Southern Gulf Islands and the lower
Vancouver Island region are made of a soft, clastic sedimentary
layer six kilometers thick in some parts. Included in this layer
are bands of sandstone, mudstone, conglomerate and
coal. These
sediments were deposited during the Cretaceous period approximately
85-65 million years ago. This material forms the aquifer
for many of the 85% of the mid-island population living near the
coast. Like the other Southern Gulf Islands, Jesse and nearby
Newcastle Island also comprise of weak sandstone.
Source: Geoscape Nanaimo,
Vancouver Island University
Weathering
Rocks on the surface of the Earth are
constantly being eroded by wind, water, temperature or chemical
action. These powerful forces are constantly reshaping the planet.
Every coastline is under assault by the relentless power of nature,
some at faster rates than others.
What
is a Cave?
A cave by definition is a cavity in the Earth.
On land, these are usually formed out of soft limestone which gets
dissolved by acids in watercourses. These can create cave networks
which can be up to hundreds of kilometers long in some cases. A sea
cave is formed through a different process.
All rocks are constantly being weathered and
some are more durable than others. Sandstone is not a very durable
rock and as a result is easily weathered. Unlike caves on land,
where passages in rock are dissolved away through the action of
carbonic acid, sea caves, also known as littoral caves, are
primarily formed through wave action. Aiding the wave action is the
fact that the sandstone of the Gulf Islands is very susceptible to
salt damage, which results in large flakes breaking away.
In order for a cave to form, there needs to be
a point of rock that is weaker than its surroundings. This is known
as differential erosion. The constant wave action striking the face
perpendicularly erodes the rock and eventually erodes the rock
away. When this powerful wave action is applied to both sides of
the point, the force is concentrated, and the caves eventually
connect through. It should be noted however that it is not just the
water which does the weathering. Contained in the water are
suspended sand and rock particles which also impact the rock. If
the cave is big enough, the hydraulic pressure resulting from the
power of the waves will create a hole is the ceiling known as a
blowhole. After a few thousand years, this roof will eventually
collapse resulting is a column of rock just offshore called a sea
stack. A sandstone cave is unlikely to ever become big because of
the weak nature of the rock. Igneous rock, formed out of cooled
magma is more resistant, and it will result in the largest
caves.
During foul weather, wind often blows into Departure Bay between
Stephenson Point and Newcastle Island and also down Newcastle
Channel. This can create very choppy conditions which pound away at
the weak sandstone. Since the waves worked away at both sides of
the headland, this created a prime condition for a cave to form.
The unique sandstone features surrounding the island, such as sea
stacks and surge channels aided the focusing of the waves on the
rock face. Another prime example of wave erosion is the Malaspina
Galleries located on nearby Gabriola Island. The soft sandstone was
worn away and created a unique and dramatic overhang. Definitely
worht checking out! Waypoint is below.
The Gulf Island coastline is rich in life. As
the tide rises and fall twice a day, different levels of shoreline
are exposed at different times. This allows for many different
types of sea life to thrive. There are many reasons why different
types of sea life choose to live where they live. The higher up
animals tend to have harder shells and be resistant to dry
conditions while the lower animals prefer to be wet at all times.
Life that lives in these locations have adapted and in fact thrives
in the intense pounding surf that creates these caves.
Logging
Requirements
This cave is not visible from land no matter
how hard you try to spot it. As a result, this Earthcache is only
accessible from boat. A kayak is definitely the best option but any
boat will do. The closer you get, the greater the reward, but I do
not want anyone risking their larger boat by getting too close to
shore. To log this cache:
- a. What bearing is
perpendicular to the cave (projects through the centre)?
b. How high is the cave from the base to the very top?
c. How wide is the cave at its base?
d. Describe the colour and texture of the rocks on the left
and right of the cave.
e. Why do you think the cave faces the way it
does?
- Record the air and water
temperature at the site and post in your log. Compare with the
readings from this
station to see how similar your readings are. When I was there,
the water temperature was 7.5 degrees celsius and the water
temperature was 8.5 degrees celsius.
- What are the two factors
(possible 4) that has led to the developement of the
cave?
- OPTIONAL:Take a picture of at least one
member of your party (and your GPS if possible). If the water is
too choppy and you’re sitting in a kayak, just a picture
proving you’ve been there will suffice.
These answers must be E-mailed to me
to qualify for the log. Please send answers to my E-mail through my geocaching.com
profile
.
References
Kimantas, John. 2007. The Wild
Coast 3. North Vancouver: Whitecap Books.