This EarthCache is based on one of
the suggested field trip stops in the book ‘Vancouver, City on
the Edge’ by John Clague and Bob Turner. This is a fantastic
book for both geologists and non-geologists alike, which
describes and explains the geological processes at work in the
Lower Mainland of British Columbia. It is available from
Tricouni Press
"Caulfield Park – the deep
Earth exposed
Caulfield Park is a ‘must see’ for geology students young and old.
As you walk along the very scenic shore in the park, you will cross
some of the oldest rocks in the Vancouver area. These rocks formed
deep within the Earth under very high temperature and pressure.
Since then, they have been elevated tens of kilometres by Earth
forces, and the overlying rocks have gradually eroded away. There
are two main types of rocks at Caulfield Park – light-coloured
granitic rocks consisting mainly of crystals of the minerals
quartz and feldspar, and dark banded metamorphic
gneiss (the “Caulfield gneiss”), which contains abundant
feldspar and black hornblende. The gneiss formed through the
recrystallization under high temperature and pressure from
pre-existing sedimentary or volcanic rock. It is cut by numerous
dykes of granitic rock of different ages, which suggests
that granitic magma filled fractures in the gneiss and, therefore,
that the gneiss is older than the dykes. But wait a minute, look
more closely: some of the granitic rocks seem to grade into the
gneiss, without a sharp boundary. Could some of the granitic rocks
have formed directly from the gneiss without actually having
melted? Some geologists think so. Examine these rocks closely and
see what you think. You can use the geological principle of
crosscutting relationships to sort out the relative ages of
the various dykes and the gneiss. This is a fun geological
brain-teaser”!
p.22-23,'Vancouver, City on the Edge’, John Clague and
Bob Turner. Tricouni Press, Vancouver, 2006
Useful Geological Terms
An understanding of the following terms, as defined in the book
(excluding photos and diagrams), may help you:
Biotite – “A common, platy rock-forming mineral of the
mica group. Biotite is generally black or dark brown because it
contains iron and magnesium. It is an important constituent of
granitic rocks and also occurs in many metamorphic and sedimentary
rocks.”
Crosscutting – “A body of rock or sediment cuts across a
second body and therefore is younger. Crosscutting commonly results
from injection of magma or fluidized sediment along fractures in
older rocks.”
Crust – “The outermost layer of the Earth, which is
composed of rocks rich in silicon and aluminum. The crust ranges
from 8 to 70 km thick.”
Dyke – “A sheet-like body of igneous rock that cuts
across the layered, folded, or other structures of the surrounding
rock.”
Feldspar – “A group of abundant rock-forming minerals of
the general formula: MAl(Al,Si)3O8, where M is potassium, sodium,
or calcium; Al is aluminium, Si is silicon; and O is oxygen.
Feldspar constitutes 60 percent of the Earth’s crust and occurs in
all types of rocks, including granites. It ranges in colour from
white to pink; some feldspar is translucent.”
Foliation – “A planar fabric [appearance of lines or
waves] in a metamorphic rock, generally produced by the alignment
of minerals under directed stress.”
Gneiss – “A banded metamorphic rock formed at high
temperature and pressure deep in the Earth’s crust by
recrystallization of pre-existing sedimentary, metamorphic or
igneous rock. Gneiss consists of alternating dark- and
light-coloured layers of different minerals.”
Granitic rock – “A term loosely applied to any
light-coloured, medium- to course-grained igneous rock consisting
mainly of quartz and feldspar. Geologists discriminate many
different types of granitic rocks on the basis of the types of
feldspar and the amount of quartz, i.e. granite, granodiorite and
tonalite.”
Hornblende – “A black, dark green, or brown mineral that
is common in igneous and some metamorphic rocks.”
Igneous rock – “Rock formed from molten or partly molten
material (magma).”
Magma – “Molten rock within the Earth, from which igneous
rocks are derived.”
Metamorphic rock – “Rocks that have formed from
pre-existing rocks by changes in mineral content and structure
under high temperature, pressure, and stress within the Earth’s
crust.”
Quartz – “Crystalline silica (SiO2), an important
rock-forming mineral. Quartz is the second most common mineral in
the Earth’s crust, after feldspar. It occurs in colourless and
transparent hexagonal crystals and in crystalline masses, and is
common in many igneous, metamorphic and sedimentary rocks.”
Sedimentary rock – “Rock formed at or near the Earth’s
surface at relatively low temperature and pressure by (1)
compaction and cementation of rock and mineral fragments (“clastic
sedimentary rock”), (2) precipitation of minerals from solution
(“chemical sedimentary rock”), or (3) accumulation and alteration
of plant and animal material (“organic sedimentary rock”). Mudstone
and sandstone are examples of clastic sedimentary rocks. Rock salt
and some limestone are chemical sedimentary rocks. Coal is an
organic sedimentary rock.”
Validating your EarthCache find
You already know you will find two types of rock on the shore at
Caulfield Park. A closer inspection of each rock will reveal more
about them. Therefore, in order to claim that you have found this
EarthCache please email me the following.
- A description of the granitic rock and at least two of the
minerals you can see in it. You should comment on:
- the general appearance;
- mineral colours;
- average size of the mineral crystals (are they: A - less than 1
mm in diameter?, B – 1-2 mm in diameter? C – 2-5 mm in diameter? D
- > 5 mm in diameter?
- lustre – what the surface of the crystals look like in the
light, e.g. shiny, metallic, pearly, frosted, waxy, dull/earthy,
shimmering, glassy;
- crystal shape – you can either go with your own descriptions or
look for a geologist’s term. There’s a good selection of examples
at
www.minsocam.org ; and
- approximate proportion of each mineral in the rock. Use the
following as a
guide:
Abundance Chart
- Describe the gneiss. You should comment on:
- colours;
- patterns;
- any minerals you can identify in it; and
- the general direction (e.g. east to west) of the
foliation.
Please include in your log a photo of you at the Earthcache
location.
Some additional resources if you’d like to use them:
http://geology.about.com
http://flexiblelearning.auckland.ac.nz
Access & Site Information
Caulfeild Park is a shore park in West Vancouver. It can be
accessed from Marine Drive, approx 8 km from Horseshoe Bay or 10 km
from Lions Gate Bridge. You can get there by:
- bicycle - Marine Drive is a popular scenic ride for
cyclists;
- bus - 250 Vancouver to Horseshoe Bay - stop at Piccadily Road N
- stop id 54638
westbound or 54576 eastbound; or
- car - if you must! Parking is limited and restricted to
designated sections of Pilot House Road and Piccadily Street.
Please take care to park in the designated parking sections
only.
The EarthCache location can be accessed from several trails on
Pilot House Road. The easiest/shortest trail is from the parking
area at N49° 20.277
W123° 15.059. The nicest way to to
reach the site is to walk along the Caulfeild Trail from N49 20.466
W123 14.907.
Pilot House Road is a residential area and the park is used most
often by residents and dog-walkers. The EarthCache site is
occasionally used for school geology field trips.
Please practice minimum impact caching – leave your
geological hammer at home and take photos, not samples.
