"As a huge
Stone is sometimes seen to lie
Couched on the bald top of an eminence;
Wonder to all who do the same espy,
By what means it could thither come, and whence;
So that it seems a thing endued with sense:
Like a Sea-beast crawled forth, that on a shelf
Of rock or sand reposeth, there to sun itself."
-W Wordsworth 1807 The Leech
Gatherer
A Glacial Erratic is a
rock or rock fragment transported by a glacier and deposited on
bedrock of different composition. Glacial erratics range from a few
millimeters to several yards in diameter.
Disc (also known as Frisbee) golf is a disc game in which
individual players throw a flying disc into a basket or at a
target. According to the Professional Disc Golf Association, "The
object of the game is to traverse a course from beginning to end in
the fewest number of throws of the disc."
Ferguson Park at Blackman Lake in the city of Snohomish is home
to both Glacial Erratics and a Frisbee Golf Course. The Erratics
present on the course are part of the landscape and the focus of
this earth cache. The unique combination of Glacial Erratics and
Frisbee Golf is what inspired me to place this cache
As the definition above basically states a Glacial Erratic is a
rock that does not come from where it is currently located. Because
of the glaciacian of the Cordilleran Ice Sheet that covered the
region as recently as 10,000 years ago, Glacial Erratics can be
found extensively in our area. In Snohomish County as well as on
nearby Whidbey Island most, of the large erratics currently present
came from either the Chuckanut Formation located near present day
Bellingham, WA or the Fidalgo Formation which occurs on Fidalgo
Head and at several localities in the southeast part of Fidalgo
Island. The following islands are composed entirely of this
formation: Burrows Island, Young Island, Allan Island, Williamson
Rocks, Saddlebag Island, Dot Island, and Hat Island. It forms the
major part of Cypress Island.
The Chuckanut Formation is a sedimentary formation of the Eocene
epoch deposited approximately 40-50 million years ago and consists
primarily of fine to medium grain sandstones. The Fidalgo Formation
is composed primarily of three different types of Dunite; large
irregular masses of extremely coarse-grained dunite, thin irregular
off-shoots of fine-grained dunite, or both types of dunite
everywhere cut by thin stringers (rarely exceeding two inches in
thickness) of serpentinized pyroxenite. These rocks date to the
late Jurassic Period, approximately 140-160 million years
ago.
From wikipedia, Sandstone (sometimes known as arenite) is a
sedimentary rock composed mainly of sand-sized minerals or rock
grains. Most sandstone is composed of quartz and/or feldspar
because these are the most common minerals in the Earth's crust.
Like sand, sandstone may be any color, but the most common colors
are tan, brown, yellow, red, gray and white. The approximate
density of Sandstone is 145 lbs./Cubic foot.
Also from wikipedia, Dunite, is a granular, green igneous rock, of
ultramafic composition, with coarse-grained or phaneritic texture.
The mineral assemblage is greater than 90% olivine, with minor
amounts of other minerals such as pyroxene, chromite and pyrope.
Dunite is the olivine-rich end-member of the peridotite group of
mantle-derived rocks. Dunite and other peridotite rocks are
considered the major constituents of the Earth's mantle above a
depth of about 400 kilometers. Dunite is rarely found within
continental rocks, but where it is found, it typically occurs at
the base of ophiolite sequences where slabs of mantle rock from a
subduction zone have been thrust onto continental crust by
obduction during continental or island arc collisions (orogeny).
Dunite is a granular, green igneous rock composed of coarse grains
of olivine, is the source of the world's supply of chromium. It
weathers to a dun brown color. Some dunites are rich in chromite,
magnetite, ilmenite, or pyrrhotite; they may also contain a small
amount of pyroxene. Dunite occurs in layered, gabbroic igneous
complexes. It probably forms from the accumulation of dense, early
crystallizing grains of olivine that sink to the bottom of low
silica magma. Intrusions of dunite form sills or dikes. Some dunite
has been altered to form serpentine. The density of the primary
constituent of dunite, olivine, is approximately 207 lbs./cubic
foot.
To log this cache you must:
1) Proceed to the posted coordinates and determine, based on the
rocks appearance if the Glacial Erratic at that location was most
likely transported by a glacier from the Chuckanut Formation or the
Fidalgo Formation.
2) Based on your observations from 1) above, determine the total
mass of the erratic. In order to determine the mass of the erratic
you must first determine the volume of the erratic in cubic feet
and multiply that by its density (lbs. per cubic foot of material
that the rock is made from). To simplify matters for you, assume
the rock is a perfect ellipsoid. The equation for the volume of an
ellipsoid is 4/3*pi*r1*r2*r3 where r1 equals ½ the length of the
ellipsoid, r2 equals ½ the width of the ellipsoid and r3 equals ½
the thickness of the ellipsoid. You must take approximate
measurements of the rock based on your observations at the
sight.
3) At the sight of the basket associated with hole number 3 on the
Frisbee Golf course is a rock of smaller size (but still
significant) when compared to the rock at the posted coordinates,
Using your GPS device, determine the distance between the larger
rock at the posted coordinates and the smaller rock associated with
the basket at hole number 3.
4) Find the Frisbee golf basket associated with hole number 2.
Determine the GPS coordinates that the basket is located at.
5) Take a picture of yourself and your party at the large rock
associated with the posted coordinates of this earth cache. Make
sure that your GPS device is visible in the picture and post the
picture with your log.
Using the “Send Message” link on my Geocaching Profile
Page, you must e-mail me with the answers to questions 1-4. Please
do not include any of the answers to the questions in your online
log, even if encrypted. Logs that contain answers to the questions
will be deleted immediately without notification. Logging criteria
for 5) is optional based on the current guidelines for logging
requirements for Earthcaches but is highly encouraged for this
cache. Failure to comply with the above listed logging requirements
within a “reasonable” amount of time will result in the
deletion of your log. A reasonable amount of time could be a week
or two but the cache owner reserves the right to delete any log
regardless of what seems reasonable to the log poster. If there are
extenuating circumstances, it is your responsibility to contact me
with an explanation as to why you are unable to fulfill all of the
logging requirements at this time and when you will be able to
complete them.
I hope that you have enjoyed this brief lesson in geology and if
you have had time to use the Disc Golf Course and the facilities of
the park so much the better. As always, thanks for taking the time
to look for one of my caches.