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I have driven or walked past this glacial erratic dozens, perhaps
even hundreds of times, and somehow never took notice of it until
recently.
To log this earthcache, you are going to have to e-mail me the
answers to the following questions and post the necessary photos.
Information is provided below to assist you with your
answers.
1) What is the rock's approximate height, diameter, length,
radius, circumference, etc. (Choose one or more)
2) What type of rock is conglomerate? (igneous, sedimentary,
or metamorphic)
3) How would you determine that this glacial erratic is
conglomerate, and not breccia?
4) The clasts (fragments) found in Jackass Mountain
conglomerate are typically composed mainly of what type of rock?
(igneous, sedimentary, metamorphic).
5) Looking at the different clasts in this specific glacial
erratic, what would you say is the average size? Would this
conglomerate be considered a pebble or cobble conglomerate?
6) There are numerous fragments of granite embedded in the
conglomerate, post a photo of one of these fragments with your GPSr
next to it, or with your finger pointing at it.
7) Do you think this glacial erratic would be classified as
monomict, oligomict, or polymict conglomerate?
Bonus Question:
Try and calculate the approximate above ground mass of the erratic.
The density of conglomerate is approximately 2000 kg per cubic
metre. Using your size measurements (length x width x height) you
can calculate the mass if you know the density (density =
mass/volume, and therefore mass = density x volume).
To confirm that you visited the site, it would also be nice to see
a photo of yourself or your GPSr with the erratic in the
background. Any logs that do not meet the required logging criteria
will have to be deleted as per EarthCache rules.
Glacial Erratics
A glacial erratic is a piece of rock that differs from the size and
type of rock native to the area in which it rests. Erratics are
carried by glacial ice, often over distances of hundreds of
kilometres. Erratics can range in size from pebbles to very large
boulders.
Geologists have suggested that landslides or rockfalls initially
dropped the rocks on top of glacial ice. The glaciers continued to
move, carrying the rocks with it. When the ice melted, the erratics
were left in their present locations.
Erratics can range in size from pebbles to large boulders. The
largest known Glacial erratic is the "Big Rock" erratic(16,500
tons) near Okotoks, Alberta, Canada. Other examples of glacial
erratics include:
A fern-covered erratic the size of a garage is found on Cougar
Mountain near Seattle. White Rock, British Columbia derives its
name from a sea-side erratic the size of a garage found on the
beach at Semiahmoo Bay, right at the US border with Washington
State. The Foothills Erratics Train is a deposit of rocks of many
sizes. These deposits stretch in a narrow belt for about 600 km
from the Athabasca River Valley in Alberta to southwestern
Alberta.
During the most recent North American glaciation, the Wisconsin
glaciation (70,000 to 10,000 years ago), ice sheets extended to
about 45 degrees north latitude. This Wisconsinian glaciation left
widespread impacts on the North American landscape.
Fraser Valley
Ice Age sediments deposited during the Pleistocene Epoch (2 million
to 11,000 years ago) underlie gently rolling uplands (15 to 250 m
elevation) of the Fraser Valley. Most Ice Age sediments in the
Vancouver area date to the last glaciation, about 25,000 to 11,000
years ago, and in particular to the period of glacier retreat when
areas below 200 m elevation were covered by the sea.
The Abbotsford Erratic
According to a geology professor at the local university, this
specific erratic is composed of conglomerate that is thought to
originate from the Jackass Mountain Group, which is found in the
Hope, BC- Manning Park area.
Jackass Mountain conglomerate erratics can also be found in
Bellingham and Ferndale, Washington. The deposition of Jackass
Mountain erratics in these different locations is a record of the
direction that the glacier flowed.
Conglomerate
A conglomerate is a type of rock consisting of individual rock
fragments or grains that have become cemented together within
material that is of a smaller grain size. The individual rock
fragments or grains are the result of the breakdown of larger rocks
into smaller pieces of rock and are known as clasts.
Look carefully at this erratic to see the rounded cobbles of
igneous rocks that make up much of the conglomerate; these are
pretty sure indicators of the Jackass Mountain rock.
Conglomerates consist of rounded fragments (clasts) and are thus
differentiated from breccias, which consist of angular (square or
triangle shaped) fragments. Both conglomerates and breccias are
characterized by clasts larger than sand (larger than 2 mm).
Clast composition
Conglomerates are classified from each other due to their different
physical characteristics (lithologies) of the fragments that make
up the conglomerate. The lithology of a rock is a description of
its physical characteristics such as colour, texture, grain size,
or composition. Depending on the number of different types of
fragments found in a piece of conglomerate, the conglomerate will
have a different classification:
- Monomict conglomerate- clasts with only a single lithology
- Oligomict conglomerate - clasts of only a few different
lithologies
- Polymict conglomerate- clasts of many different lithologies
Clast size
Like the different types of fragments found in the piece of
conglomerate you see before you, the fragments (clasts) are also of
many different sizes. Conglomerates are also classified by the
dominant (most common) clast size that is present in that specific
piece of conglomerate. The different clast ranges are:
- Granule conglomerate - most fragments range from 2–4
mm
- Pebble conglomerate - most fragments range from 4–64
mm
- Cobble conglomerate - most fragments range from 64–256
mm
- Boulder conglomerate - most fragments are greater than 256
mm
"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
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