The Cordilleran Mountain orogeny , which is the
northern portion of the Rocky Mountains, occurred during the
Cretaceous era (100million - 65million) years ago. It is a
geologically complex range, with remnants of an ancestral Rocky
Mountain system and evidence that uplift, which involved almost all
mountain-building processes occurred as a series of pulses over
millions of years. The mountains have since been eroded to expose
ancient crystalline cores flanked by thick upturned layers of
sedimentary rocks. Glaciers and snowfields, which covered portions
of the northern ranges were at one time more extensive; throughout
the system the erosional features of alpine glaciations are
apparent. The Rocky Mountain system has been an evolutionary and
complex event over millions of years. At one time the mountain
region was the lowest part of the North American continent. There
was a large shelf of sedimentary deposits that over time under
pressure and heat, the bottom layers “morphed” into
solid crystalline formations as part of the tectonic plates.
These large and heavy deposits were subjected to massive
deformations by an unstable earth. Mountain building occurs along
mobile belts... places where colliding tectonic (lithopheric)
plates disrupt the continental crust. These belts are known as
orogens. There are different types of mountain formations but they
all generally involve some form of colliding massive lithopheric
plates and the internal forces lifting the earth crust tilting and
folding the sedimentary and metaphoric structures. In some instance
the plates will slide over one another along fault lines while
others will rise in the form of a massive arc. The formation of the
mountain areas can change from location to location. In this area
it appears there has been an upward thrust that has exposed
metamorphic rock along with sedimentary. The canyon has been carved
out of a solid metamorphic formation and over time glaciation and
erosion has transformed the area to what it is presently. The
Natural Bridge is located on the Cross River that has the Mitchell
River as a tributary which has its source in the Mitchell Range of
British Columbia. (see map)
Cordilleran refers to the entire mountain formations of
Western Canada.
Orogeny: Greek word for study of mountain building. Periods
of mountain building are known as orogenies and in this area,two
have been responsible for the mountains we see today. Prior to
these, the North American Plate had been moving in a westerly
direction and the neighbouring Pacific Plate trending
northward.
Plate tectonics: According to this theory, the surface of
the Earth is made up of a series of plates, each of which move
relative to the others.
The Cross River Natural Bridge is one incredible landscape
that geocachers should visit. This is a two for the price of one
incredible geological site created by moving water. Firstly, there
is the very narrow mountain canyon and secondly, there is a natural
bridge.
The formation of mountain canyons is basically the natural
tendency for water to flow to a natural baseline. The lower the
baseline the steeper the gradient, the faster the water flows and
more energy will be used for erosion at a quicker pace. The water
will find weak areas in the landscape and begin to dissolve the
minerals and exploit fractures in the rock formations to develop a
path. Eventually the flowing debris in the water will carve a
narrow path or a wide path depending on many factors. Water
velocity, volume, turbidity, rock particles and temperatures all
affect erosion capabilities. The type of rock, formations, attitude
and glaciation also affect the channel development and route the
water will take. In this case on the Cross River there is a
metamorphic formation the water has broken through making a very
narrow canyon.
The natural bridge is splendid in its arching ways. These
bridges are usually formed in mountainous areas when water/freezing
erosion penetrates fractures in front of hanging waterfalls. Over
time this action sculpts out a larger hole or even a cave behind
the waterfall. These areas have a weaker structure and material
content which is more dissolvable. Eventually the water finds its
way under the hanging waterfall through cracks and dissolving the
substrata to gradually create a hole under the waterfall. In time
the hole will be enlarged sufficiently to allow all the water to
flow through it leaving rock above which becomes a “natural
bridge”.
Please email answers to the following questions.
a. Drop a stone from the unnatural bridge to determine drop time.
Now multiply your drop time by 32ft/sec to find the depth of the
canyon in feet.
b. Estimate distance across the natural bridge in meters.
c. How many plank widths are there on the unnatural bridge?
d. Take a picture of your GPS at the bridge. (Photos to be
submitted within three weeks or log will be deleted.)