By Keith Douville, Geology Class Student and Scholarship Volunteer
Our final class of the
Geology of the Pacific Northwest class was on
October 1, 2012. The first thing we
discussed was a bit of review, to reiterate some of the concepts that we went
over earlier. Questions on the Kootenay,
the Omenica arcs, orogeny episodes, The Coast range episode, the breakup of the
Kula plate, the timing of the Cascade episode, and the differences in the
formations north and south of Snoqualmie pass were rewarded with
chocolate.
John Nickel from the class
had found a fossil at our second field trip, and George Wooten had written a
small interpretive piece on it. The
fossil is on display at the Methow Valley Interpretive center.
Some questions from the
class were discussed and we talked about challenges
of geology in general. Often geologists
are forced to interpret a landscape with very little of the original material
intact, say 10% or so. This material has
been faulted, broken, and accreted.
Follow this with multiple glaciations events and subsequent
flooding. Finally, our ability to
replicate these processes is limited at best.
These challenges make interpretations difficult, but it is the challenge
which can make it so rewarding as well.
Following the review and
questions, we discussed the Ice Ages and what they meant to the area. These events over the last 5 million years
have had a profound impact on the landscape, sculpting it into the landforms
which we see today. While we are still
in the Cascade Episode, the last 5 million years have been most instrumental in
changing the shape and function of our landscape and are worth additional
scrutiny.
We are still in this recent
Ice Age by most accounts. The ice ages
have been found to correlate with the Milankovitch cycles, which describe the
changes to the Earth’s climate from the regular changes in the tilt and orbit
of the Earth. Other theories to the
induction of the current ice age are the India
plate collision which gave rise to the Himalayas,
in turn changing the amounts of atmospheric dust, ocean circulation, and
atmospheric CO2 removal as rock weathering increased the amount of calcium carbonate in the oceans. The PBS
series NOVA produced “Cracking the Ice Age” which can help describe some of
these changes. The advent of the ice
ages allowed for an increase in two types of glaciations: Alpine and
Continental.
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| A classic glacier-carved U-shaped valley in Mazama. |
Alpine glaciation has had a
significant impact on the Methow and the Pacific Northwest.
Several landforms are commonly seen from
the movement of alpine glaciers in the area and were discussed in class. Cirques are bowl-shaped depressions formed at
the head of glaciers as the weight of increasing ice carves out the side of a
mountain. These bowls often contain a
mountain lake after the ice melts. A
Kame forms when debris, usually rounded outwash, forms laterally along a
retreating glacier and a terrace is left behind. Examples of Kame terraces are found near Sun Mountain,
Pug flat, and Rooster flat. Distinct
U-shaped valleys are formed as a large alpine glacier scours a valley. This is different than the V-shaped valleys
that form with stream erosion. The
Methow valley’s wide U-shape with steep walls and a wide flat bottom is a
classic example of this type of alpine glacier erosion. Lake Chelan
is an example of a fjord lake that has formed from another U-shaped valley or
glacial trough. A Nunatak is exposed rock that protrudes above the
surrounding ice, and many of the horns that are present in the North Cascades
are pinnacles of rock that stayed above the ice. Erratics are rocks that are carried by
glacial ice and then deposited as ice retreats, or moved by glacial
floodwaters, and can be seen throughout the valley floors. Erratics are often composed of different rock
than the surrounding area and are easily spotted.
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| The Stauning Alper Glacier of East Greenland is a classic alpine glacier with the main ice sheet separating peaks of granite from the heavily eroded valley. This is probably what the Methow looked like 15,000 years ago! |
As glaciers melt and
retreat the landscape is changed as well.
Outwash plains and lakes can deposit large amounts of silt, and this is
common in valley floors. These outwash
plains typically have sorted material, with distinct layering or
stratification. This differs from
glacial till, which is deposited in moraines usually at the ends or lateral
edges of glaciers and is poorly sorted, with angular chunks surrounded by finer
materials. Drumlins can be found in the
area as well, but usually in the lowlands and not in the mountains. These mysterious features are elongated
hills, oriented along the long axis with the direction of glacial travel and
are thought to form within glaciers as till is repositioned and deposited. Striations or scratches in bedrock can
indicate the direction of ice travel as well.
Continental glaciation
occurred in Washington
in addition to the alpine glaciers. Continental glaciers are huge, one of the major differences from alpine
glaciers, in the range of 50,000 sq. km or so.
The Frasier or Cordilleran ice sheet reached its maximum in the Methow
about 15,000 years ago. The Winthrow (yes, Winthrow, which is in the Waterville Plateau area of WA)
moraine marks the terminal of one of its lobes.
As the ice retreated, sometimes large chunks of it remained in debris
and as it melted kettle ponds were formed, such as Dead Horse Lake.
In Western Washington the Puget Lobe extended east of Olympia
and the Juan de Fuca lobe extended further north between the Olympics and Vancouver Island.
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|
Fine silts forming from
glacier erosion can be moved by self generated winds off the cold
glaciers. When wind deposited silt
collects in dunes it is called loess, a major feature of the Palouse landscape
in southeastern Washington. Loess is known for its high moisture
retention and nutrients, and makes for great farmland.
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| Pipestone Canyon here in the Methow Valley |
As huge amounts of water
are generated from melting ice, coulees can be formed, such as Alta or Elbow
coulee locally. Pipestone canyon is probably another coulee formed in the same
way, from meltwaters or flooding as opposed to long lasting stream cuts forming
deep flat bottomed canyons. Sometimes
these meltwaters would become impounded by ice and when these dams fail the
results can be catastrophic. J. Harlen
Bretz won the Penrose medal, a prestigious award in geology, for his lifetime
of research which led to the acceptance of the Missoula floods (aka the Ice Age Floods). Lake terraces formed high on the hills
around Missoula show huge, ancient water
bodies, and the channeled scabland landscape in Eastern
Washington shows how multiple flood events fought their way to the
ocean following collapse of the dams. Crescent Bar shows how water can drop huge deposits, and the Wallula Gap
shows how stronger rock formations can create choke points for the waters. Bruce Bjornstad with the Ice Age Floods Institute will speak about the Missoula Floods
on November 25th 2012 at the Methow Valley
Interpretive Center.
We concluded talking about
other geologic phenomenon that is very recent, geologically speaking, and some
open ended questions. The Little Ice
Age, from about 1500 to 1850, left a mark on the Methow with small moraines
left from glacial retreat. Every 300-600
years a subduction earthquake can be a possibility. One near Chelan in 1872 was estimated to be
about 6.8 and created landslides throughout the Cascades, with fissures opening
and water geysers erupting for days.
Rain is a common occurrence in the mountains, especially on the
Westside, and mass wasting events are common here with the ground slumping and
sliding down slope. We discussed the
long term challenges of maintaining roads such as Highway 20 in a mountainous
landscape. As our glaciers continue to
shrink, what will it mean for the Methow?
When will our next ice age arrive?
What will we call our next supercontinent?
This class covered
a fairly broad range of materials, and did so in few class periods. Our instructor Eric Bard’s ability to
synthesize and present material from introductory geology to specifics on the
Pacific Northwest was second to none, and we were lucky to have the opportunity
to have such a knowledgeable and personable expert right here in the
valley! Class participants in this
Geology of the Pacific Northwest class emerged
knowing a bit more about the landscape they call home. We learned to think like geologists, and we
are a little more excited about rocks than we were before. We have become time travelers, by our ability
to interpret rocks that are from the ancient past. The Methow Conservancy set up this great
course, and the Methow
Valley Interpretive
Center provided a
wonderful place for us to meet for these 4 weeks. Most of all I would like to acknowledge our
instructor Eric Bard for teaching us these great skills! A big thank you to all of these folks! Happy rock hounding out there in the Methow!
For further info on the
Methow area rocks, check out the Methow Block Field Guide at: http://www.nwgs.org/field_trip_guides/6.%20Methow%20Region.pdf
