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.
|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.
|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.
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.
|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