On September 22, we had our first field trip of the class, focusing on the lower parts of the valley. Due to smoke from the Okanogan complex fires, we decided to focus our trip on the Twisp River valley. We began with older rocks and as we moved up the valley, we moved up in time as well.
|Newby rocks at the junction of Poorman Creek are discussed|
Our first stop was at the junction of Poorman Creek, at an outcropping of rock. The rocks observed here were of the Newby formation, the earliest rocks we could observe as the Hozameen formations are typically only visible further to the west. Newby rocks are a combination of igneous and sedimentary rocks, formed from island arcs and their surrounding oceanic areas. Because they were located near a fault zone, they were squeezed and often metamorphosed. The green color of these rocks is an indication of this metamorphic activity, and can hide the original volcanic texture. Quartz veins in these rocks were later added as an intrusion after pressure was released and liquid rock flowed into the cracks. These breccias were more pyroclastic than basalts, evidenced by the larger angular pieces surrounded by sediments. Other formations near this area include intrusions (and associated mines) such as the Alder Creek and Spokane grade areas.
Our next stop was at our instructor Eric Bard’s house. Here we examined sediments that were laid down around the volcanic islands. These sedimentary rocks are known as argillites, or more commonly as shale. Because of the enormous amount of activity following deposition in this dynamic geologic landscape, these shales were “cooked” and became argillite. This “cooking” process has destroyed many but not all of the fossils that you would expect to find in sedimentary rocks. Despite that fact that these rocks formed all around volcanic islands, some of the parent rocks that the sediments originated from are not volcanic in origin but may have travelled longer distances and were laid here by ocean current movement. The rocks here differ from the Newby formation with smaller crystals and finer sediments. This argillite can be a challenge for current valley residents because of its inability to hold water and makes a challenge when drilling wells when compared to drilling in glacial tills.
|Outcrop of high energy conglomerate along the Twisp River|
Our third stop, moving up valley ahead in time again, was at an outcrop on the side of the road. These rocks were a high energy conglomerate formed from alluvial fans extending beneath the ocean surface. Adding to these fans of sediment from the mainland were again oceanic deposits that may have travelled great distances. The larger fragments in these rocks are more rounded, indicating the energy of moving water that smoothed them before they were buried and eventually became rock. The shales around the edges of these rocks show the less energetic environment expected of deeper ocean environments that surrounded these fans.
|Low grade coal seams along the Twisp River are described|
When I originally thought of geology in the Pacific Northwest, I thought of volcanoes and great eruptions. Our next stop showed us a bit of evidence of that. We stopped at a cut bank along the river to look at Mazama ash, which was deposited from the great eruption of Mt Mazama which collapsed to form Crater Lake in Oregon. This was a relatively recent event, and the ash can be found throughout the Pacific Northwest. Look for the bands of white, extremely fine grained, slippery to the touch ash in cut banks as you travel about.
Next, we looked at some Midnight Peak volcanic formations. These newer rocks formed from the subduction of Cascade episode lavas, although we don't know exactly where all of the volcanic centers for these flows were. These rocks formed at the same time as the Oval Peak batholith, but instead of slow cooling underground as occurred in that batholith, the liquid rock flowed to the surface as lava and cooled faster. White crystals in the rock show slower underground cooling; after eruption the surrounding rock cooled faster.
(Photo: Midnight Peak formation volcanics. Note the difficulty of viewing the rock through weathering and lichens. To ID your rocks more effectively, try to crack it open with hammer (use safety glasses).)
|Twisp Valley schist at War Creek|
Our final stop of the trip was at War Creek. The creek had a mass wasting or washout event in 2011, exposing the Twisp valley schist beneath. While stream blowouts such as this can be a pain for road and trail users attempting to access the backcountry, they can be a blessing for geologists by removing the pesky overburden, in this case 13,000 years of soil covering the rock below. The rocks here have distinct linear foliations from metamorphic activity. As pressure was placed on the rock from a consistent direction, the particles in the rock aligned themselves perpendicular to that pressure and formed the foliations. To see this process yourself, try mixing mica flakes in play-doh and then squeezing a ball of it between your palms. Break the flattened ball in half and see how the flakes have aligned perpendicular to the pressure. Veins in this rock often contain quartz intrusions amid the sedimentary and oceanic parent rocks which have been metamorphed.
|An intrusion of quartz within the War Creek schist|
This field trip was great with many interesting questions from the class. The areas we travelled to and the rocks observed raised questions about the Earth, plate tectonics and the crust, the formation of planets and cosmology, the formation of minerals, and some of our unique minerals such as Oakanogenite. We were left with an excitement for our next field trip, which occurred further up the valley.