Geology Rocks! Days 4 -7
Day 4 – Halfway to Wallowa Lake
Halfway Oregon sits in a lush green valley backed by the jagged peaks of the Wallowa Mountains. The broad flat valley is bounded on the south by the Pine Valley fault, along which the valley floor has subsided over the last million years. This subsidence happens 3-6 feet at a time, during large earthquakes along the fault. Small earthquakes are still occurring on the fault today. Although the hills around the valley are made up of Columbia River Basalt, the high Wallowas are composed of metamorphic rock and granitic rocks of the Wallowa Batholith. The Batholith formed when huge blobs of magma were injected into the rocks of the exotic terranes about 130 million years ago. Because the magma was injected many miles below the surface, the molten rock cooled slowly, allowing the characteristic coarse salt and pepper crystal texture of granite to form.
For most of the rest of the day, we will be riding through stacks of Columbia River Basalt flows. You should not miss the side trip to McGraw Overlook, which will give you a spectacular view down into Hells Canyon, overlooking the stretch of road you took on the way to Hells Canyon Dam. At this point the Canyon is 3610 ft deep, dropping to the river at an average 30% grade.
On the Oregon side, the upper part of the Canyon is composed of layers of Columbia River Basalt, again forming the striking cliff and bench topography. You will also see that the trees prefer to grow on the benches. This is because the benches are formed on the broken lava rubble layer that separates successive flows, and it is easier for trees to root there. Part way down the slope you will see a sharp break between the basalt layers and smoother topography below. The smoother surfaces are formed on the metamorphic rocks of the exotic terrane, and the line of separation with the basalt, called an unconformable contact, represents a time gap of at least 200 million years, between the formation of the exotic terranes and the eruption of the basalt. Across the canyon, the Seven Devils range is formed of more of the exotic terrane rocks.
From the outlook, we drop into the Imnaha River canyon, where you will see some beautiful mature yellow pine forest. These trees are fire-adapted, and often form park-like groves of widely-spaced old-growth trees with a grassy understory. Periodic lightning-sparked fires clear out the undergrowth, while the big trees are protected by their fire-resistant bark.
We climb out of the Imnaha canyon and cross a few more small valleys before we arrive at the broad flat upper Wallowa Valley, and the last delightful miles as we ride into the heart of the Wallowa Mountains along the shore of Wallowa Lake.
Day 5 – Wallowa Lake to Enterprise
The optional layover day route starts from our camp at the Wallowa Lake state park, one of the gems of the Oregon park system. Don’t miss the Gondola ride to the top of Mt Howard, where you will get great views across the Wallowa valley and Hells Canyon to the east, and into the Eagle Cap wilderness to the west. The rocks straight across the canyon are grey, white and reddish metamorphic rocks, originally shale, sandstone and limestone on an ancient seafloor. You can see several narrow bands of brown rock that cut across the metamorphic layers. These are bits of Columbia River Basalt that formed as the magma forced its way through fissures in the surrounding rock on its way to the surface to feed the giant eruptions. Some of the magma remained in the fissures and cooled to form a lava body called a dike.
The top of the tram also provides a great view of Wallowa Lake, a classic example of a glacial moraine lake. The long high ridges that hem in the lake are lateral moraines, huge piles of rock rubble that accumulated around the edges of a glacier that completely filled the area of the lake just 17,000 years ago.
If you take the day ride to Enterprise, you will be rewarded with great views of the steep, straight edge of the Wallowas. The front of the mountains is so abrupt because it is formed by another fault line, along which the mountains have been uplifted in a series of 3 to 6 foot jerks accompanying large earthquakes. The lava flows of the Columbia River Basalt on which Enterprise sits match those on the top of Mt Howard, having been separated by almost 4000 feet vertically.
Day 6 – Wallowa Lake to La Grande
We start the day with one more spectacular ride along the shores of Wallowa Lake, then head down the Wallowa valley. As we descend through the towns of Lostine and Wallowa, we pass several more big canyons coming out of the Wallowa Mountains, and will continue to see the Wallowa fault that forms the sharp front of the range. The Wallowa valley narrows to a canyon, as the highway wraps around the north end of the Mountains, and shortly after we pass the confluence of the Minam and Wallowa Rivers, we start a long steady climb out of the canyon. This climb takes us up through a series of layers of Columbia River Basalt, each layer formed during a massive eruption that blanketed much of eastern Oregon and Washington in molten lava. You will notice that there are layers of rubbly rock, often colored pink, red or orange, that separate layers of more solid brown or grey rock. These colored rubbly zones mark the tops of successive lava flows. As the lava flows across the landscape, the upper surface cools and solidifies, forming a rocky crust that is then broken into rubble by the continued movement of the underlying lava.
As we descend into the town of Elgin you can see a tree-covered conical hill to your right. This is Jones Butte, one of the youngest volcanoes in eastern Oregon. Most of the volcanic eruptions in this area ended 13 million years ago, but Jones Butte is only 2 million years old.
We leave Elgin and ride west to the foot of Mt Emily, which rises over 3000 feet above the Grand Ronde valley floor. The long straight front of Mt Emily is another large fault; the West Grande Ronde Valley fault. Geologists have been able to match the Columbia River Basalt layers exposed in the steep face of the mountain with identical layers from a 3,000 foot deep water well drilled in the middle of the valley floor, proof that the fault has uplifted the mountain by thousands of feet. At mile 75, just past Woodell Lane there is a small hill on the valley floor to the left. This hill is made of the same lava flow that caps Mt Emily, 3200 feet above you to the right. The Grande Ronde Valley is a great example of a geologic feature called a graben. A graben (German for grave) is a basin that is formed when a block of the earth’s crust sinks down between two large faults. The West Grande Ronde Valley Fault forms one side of the Grande Ronde graben, while the East Grande Ronde Valley Fault on the opposite side forms the other. The Grande Ronde river has not eroded out this valley, and instead is slowly filling the valley with sediment as the graben subsides. As we enter La Grande, you will notice that the Grande Ronde River occupies a steep, narrow canyon west of the City. The mountains to the west are being actively uplifted along the West Grande Ronde Valley Fault, and the river is rapidly cutting its canyon deeper in an effort to keep up.
Day 7 – La Grande to Baker City
We start across the flat floor of the Grande Ronde Graben and pass Hot Lake on our way to the town of Union. Hot Lake is a high-volume hot spring, where water at 180 to 200 F (82-93 C) gushes up along a fault line that runs along the base of the mountain. Hot springs either form around young volcanos, where there is hot magma near enough to the surface to heat groundwater, or in areas where large faults allow cold ground water to circulate deep in the earth where it is heated and then rises back to the surface. Hot Lake was developed as a grand resort and sanatorium in the 1920’s and 1930, but fell into disrepair over the years. The resort has been restored to much of its original glory over the last decade by the Manuel family. There is another hot springs at the town of Cove, a few miles east of Union, and we will pass a few miles from Radium Hot Springs in the town of Haines as we ride through the Baker Valley.
We climb a low pass out of the Grande Ronde Valley at Union, and enter the Baker Valley at North Powder. As we ride up the North Powder river, you may notice a large quarry on the low hill to your left. This quarry provides much of the ballast rock (gravel that supports the railroad ties) for the Union Pacific Railroad in the region, because of the high strength and uniformity of the crushed rock it produces. We turn south across the North Powder and after a few miles riding straight south, the road takes a short jog to the left. As it does so, it crosses yet another fault, the Mansfield fault, which has uplifted the eastern side of the valley a few feet higher than the western side. This fault has moved since the glaciers melted off of the Elkhorn Ridge (so named because it is shaped like an elk horn when viewed from above), probably 5-10,000 years ago. Don’t be surprised if you don’t see it, as it was only recently discovered when high resolution laser topographic maps of the area were produced.
From here you can enjoy the views of Elkhorn Ridge as you finish the ride, and may even see the Wallowa Mountains to the east.