Geology Rocks! Days 1-3
Day 1 – Exotic Terranes
We start Day 1 in a broad green valley hemmed in by Elkhorn Ridge, which rises to over 9000 feet to the west. Elkhorn Ridge, and most of the rocks we will ride through today are part of an exotic terrane, a slice of an ancient landscape that originated far from its current location. These scraps of the earth’s crust may have originated as a volcanic island chain like Japan, located as far away as the tropics. Over millions of years of plate tectonic movement, these rocks traveled to their current location, where they collided with the western edge of the North American continent and were scraped off and welded into place. Several different slices arrived about 150 million years ago and now underlie most of Eastern Oregon.
We will begin to see the exotic terranes up close when we get on to Interstate 84 at the Ash Grove cement plant. This facility mines high purity limestone from a quarry above the plant, then roasts the crushed rock in the long rotating cylinder you can see from the road. This drives off carbon dioxide, leaving behind calcium oxide, or lime, which is the essential ingredient in cement.
As we descend Burnt River Canyon on the freeway, we will see sedimentary and volcanic rocks of the exotic terrane, sheared by the violence of their arrival and metamorphosed by deep burial into a range of bright colors and interesting textures. We will see them again in Hells Canyon.
Just before we turn off to Huntington, we pass a derelict cement plant at Lime, and you can see the surrounding landscape stained white by years of limestone dust. As we leave Huntington, we will pass by a large landslide on the left, and you will see cracks in the hillside, evidence of recent movement. The day ends in Farewell Bend State Park – a popular stop for pioneers heading further west.
Day 2 – Columbia River Basalt
On Day 2 we leave Farewell Bend and head north along the Snake River towards Weiser. The road leaves the Snake River for a short climb across low hills carved in soft claystone and sandstone that were deposited in a huge lake several million years ago. Lake Idaho filled a 200 mile-long swath of Southwest Idaho, leaving distinctive white and tan lake beds. As we descend back to the Snake River plain, we cross a big landslide where the river has undermined the slope, causing the soft rocks to slump.
The peaceful valley of the Snake that we are riding through was the site of an incredible catastrophe 14,500 years ago. At that time, the last ice age was starting to fade, and the basin now occupied by the Great Salt Lake was an inland sea called Lake Bonneville. Melting ice raised the level of this enormous lake until it spilled over a low divide into the drainage of the Snake River. The soft earth at the divide eroded quickly, and a huge flood ensued, as 1200 cubic miles of water swept down the Snake, an amount equal to the volume of Lake Michigan. The great Bonneville flood lasted weeks, and the water at Weiser was hundreds of feet deep and moving 70 mph.
Leaving Weiser, we begin a long steady climb towards Cambridge, and start riding through low hills made of dark grey, black or reddish brown basalt lava. These lava flows are part of the Columbia River Basalt, the result of a cataclysmic series of eruptions that occurred about 16 million years ago. Highly fluid basalt lava erupted from miles-long fissures along the Oregon-Idaho-Washington border at a rate sufficient to cover most of eastern Oregon and Washington with a lava sea 50 to 100 feet deep. This happened dozens of times, building up lava layers that are 10,000 feet thick in places. Some of the larger flows traveled hundreds of miles following the ancestral Columbia River until they entered the sea, where they burrowed into soft seafloor sediment and traveled nearly 100 miles underground before erupting again near Newport, on Oregon the coast. You may see some of the distinctive six-sided columns that often formed as the lava cooled. Shortly after we leave Mann Creek Road and start climbing on Highway 95, you will see colorful layers of basalt and sedimentary rock in the roadcut on your right.
Day 3 – Hells Canyon
Day 3 starts with an incredible descent into Hells Canyon through layer after layer of Columbia River Basalt. Each layer represents a single enormous lava flow, and you can see the layering in the terrain as a series of benches and cliffs. We reach the bottom of Hells Canyon along the shores of Brownlee Reservoir, and proceed downstream to Brownlee Dam which is 420 feet high and generates almost 600 MW of hydroelectric power. After riding on past Oxbow Reservoir and Dam, you reach the community of Oxbow, where you can decide to go straight to Halfway, or take the trip to Hells Canyon Dam and back. The ride to Hells Canyon dam is truly worth it, as the canyon just gets steeper and deeper the further you go (Note: Riders must begin the option by noon). Hells Canyon is said to be the deepest Gorge in North America, with an elevation difference of 6800 feet between the summit of the Seven Devils mountains in Idaho and the river below. The gorge is deep and steep because it is only a few million years old, and the Snake River is still aggressively cutting down through the rock. As you ride along the shores you may hear the faint echoes of rapids that once rivaled the Colorado River in the Grand Canyon, now silent beneath the still waters of Hells Canyon Reservoir. The rocks here are ancient seafloor and volcanic islands, part of the exotic terranes, and as you approach Hells Canyon Dam, you will see a band of grey limestone on the opposite canyon wall, folded into a great arch when the terrane docked with North America.
Below the dam, the Snake runs free for over 100 miles to Lewiston, Washington and Clarkston Idaho. If you are up for the additional 330 foot climb, ride down to the boat ramp at the end of the road and walk back up the narrow creek that enters there.
Above the dam, the Columbia River Basalt that we saw at Oxbow caps the very top of the canyon wall, which means that when the basalt erupted, it encountered a canyon here nearly as deep as the one we see today, and the lava flows eventually buried that ancient gorge without a trace.
After returning to Oxbow (hopefully with a tailwind!), we will climb up Pine Creek through more Columbia River Basalt into the beautiful Pine Valley and the town of Halfway. Rising behind the town you will see the Wallowa Mountains, our goal for the next day.