Ian Madin Rocks Geology — Lava Landscapes
Overview of the Day Six route
We start Day Six riding across a broad plateau with a few dips where little canyons cut into the plateau. At about Mile 11, we descend into the valley of Antelope Creek, where we start a long climb. As we drop down to the creek, we once again enter an area of the John Day Formation, which is made up of a wide variety of 30- to 40-million-year-old volcanic rocks, and we make another big geologic transition.
The landscape up to Antelope is quite varied topographically, with numerous knobs and bumps along the route and off to our right. To our left we will periodically see the edge of a higher plateau. These two different landscapes are the result of the very different nature of the rocks underlying them. In the John Day Formation, there are layers of ash and lake sediment with little blobs of lava from small volcanoes. This variety means that there is a wide range in the hardness of the rock, which is why the landscape is so varied, with the ash layers making low spots and the lava making knob and knolls. We’ll see some of the bright white ash in road cuts, and about halfway up the climb we’ll cut across a distinctive reddish-brown ridge of lava.
The high plateau to our left is made up of a stack of lava flows that are part of the Columbia River basalt. This hard, uniform lava makes for a very different landscape and typically forms broad, flat plateaus cut by steep-walled canyons. As we leave Antelope and start the steepest climb of the day, we’ll see the dark lava of the Columbia River basalt capping the ridges on either side, and as we make the final hairpin turns and reach the very top of the climb we’ll once again be riding through the basalt.
This view shows our route climbing from Antelope up to the basalt plateau. Notice the layer of dark basalt that is visible along the edge of the plateau.
When we cross the boundary between the ash of the John Day Formation and the Columbia River basalt, we are taking a step through time of some 20 million years. In many places you can put your finger on the exact spot on the ground where that huge gap in time occurs. The Columbia River basalt is a very useful tool for geologists in the Pacific Northwest. It is found in many parts of Oregon, Washington and Idaho, is very distinctive, and was erupted in a very short period of time, about 15.4 million years ago. This makes it an excellent milepost of time for geologists; if you’re in rocks on top of the basalt, they must be younger than 15.4 million years, and if they are below, they must be older.
As we descend across the basalt plateau toward Shaniko, we’ll be passing through an area that is covered with beautiful Mima Mounds. This area has not been plowed for agriculture like the areas around Dufur and Tygh Valley, so the mounds are well preserved. They make beautiful patterns on the ground, and will be quite obvious from the road.
From Shaniko we continue across the basalt plateau, and if conditions are right we should get good views of Mt. Hood, Mt. Adams, and quite possibly Mt. Rainier in the distance beyond Mt. Adams. We will eventually descend to the Deschutes River at Maupin. As we proceed down the river, we will be riding through a canyon cut into the Columbia River basalt, and where we cross the river at Sherars Bridge, a spectacular series of falls has developed as the river struggles to erode through a particularly hard layer of basalt.
This image shows beautifully developed Mima Mounds along the road between Antelope and Shaniko. One theory suggests that they are formed by thousands of years of burrowing by ground squirrels.