Ian Madin Rocks Geology – Day Two: Geology, Faults, Flows and Floods
Ian Madin is the chief scientist at the Oregon Department of Geology, and he will be providing us with information about the areas we’ll be visiting each day.
As we leave Glenwood and head south across the valley, we’ll see a long straight escarpment forming a wall along the edge of the valley. We came down this ridge at the end of Day One, but from this perspective it’s easier to see that this is a major fault. Faults are breaks in the Earth’s crust along which movements occur. In this case, the valley floor has dropped along the fault, while the land on the other side of the fault has risen; we call this type a “normal” fault. There has been at least 1,000 feet of vertical movement on this fault, just to account for the height of the escarpment ahead of us. We don’t know the depth beneath the valley of rocks that matches the top of the escarpment, but we would need to add that depth to assess the total amount of movement on the fault. This is the first of several major faults we’ll encounter today. Off to our left, the low rise is a young volcano, which erupted about 3,400 years ago. We’ll ride up and over the edge of a lava flow from this volcano shortly before we start to climb the fault escarpment.
This Google Earth image shows our route in purple and the fault along the base of the escarpment in red. Mt. Hood is visible in the distance. The small bump in the route before it begins the big climb occurs when the road climbs over the edge of a 3,400-year-old lava flow.
Once we get to the top, the rolling ride to Lyle once again crosses the Columbia River Basalt. It’s not very interesting in this kind of landscape, but it is a very important geologic marker in the Pacific Northwest because it allows geologists to get oriented in geologic time. The lava flows covered much of Washington and Oregon in a very short time span, between 15 and16 million years ago. That means that whenever you come across these lavas, you know approximately where they are in the vast expanse of geologic time. Geologists have learned to identify most of the dozens of individual lava flows that made up the flood basalt, using a combination of subtle chemical differences and differences in the direction of the north magnetic pole that each flow preserves. When a lava flow cools, microscopic crystals of iron-rich or magnetic minerals orient themselves to the direction of the magnetic pole at the time when the rock solidified. The north magnetic pole wanders constantly with respect to true north, and so each successive flow records a slightly different direction. Sometimes the north and south poles rapidly switch places, in a geomagnetic reversal, and they will then stay stable for hundreds of thousands of years before flipping again.
Along the way we’ll catch views of Mt. Hood in the distance. As we descend the final hill into Lyle, we’ll get a great view of dramatic evidence of the great floods from Day One’s route. The road passes a large gravel pit on the right, and you can clearly see that the top of the slope is composed of gravel sitting on top of reddish-brown rock and soil. This is a gravel bar left by the many ice age floods, which means that the ridge ahead must have been deep underwater. Past the gravel pit we’ll come around the corner and be looking at Rowena Crest, the mid-point of the day’s optional ride. The cliffs there are again made of successive lava flows of the Columbia River Basalt.
As we descend that last mile into Lyle, we’ll pass this gravel pit, in which we can see loose gray river gravel deposited by the ice age floods 15,000 years ago on top of the reddish-brown Columbia River Basalt.
After we make the climb to the top of Rowena Crest we’ll be rewarded with a spectacular view both ways of the Columbia River Gorge, and, again, most of what you see is Columbia River Basalt. Rowena Crest is famous for its spring wildflowers and for its vernal pools – short-lived ponds that harbor a wider range of life in spring. After we descend Rowena Crest and ride about one mile, we’ll start to get great views across the river of the second big fault of the day. The normal fault we climbed this morning formed by moving one side up, but this fault is a strike-slip fault, which moves one side horizontally past the other. The result is not an escarpment but an area of strongly bent and tilted lava layers. These layers were originally flat, but right against the fault they have been turned nearly vertical. The climb up Rowena Crest was actually crossing the same fault after it crosses the Columbia.
Looking across the river as we return from Rowena Crest, we can see the lava flows of the Columbia River Basalt tilted and folded by movement along a major fault. The red line marks the path of the fault; blue lines show how individual lava layers, originally horizontal, have been tilted.
From The Dalles to Dufur we’ll be riding through rolling uplands and shallow canyons carved into the Dalles Formation, which is composed of layers of river sediment (sand and gravel) and layers of volcanic mud flows, all of which originated 8 to 10 million years ago from Cascade volcanoes to the west – these are now long eroded away and buried by younger volcanoes.