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Ian Madin Rocks Geology — Over the Hump and Heading Home.


Overview of the Day 7 route (purple line)

Most of the day will be spent in beautiful, gentle descents, with spectacular views of Mt. Hood and Mt. Adams jumping out at you, so most of the time you won’t be thinking as much about rocks. We’ll climb across Tygh Ridge, which is a giant fold in the basalt layers of the Columbia River Basalt. Between The Dalles and the northern side of Tygh Ridge, the lava layers slope gently down toward the Columbia River, then at Tygh Ridge they bend in a sharp wrinkle. On the Tygh Valley side of the ridge, the lava layers are tilted steeply toward the south. In the Google Earth image below, you can see where the upturned edges of a steeply tilted lava layer make a series of upside-down “V” shapes where they intersect ridges.


Inverted “V’s” on the southern slopes of Tygh Ridge are the edges of a steeply tilted lava bed

The climb through Butler Canyon to the top of Tygh Ridge is relentless, so for the first hour of the day you may need something to occupy your mind. Here are a few things to watch for as you slog your way up Highway 197:

As we enter Butler Canyon, we will see pronounced stone stripes on the right-hand side. The stripes extend from the top of the hill to nearly the bottom, and the dark stones stand in stark contrast to the dried grass that makes up most of the slope. Although geologists are still not sure how these features form, they are very common in areas of eastern Washington and Oregon where Columbia River Basalt makes up the local terrain. Like the Mima Mounds we saw on previous days, the stone stripes may be the result of the burrowing activity of pocket gophers.


These stone stripes that run down the steep grassy slopes may be the result of gophers

About a third of the way up the canyon, look for some beautiful basalt columns on the left-hand side of the road. Basalt often forms crude hexagonal columns, which form as the lava cools from nearly 2000 degrees F to room temperature. The liquid lava solidifies at temperatures a bit below 2000 F, and then the hot, solid lava shrinks as it cools to room temperature and cracks into these hexagonal columns to accommodate the shrinkage. In some places like the Giant’s Causeway in Britain, or the Devil’s Postpile in California, the columns are so regular and perfect that it’s hard to believe they’re natural.


These are nice examples of the basalt columns that form as thick lava cools

A bit beyond the columns, watch for a distinctive light-gray rock layer in the road cut on the left-hand side. This is a layer of sandstone caught in between two lava flows of the Columbia River Basalt. Although most of the Columbia River Basalt flows were erupted in a geologic blink of an eye, there are some places where enough time passed between eruptions that rivers and streams were able to establish courses across the barren lava surface. In this instance, the stream left a layer of white-to-gray sand, probably brought from the granitic mountains of Idaho to the east.


This gray sandstone layer between two basalt flows marks a river that flowed between eruptions

As we near to the top of the climb, watch for basalt pillows in the road cut on the left-hand side. Basalt pillows form when basalt lava flows enter deep water. The lava freezes solid when it hits the water, until the pressure of the liquid flow cracks the frozen surface and a glob of liquid lava squirts out like toothpaste from a tube, then instantly freezes in the water. The lava flow advances by building a pile of these long tubular lava pillows. In the road cut you will see rounded bodies of dark lava in a yellow-gray mass of clay-like material.

You will start your final long downhill with a spectacular view of Mt. Hood. As you wind your way back to The Dalles, in addition to Hood you will see Mt. Adams to the north, and possibly Mt. Rainier (near Seattle, with altitude over 14,000 feet) looming beyond it. You may even see a log-gray mountain between Hood and Adams, which is Mt. St. Helens.


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