M spent only one day exploring the Grand Coulee, which wasn’t nearly enough. Most of this limited time was spent on two hikes, both of them in the area of Sun Lakes Dry Falls State Park. He chose these on the basis of information in a wonderful book by Bruce Bjornstad called On the Trail of the Ice Age Floods–The Northern Reaches. Bjornstad’s guide includes a great deal of geological information as well as detailed instructions about how to get to places where you can see real examples. M is very grateful to his neighbor M for telling him about the book. It is out of print, but used copies are available here and there. On this trip M carried a copy from the Corvallis Benton County Library.
Hike #1 – Caribou Trail
Accessible from State Park Road, this trail starts in the bottom of the coulee and climbs up to rim, offering a nice long distance view down the Grand Coulee and also providing a good angle to see what is called the Pothole Field on the coulee floor. Not far from the trailhead the route passes just a few feet from the edge of one on these holes.
From a little farther up the trail you can see that there are many such holes in the coulee bottom, some of them more circular than others. Bjornstad says that the deeper ones go down 30 feet or more.
It is thought that the potholes were created by the end of ice age floods, but what was the mechanism? If we can imagine the coulee full to the brim with a rushing river 400 feet deep, we might next try to imagine what things might be like at the bottom of that river as it falls down over a 400 foot cliff, as it does at the spot now known as Dry Falls. There would have been a tremendous amount of erosion going on and also a tremendous amount of turbulence. Geologists now believe that this environment created underwater whirlpools, whose erosive force drilled down into the basalt.
About halfway along its length the Caribou Trail makes a hard right turn and enters a side canyon–a very rare break in the normally vertical coulee walls. At the top, as Bjornstad explains, hikers have to leave the trail and stroll left across a field of rocks and grass to get to the coulee rim. There are a number of great views there, including this one looking back south down the coulee.
Another thing you can see in the photo is how erosion has revealed the layer cake nature of the basalt. Each layer was laid down by a separate eruption event, with each of these events occurring hundreds of thousands of years apart–maybe millions. The period of greatest volcanic activity is said to have been from 16 million years ago up until 8 million years ago, so there was plenty of time for long pauses between layers. Also visible in the photo are a few potholes over on the right.
Hike #2 — Umatilla Rock Trail
Accessible from another branch of State Park Road, this trail starts at the base of Umatilla Rock and continues alongside that formation north up the coulee toward Dry Falls. Umatilla Rock is a striking feature–a long narrow strand of basalt that somehow remained after the rock on either side had been eroded away. It is an example of what are called rock blades or goat islands.
The trail goes along the right side of formation and makes for an interesting journey.
Eventually the trail crosses to the other side of the coulee where you can get a closer look at the west facing walls. Here’s photo from there. Notice that there are various shapes and patterns in the layers of basalt that the erosion has exposed. We also see a gravel slope at the base of the upper wall. This gravel, which Zentner called talus, comes from natural weathering processes that have occurred in the millennia that have passed since the floods occurred. During flood times, the talus wouldn’t have been there; it would all have been bare rock, scrubbed clean.
Although basalt is hard rock, it is more easily eroded than most other types of rock. This is due to the fact that it is formed by cooling lava. Lava does not cool evenly. Instead the process of cooling causes cracks to form, lots of cracks. Geologists have distinguished three or four different types of cracking, each of which forms a different looking basalt. In the center of the above photo, you can see one of the more spectacular kinds of basalt cracks–long vertical ones. When this kind of basalt is sliced open by erosion, the result is what appears to be a multitude of rock columns. Nick Zentner says that basalt comes pre-cut, like it was specially designed to fall apart into smaller pieces if you put pressure on it. In the case of the Grand Coulee, the floods would have applied plenty of pressure. They would then have whisked the loose pieces away and deposited them thirty miles south somewhere in the Ephrata fan.
The Umatilla Rock Trail also leads to views of Dry Falls, which many think is the most impressive feature of the whole system. The rim of Dry Falls is 400 feet above the coulee floor and three and a half miles wide. Over this rim flowed the great floods. You might think that this would have created quite a waterfall. But if we try to imagine it, we have to remember that the floodwaters flowing toward the falls were themselves hundreds of feet deep. So if we could have seen it all from above, it might have looked more like some large scale splashing and major turbulence rather than an actual waterfall. (Would a good kayaker been able to run it? Hmm. Maybe a really big kayaker.) Click the link to see a very nice photo of what Dry Falls looks like now–in the winter.
For more information, do check out Nick Zentner, who teaches Geology at Central Washington University. Known for his PBS series Nick on the Rocks, he has also made a large number of YouTube videos. One that is especially relevant to the Grand Coulee is Dry Falls and Palouse Falls. The falls may be dry, but Nick is not! And finally, if you don’t know him already, you might want to learn about J Harlen Bretz, the person who first understood what must have happened so long ago to create the Central Washington landscapes we see today.
All I can say is “WOW!!!”