Geology 101: Northeastern Oregon


To paraphrase the great John McPhee in Assembling California, most of Oregon, like most Oregonians, originally came from somewhere else. Specifically, the Wallowa, Baker, Izee and Olds Ferry Terranes originated out in the Pacific Ocean and over the course of millions of years and the inexorable forces of plate tectonics they collided with the North American core – the craton – and became a part of the continent. The Wallowa Mountains, the Blue, the Strawberry and the Ochoco Mountains are all products of those collisions.

In many ways, those mountains are the least of it. The whole messy process was attended by much volcanic activity. Excellent examples of that volcanism are preserved and on display at John Day Fossil Beds National Monument. It’s worth a trip. Some 40 million years of volcanic action is exposed and even for a non-geologist, it’s pretty awesome.

WC wrote yesterday about the Thomas Condon Interpretative Center. A map there displays in overlays the six major volcanic formations that occurred from 44 million years ago to about 7 millions year ago.

Display at Condon Interpretative Center, Oregon

Display at Condon Interpretative Center, Oregon

On the left side is a legend, showing the shapes and boundary color of the six major events. In the legend, the youngest, the Rattlesnake Formation, is at the top, occurring “just” 7 million years ago. At the bottom is the Clarno Nut Bed Formation, which was about 44 milions years go. In the map, the approximate boundaries of the six events are shown. If we zoom in one those.

Detail of the map of the six formations, Condon Interpretative Center, Oregon

Detail of the map of the six formations, Condon Interpretative Center, Oregon

You can see that the layers are indeed stacked, with the Rattlesnake Formation on top and the Clarno on the bottom. Each of those ash layers is present at John Day Fossil Beds, still mostly stacked in the order in which they were laid down, although tilted in many places by subsequent geological activity.

In the field, it sometimes looks like this.

Cathedral Rock, Sheep Rock Unit, John Day Fossil Beds Nat'l Monument, Oregon

Cathedral Rock, Sheep Rock Unit, John Day Fossil Beds Nat’l Monument, Oregon

And sometimes it looks like this:

Painted Hills, Painted Hills Unit, John Day Fossil Beds Nat'l Monument

Painted Hills, Painted Hills Unit, John Day Fossil Beds Nat’l Monument

And sometimes the formations look like this:

Clarno Palisades, Clarno Unit, John Day Fossil Beds Nat'l Monument

Clarno Palisades, Clarno Unit, John Day Fossil Beds Nat’l Monument

Those are school kids up on top are on an Oregon Museum of Science & Industry tour. They almost certainly aren’t supposed to be up there, but they do give a sense of scale.

One last photo:

Blue Basin, Sheep Rock Unit, John Day Fossil Beds Nat'l Monument

Blue Basin, Sheep Rock Unit, John Day Fossil Beds Nat’l Monument

In WC’s Geomorphology class, on the first day, the professor (whose name WC has forgotten) told us, “If you only take one thing away for this course, it should be this: Erosion always wins.” And it’s true. You can see it in these photos. But while erosion may win, the rocks make some pretty impressive scenery – and amazing fossils – along the way.

 

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