Ellen Morris Bishop: In Search of Ancient Oregon

Guest Review by Kit Bradley
October 6, 2008

The last book I reviewed was Oregon’s Greatest Natural Disasters, which, if you think about it for a moment, is a geology book. So it is not surprising that I would be attracted to Ellen Morris Bishop’s In Search of Ancient Oregon: A Geological and Natural History, which is a much more detailed geology book, but still targeted at the interested layman.

I came upon this book in an unusual way. I was swimming in a pool with my mother-in-law in an adult trailer park, when I looked out the window and saw the local library bookmobile drive up. I dried off, got dressed, and went to see what they had. Almost immediately I picked up In Search of Ancient Oregon, and the pictures fascinated me. Every geological formation or event was illustrated with a picture of a place in Oregon. (The author took all of the pictures.) It is fascinating that many of the pictures are of places I have seen in my travels, and the rest I could easily see if I made the effort. This ability to see with my own eyes the evidence of the many geologic events that formed today’s Oregon is captivating.

So I went home and bought the book from Amazon. And a couple weeks later I met Ellen Bishop at a lecture and got a belated autograph.

In Search of Ancient Oregon has a chapter for each of the major geologic eras and epochs from about 400 million years ago through to today. A lot happens in ten million years, and a lot disappears in ten million years. It is very interesting to read how geologists follow the most obscure and unlikely of clues to reconstruct the history behind the terrains and rocks we can all see. And it’s all far more complicated than anything I can summarize here.

The oldest rocks in Oregon are limestone created during the Devonian epoch, roughly 400 million years ago, but the rocks weren’t in Oregon at the time. It’s a story of plate tectonics. Subduction zones in the middle of what is now the Pacific Ocean created arcs of volcanic islands (like the Hawaiian islands). In the course of a lot of time the Pacific plate containing these islands moved eastward and eventually ran into the North American plate. The ensuing subduction scraped off random sections that ended up as “exotic terranes,” large masses of rock that have no relationship to the land surrounding them. These 400 million year old rocks are found today in the Blue Mountains in northeastern Oregon.

There is a lot to be learned from fossils, and each chapter of Ancient Oregon includes a section on the flora and fauna of the period. The climate impacts the flora and fauna, and thus the fossils tell a lot about the climate. But something happened 248 million years ago that killed 96% of all living species—the Permian extinction that ended the Paleozoic era.

We now enter the Mesozoic era, the time of the dinosaurs. The island volcanoes were still active and were starting to drift into the Oregon coast, and by 100 million years ago Oregon was firmly joined with the Blue Mountain island arc. The formations we see today are varied and jumbled, and many of the early volcanic rocks are not on mountaintops, but are buried and only visible where exposed, as in Hells Canyon.

This was the era of dinosaurs, but apparently none lived in Oregon. One duck-billed dinosaur was found in southern Oregon, but it turns out it was a migrant from California (the first of many Californians to come)—when the bedrock containing its remains faulted and drifted into Oregon. The Mesozoic era ended abruptly 65 million years ago when a meteor (probably) caused the extinction of 70% of all species, including the dinosaurs.

Now comes the Cenozoic era, where the mammals we know today appeared. Oregon’s first “native” volcanoes erupted and grew around 50 million years ago. Around 30 million years ago the first grasses appeared. (Who would guess grass wasn’t part of the originally evolved flora?) And this enabled early horses to evolve from brush eaters to grazers, and to live in herds on open prairies.

15 million years ago, eastern Oregon sat over a hot spot in the earth’s mantle (which today is under Yellowstone National Park), and this caused extensive basaltic lava flows that covered a vast area of Oregon, reaching all the way to the Pacific Ocean. And over the next 10 million years much more volcanic activity and faulting and moving of plates occurred, forming the Oregon of today.

Finally 1.8 million years ago we entered the great Ice Age, and this is also when the high Cascade volcanoes we can see today were formed. This was a time of battle in Oregon—between the volcano building forces and the glaciers’ destructive forces. Only a few volcanoes (like Mt Hood and the South Sister) won—by erupting again after the last glaciers receded.

It has been hard to summarize this 288-page book, since there are so many interesting and complex events that formed today’s Oregon. I apologize to any geologists reading this for all my inaccuracies and over simplifications. To get it straight, read the book! It contains a minimum of technical jargon, and every geological term is defined in the glossary—which I referred to very frequently!