More About Glacial Erratics

Two of my favorite Washington State erratics are not even in Washington – but they were. They’re now down in Oregon, south of Portland, and how they got there… Well, there’s a story I’d like to tell.

Perhaps the most exotic and unlikely erratic in the world got started on its journey a long time ago from a place in the dark emptiness between the stars. It is not only an erratic, it is an iron-nickel meteorite, and it is big. It is the largest such traveler in North America and is a member of the top-ten largest meteorites in the world. We can’t know where or when it fell to Earth as there is no tell-tale crater near it today, so we’ll pick up it’s journey 13,000 years ago when Lake Missoula was formed in Washington, Idaho, and Montana. That lake is going to need an introduction for this tale to make sense.

Thousands of years ago when climate change was far less shy and entirely apolitical, great ice sheets covered much of what would become Canada and several of the northern-most states of the US. This ice is thought to have been more than a mile thick in places, and was continually in motion. Through processes of expansion by way of precipitation and simply grinding across the surface, the ice sheets at times became dams for many age-old rivers. Those dams caused lakes to form behind them, and one such was Lake Missoula. It reached a depth of 2,000 feet and a capacity greater than most of the Great Lakes. And all that water was held back by a bit of ice.

The river that was blocked is the Clark Fork River that runs quietly through Montana and Idaho to Lake Pend Oreille in Idaho. That lake drains via the Pend Oreille River which wanders across Idaho, Canada, and Washington State where it meets the Columbia River. The river was dammed when the Cordilleran ice sheet cut off the river valley, and the result was Lake Missoula.

The battle of water and ice was finally won by the water. It reached a depth where it was able to lift and scour the glacial ice and the earth below it. A trickle quickly became a megatorrent and Lake Missoula was unleashed. The waters raced across Washington State at depths exceeding several hundred feet. And with it came huge ice bergs that broke free of the glacier and with them came massive boulders, some larger than houses. There are still ripples in the Scablands area of Washington that from a distance look exactly like ripples one would see on a beach or river bottom – but up close they are huge. Clearly the water raging across the area was very deep and very fast.

One of the great ice bergs that was swept away carried a house-size rock. The ice managed to survive the trip from the glacier, westward down the Columbia River Gorge to where Portland, OR is today. The water depth was about 300′ where the Williamette River is now. Here it turned south following flood waters that were filling the Wiliamette Valley as far as Eugene. It finally ended it’s voyage near McMinnville where the boulder came to rest in what is now Erratic Rock State Natural Park. Owing to vandalism it is today a fraction of it’s original size, but it is still very impressive. Particularly when you consider this was once at the bottom of the ocean in the southern hemisphere.

Photo by M.O. Stevens, Portland, Oregon ~ Erratic Rock State Natural Site
Photo by M.O. Stevens, Portland, Oregon ~ Erratic Rock State Natural Site

Geologically, the rock comes from Canada and is the largest glacial erratic rock in the Willamette Valley. The rock is argillite believed to be 600 million years old and originally part of the sea-floor. It is also the only rock of its type outside of Canada. And what is very interesting is 600 million years ago that sea-floor was south of the equator!

The next traveler is very likely a record holder for distance traveled, even for erratics. From Wikipedia we learn the following:

The Willamette Meteorite, officially named Willamette,[3] is an iron-nickel meteorite discovered in the U.S. state of Oregon. It is the largest meteorite found in North America and the sixth largest in the world.[4][5] There was no impact crater at the discovery site; researchers believe the meteorite landed in what is now Canada or Montana, and was transported as a glacial erratic to the Willamette Valley during the Missoula Floods at the end of the last Ice Age (~13,000 years ago) ~ Wikipedia

Photo by Dante Alighieri ~ Willamette Meteorite at the American Museum of Natural History
Photo by Dante Alighieri ~ Willamette Meteorite at the American Museum of Natural History