Published 07th January 2023 | Experiences

Geology at Sagecliffe: Frenchman Coulee

With Old Rock Day on the 7th of January, we thought that it’d be a good time to take a dive into the geology of our local area. At Sagecliffe Resort, we’re lucky to be surrounded by some stunning natural landscapes and rock formations like nowhere else on earth. One such place is the Frenchman Coulee – a beautiful feature left behind by ice age floods.

How it came to be

After decades of careful research by field geologists, it’s clear that this part of Washington State was hit hard by dozens of floods between 17,000 and 15,000 years ago.

As North America’s ice sheet was slowly retreating to the north, enormous volumes of meltwater cooled along the ice front and periodically emptied as a raging torrent across Eastern Washington.

Glacial Lake Missoula in Montana was the source of many of these floods and attained a depth of 2,000 vertical feet and 500 cubic miles of water. Research has shown that at least forty major floods came from this lake in Montana. However, there is mounting evidence that the melting ice sheet that covered northern Washington was also another source of floodwater.

What we can see now

The different coloured stripes that sit in the rock tell us an awful lot about the history of it. For instance, there is an area that has basalt bedrock (the darker-coloured stripe), then lighter-coloured rocks on top that make a very subtle hill. These rocks wouldn’t have been here without the ice age floods that transported them hundreds of miles. You’ll also be able to spot our local bedrock, which is dark brown to black.

Each coulee in Eastern Washington has a similar story – none of them would exist today without the ice age floods, which also means that none of them existed before then too. Back then, flood water reaching up to 50mph hauled off millions of tons of basalt bedrock, which have now cut deep coulees that we can marvel at, climb up and enjoy today. On the other hand, the gentle rolling hills that also cover much of our local area were missed by the floods, meaning that they didn’t experience the same catastrophic amount of damage, and can thankfully be used for grazing nowadays.

Geology at the Frenchman Springs Coulee

Sitting just a stone’s throw from Interstate 90 between Vantage and Quincy, the Frenchman Coulee is one of the most important pieces of evidence geologists have of the ice age floods that occurred across Central Washington. The geology of the rocks that make up this coulee, as well as those nearby, have been able to give hints of the historical timeline of the area and tell us more about these periods of intense flooding.

For example, the fact that the coulee forms an almost vertical wall against a flat, horizontal floor is really significant. Usually, in normal geological circumstances, a V-shaped valley would form from a river cutting through the rock over time. However, the presence of a box-shaped valley tells us that there was once no valley here at all – just solid rock with a waterfall in the distance in the Columbia River. Though as repeated flooding came through the area, the lip of the waterfall started to backtrack or retreat. So, with each passing flood, the length of the coulee was enlarged. Over time, this led to the formation of what we know as the Frenchman Springs Coulee today.

Visiting the Frenchman Coulee

Now, the coulee is well known for hiking, mountain biking and rock climbing – as well as geology.

Hiking trails in the area:
Frenchman Coulee

Rock climbing locations:
Feathers Sunshine Wall

Map and Directions can be found here.

Since the coulee is less than 8 miles from Sagecliffe Resort, we’re in just the right spot for you to spend the night or two after a day out exploring the area. Reserve a room or stop by for dinner at our restaurant, Tendrils – we look forward to welcoming you!

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