Let’s Talk About Loess

If you’ve been living in the Northwest long enough, you have probably heard the Ice Age Floods story; 2000-foot-deep Glacial Lake Missoula drained catastrophically into Idaho, Washington, and Oregon, permanently scarring the landscape and wreaking havoc for eons. More than 15,000 years have passed since these monstrous events, but the evidence is still all around us, as shown in Washington’s bountiful wheat crop and the incredible soil that makes it possible.

The proof is in the dust! You’ve seen it, right? This superfine, silty, windblown sediment is called loess. (Don’t try to pronounce it because even geologists can’t agree.) Loess is the Goldilocks of soils, with perfect-sized particles for growing crops. It is hundreds of layers thick in the Palouse Hills, helping the southeast corner of the state produce more wheat than the rest combined. Washington’s plentiful agriculture is the result of millions of years of geology, a little bit of luck, and just the right soil for the job.

How did this loess sediment end up blanketing Eastern Washington? For the answer, let’s go back 100,000 years to the Pleistocene epoch when enormous, continent-sized sheets of ice covered most of the northern United States. The crushing weight of the Cordilleran Ice Sheet ground the bedrock into silty glacial flour and left it behind. This happened over and over for millennia until one day… catastrophe struck.

At the end of the last Ice Age, several massive glacial lakes existed in America. Glacial Lake Missoula sat in the Bitterroot Valley in Montana as the Clark Fork River backed up against a dam of ice, formed by the Purcell Trench lobe of the ice sheet. As the lake grew, eventually the ice dam broke open, sending thousands of tons of water, trees, rock, and debris flowing across the landscape at highway speeds of up to 70 cubic miles per hour. The first flood was the biggest, carving the Grand Coulee and the Columbia River Gorge. Geologists use core samples taken from the Astoria Fan in the Pacific Ocean to estimate that the ice dam failed, refroze, and failed repeatedly more than 80 times. All that water and glacial silt had to go somewhere.

The cataclysmic aftermath of the floods scoured away the topsoil down to the basalt bedrock, carving the Channeled Scablands—literal scars on the Earth. Water pooled into deep channels and gullies, with the silty glacial sludge settling on the bottom. Over the next few thousand years, the planet warmed, evaporating the lakes and leaving behind the dusty, fertile loess—layer upon layer of amazing soil for growing wheat.

But wait… according to the map, the Palouse didn’t even experience the floods! That’s correct. It’s the reason why southeast Washington is such an agricultural powerhouse today with all its wineries and wheat fields.

Simply because the floods didn’t scour the precious topsoil away, the soil in the Palouse is 250 feet thick in some places. Instead, the powerful winds from the Cascades whipped the dry, fertile loess deposits into dunes, creating the rolling hills we see today. Silty soil like loess is highly desired for agriculture, as it contains important minerals and prevents water erosion. It’s the Goldilocks soil, with just the right-sized particles for retaining moisture and stabilizing roots.

For most people who aren’t farmers, dirt is just, well, dirt. We don’t put much thought into why crops grow well in certain places, and we definitely don’t focus on the soil as the reason. In Eastern Washington, however, we should think about our soil and how it got here. We should explore our geology and history to help us learn how to grow better wheat. But just think… how lucky we are to have this geologic masterpiece right in our backyard, and farmers growing some of the highest quality wheat in the world!