750 million years ago, Seattle as we know it would have been in deep, deep ocean. That’s because the coast of Washington was actually located near modern-day Spokane and Pullman.
Geologists know this because northeastern Washington has entire mountains made of quartzite, a metamorphic rock. Its beautiful, sparkling grains of sand from Washington’s original beaches were buried, squeezed and compressed into quartz rocks.
Over the past 200 million years, tectonic activity built most of the Pacific Northwest. The ancient North American plate moved westward inch by inch, merging with volcanic islands along the way.
From 6 million to 17 million years ago, lava oozed from long fissures in the Earth’s crust, flooding the Columbia Basin.
Repeated lava floods filled the basin like thin layers of a chocolate cake. The basalt layers were so heavy they caused the Earth’s crust to sink.
From 1 million to 10 million years ago, a powerful, wild Columbia — in some places a mile wide and 500 feet deep — transported snowmelt from the mountains to the Pacific. Lava flows pushed the river from its path, but it always found a way around, carving channels and dropping waterfalls over basalt.
As recently as 12,000 to 18,000 years ago (when mammoths were living here), Washington experienced a series of cataclysmic ice age floods. Huge glacial lakes — Lake Columbia in northeast Washington and Lake Missoula in west Montana — would periodically break through their ice dams and send a torrent across the basin. Within the span of a week, 80 mph floodwaters scoured the landscape, transforming everything in their path.
The water would back up behind narrow basalt channels, like at Wallula Gap. Water rose as much as 1,250 feet above sea level, creating temporary Lake Lewis. It filled the Pasco Basin and is estimated to have covered 4,500 square miles and reached a depth of 900 feet. The entire Tri-Cities area was under water in what would have resembled a frothy chocolate milkshake, complete with floating icebergs.
Fast-forward to the present… A roadside cut near Granger, Yakima County — 15 miles away from the current path of the Columbia River — reveals layers of river cobbles and fine silt. Geologist Nick Zentner recognizes the quartzite cobbles like a trail of breadcrumbs, jelly beans or … potatoes.
Quartzite cobbles are identified by their off-white or blond exterior, and sparkling white interior. Quartzite is rare in Washington, and can only come from one place — the northeast corner of the state. The trail of cobbles moved by the ancient Columbia, still there for us to see 10 million years later, reveals the path of the ancestral Columbia River.
In the same roadside cut, Zentner also sees evidence of the ice age floods. Layers of superfine silt and sand, powdery to touch, are interspaced with river cobble. Called rhythmites, these were deposited from the relatively still, icy cold water that once filled the basin.
Lighter bands in the rhythmites are windblown deposits called loess, indicating dry periods following each flood.
Loess is the Goldilocks of soil — containing important minerals and just the right particle size to retain moisture and support roots. Wheat, grapes, fruit orchards and other crops thrive in the superfine, silty loess, first deposited by ice age floods and then blown by wind into soft, rolling hills. In the Palouse region, the loess is up to 250 feet thick.
Across from the Hanford nuclear site, in the original territory of the Wanapum people, the White Bluffs tower 300 feet above the Columbia River. In one section, geologists can see a dozen stacked rhythmites, each indicating ice age flood deposits topped by lighter bands of wind-blown loess.
These rhythmites, only thousands of years old, fill a channel within the 3-million- to 8-million-year-old Ringold Formation. Deposited by slow-moving tributaries of the ancestral Columbia and Snake rivers, the Ringold layers are rich in fossils, including traces of prehistoric horses and camels.
Evidence of the ice age floods can be seen on the surface of the White Bluffs and all around the Columbia Basin. Ice-rafted erratics — exotic boulders made of material different from the surrounding terrain — were transported and left behind by melting icebergs.
Giant current ripples can be seen from the air on West Bar, located near the resort community of Crescent Bar. These are no ordinary ripples. Measuring 20 to 40 feet high and 300 feet apart, they were created by ice age floodwaters roughly 300 feet deep, moving 40 mph.
Basalt layers under the White Bluffs (part of the Columbia River Basalt Group) are more than 13,000 feet thick. When the lava flows cooled and cracked, columns formed, often in hexagonal shapes. Called columnar basalts, these unique geological features are visible today, particularly where they have been exposed by erosion along the banks of the Columbia River.
The fury and power of the ice age floods — and the undammed Columbia River — carved steep canyons and waterfalls and plucked columnar basalts from their moorings. Sometimes a few pinnacles were left standing, such as the Twin Sisters that overlook Wallula Gap.
Lava floods, ice age floods and the ancestral Columbia created the canyons and coulees we see in the Northwest today. Within the past 95 years, humans used these natural formations to harness the Columbia’s mighty power to produce electricity, irrigate farms and provide navigation through locks at the dams.
Sources: Geologists Nick Zentner and Walter Szeliga of Central Washington University, The Geologic History of the Pacific Northwest (Shaw, 1998), On the Trail of the Ice Age Floods (Bjornstad, 2006), Ice Age Floods Institute, Washington Wheat Foundation, USFWS, USGS, NPS, in-person surveys.
