Ice Age Floods Geological Trail-Wenatchee, Washington
By Wenatchee Valley Convention & Visitors Bureau
06/19/2008
Driving through central Washington, one can’t help but notice an extraordinary geological panorama unfolding along the roadways. What most people fail to realize is that the entire terrain—hillsides, cliffs, valleys, and canyons—created over millions of years, was dramatically reshaped by an unusual series of events, which took place during the end of the last Ice-Age.
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- This itinerary can be customized for your groups timing needs 1-3 days
Approximately 17,000 years ago near the end of the Pleistocene Epoch enormous glaciers covered nearly one third of North America. The western portion of the glacial ice was known as the Cordilleran Ice Sheet and the eastern portion as the Laurentide Ice Sheet. East of the Cascade Mountains, the Okanogan lobe of the Cordilleran Ice extended south on a line between what is now Chelan and Coulee City, WA.
Farther east a lobe of advancing ice blocked Montana’s Clark Fork River at the point where the river passed through the Bitterroot Mountains. This buildup of ice blocked the drainage of a large portion of what is now northwestern Montana. The subsequent backup of water from the melting glaciers formed an enormous lake known today as Glacial Lake Missoula. Although we don’t know the eventual height reached by the ice dam, high water marks on the mountainside indicate a depth of approximately 2000 feet.
Exactly what happened next is still open to speculation; however, evidence shows the ice dam was eventually breached. When the dam collapsed it permitted catastrophic flooding of the landscape to the west. With incredible force, approximately 500 cubic miles of water and glacial ice burst through the channel gap, sweeping all before it, as it roared westward.
This massive flow of water scoured the soil from the landscape, breaking loose enormous chunks of rock, which it rolled and tumbled along for miles. Iceburgs with incorporated large boulders or “erratics” floated on the muddy turbulent flood-waters until finding a quiet resting place sometimes hundreds of miles from their ice dam origin.
It is believed the force of the rushing water was so great a mere four days were required to drain the entire lake. Today, we can see evidence of how the floods carved out more than 50 cubic miles of earth, deposited mountains of gravel, and scattered 200 ton boulders across the landscape. This astounding process is believed to have happened not once, not twice, but again and again, possibly as many as 100 or more times.
The Wenatchee Valley and the surrounding area contain one of the largest collections of these cataclysmic flood remnants in the U.S. A self-guided driving map is available that includes the 30-mile and the 164-mile trails. Step-on guide services are available upon request for coach groups.
The following points of interest encompass the approximately 30-mile trail which includes 18 individual stops.
Points of Interest – Local Ice Age Floods Features
(Wenatchee/East Wenatchee Area)
1) Large Bed-Load Erratics transported by floodwaters from up Columbia River Valley
Location - Loop Trail –East Wenatchee (9th Street-Irrigation/Foot Bridge)
2) Pangborn Gravel Bar – Enormous Gravel Bar left by floodwaters (in excess of 900’ deep) as water made the corner.
Location – Much of East Wenatchee – Fancher Heights
3) Giant Current Dunes “Ripples” – Large ripples on top of Pangborn Bar.
Location - Grant Road (west of Pangborn Airport) – 2nd Street EW – Highlander Golf Course.
4) Richey Clovis Cache – Archaeological Site that contained numerous “Clovis” people
spearheads and other artifacts.
Location – North of Pangborn Airport off Grant Road
5) Ice Rafted Erratics located several hundred feet above Pangborn Bar-north on hillside
Location – Grant Road – East Wenatchee
6) Overlook of Jump Off Joe Ridge, Malaga Slide and Moses Coulee Bar
Columbia River Basalt Group (basalt flows) visible to the east are part of the Grand Ronde Member (15.5-17.5 million years old). Rhythmites – temporary Lake impounded by blockage of Columbia River drainage by Moses Coulee Bar south of Rock Island. Rhythmites are layers that formed as sediments settled out of the impounded lake.
Location – Intersection of Batterman Road & Saunders Ave. NE of Rock Island.
7) Malaga Slide and Giant Current Dunes “Ripples”
Location – Large pull-off along Highway 28 (view south across river).
8) Large Erratics in yard.
Location - Intersection of Rock Island Road and SR-28 (on right-North)
9) Foreset Bedding in gravel deposit (sloping down current to east) is visible in gravel
pit on Nile Road.
10) Deltas located at the heads of Squilchuck and Stemilt Canyons. Ice-Age
Floodwaters periodically ponded by Moses Coulee Flood Bar that blocked the Columbia River Drainage.
Location – Douglas County PUD Park – East Wenatchee.
11) Panorama of Geology on west side of Columbia River.
Saddle and Castle Rocks, etc. Intrusive igneous rock formations (resistant to weathering) that is visible to the west along the Wenatchee skyline.
Chumstick Formation (sandstone) visible to the south below Wenatchee Heights.
Location – Park at end of Grant Road (Fred Meyer Parking Lot).
12) Enormous Basalt “Ice Rafted?” Erratic
Location – East end of 27th Street – East Wenatchee
13) Blue Grade – Area where historic clay mining occurred in the Wenatchee Formation
(34 million years old) to make bricks for buildings in the Wenatchee Valley.
Location – Northeast of SR 2/97 and SR 28 intersection
14) Ohme Gardens – Area stripped of topsoil down to bedrock by floodwaters. Now a
garden administered by WA State Parks. Bed rock here is Swakane Biotite Gneiss (pronounced nice) is the oldest rock formation in the Wenatchee area (as old as 1.6 billion year old).
Location – North of Wenatchee – Ohme Garden Road
15) Longitudinal Gravel Bar – Large Gravel Bar formed in northwestern Wenatchee
Location – Chatham Hill Area (North of Wenatchee River)
16) Overview of Pangborn Bar – Saddle Rock and Castle Rock
Most of East Wenatchee resides on Pangborn Bar. Fancher Heights marks the upper portion of this enormous feature.
Saddle Rock and Castle Rock are resistant remains of volcanic “intrusions” along the western skyline of Wenatchee.
Location - Walla Walla Point Park
17) Concentration of Large Flood Erratics
Location – Intersection of Mission and Chelan Streets – South Wenatchee
18) Chumstick Formation – Eocene Sandstone (42-46 million years old) that makes up
bedrock in Wenatchee Valley area.
Location – Behind business along west side of south Wenatchee Avenue.
