During multiple ice ages, enormous glacial rivers swept through central Washington and neighboring areas, gouging out enormous and smaller channels, canyons, and coulees (known collectively as scablands). Over 13,000 years ago, floods carved the Grand Coulee when a glacial lobe dammed the Columbia River where the Grand Coulee Dam stands now, forcing the water to re-route (Amara and Neff 2008:17-18).
Banks Lake of Grand Coulee by Shari Maria Silverman is licensed under a Creative Commons Attribution 3.0 Unported License. Banks Lake is a human-made reservoir that fills a portion of the Grand Coulee upstream (S-SW) of Grand Coulee Dam. Although the dam was completed in 1941, it is in the same spot that blocked, then re-routed the Columbia River over 13,000 years ago, carving out the Grand Coulee in the first place. While south of Banks Lake, the tilted rock is from a monocline; underneath Banks Lake and the dam itself, the tilted rock formed from landslides. Blocks of basalt slid in a rotational curve at their bases, called rotational slides (Amara and Neff 2008:13, 17-19, Diagram 4).
This mass of water, icebergs, and debris poured into an area called the Coulee Monocline. The visible basalts of this monocline are Wanapum basalts, which formed from lava smothering the landscape between 15.6 and 14.5 million years ago (Bjornstad et al. 1997:211, Fig. 2). However, these lavas covered other magma; and, in turn, were blanketed by still more basalt flows. The entire process occurred between 17.5 and 6 million years ago. Amara and Neff (2008:23) suggest that the monocline, which is a sideways “S”-shaped warped landform, resulted from “deep-seated faulting and settling of the basalt lava flows in response to the thousands of feet of lava that erupted onto the earth’s crust from the earth’s interior.”
To give you a better picture of a monocline, this is the sideways “S”, kind of flattened out. The cracks are at bends . The bends endure the greatest strains and stresses. Therefore, they break apart the most.
This portion of the Association of Women Geoscientists (Pacific Northwest Chapter) field trip I took on Saturday, August 13, 2013, led by geoscientist Mark Amara, focuses on the Grand Coulee glacial floods and the Coulee Monocline.
Definition of Coulee
Before I go any farther, let me pull the definition of coulee from the post of the first portion of the field trip: Basalts and Ice-age Floods of Grant County, Washington.:
“A long, dry, steep-walled, trench-like gorge or valley representing an abandoned river channel. In south central Washington, the term coulee is mostly used for an abandoned ice-age flood channel,” (Ice Age Flood Institute 2002-2011).
Dry Falls with Pothole
The first of these Dry Falls slides appears in part one of the field trip, Basalts and Ice-age Floods of Grant County, Washington, but the following one would seem incomplete with out it. It is a dramatic element of the Grand Coulee. Besides, it’s a pretty view.
Dry Lake by Shari Maria Silverman is licensed under a Creative Commons Attribution 3.0 Unported License. When they flowed over basalt rock 14,000 years ago, these falls dwarfed Niagara. They spanned 3.5 miles and plummeted 350 feet. The water may have been 300 feet deep before cascading over the cliffs. The water originated from an ice dam break, which released glacial Lake Missoula to pour over central Washington. The gargantuan glacial river carried huge icebergs and boulders, as as sands, mud, and gravel. All these scoured the landscape (Amara and Neff 2008).
The next slide is the reason for the previous slide: the pothole. Potholes (also known as kettles) are circular depressions carved by violently swirling vortices called kolks (Bjornstad 2008:17). These underwater whirlwinds occur in extremely abundant, powerful water. The pothole pictured below is located just downstream from Dry Falls, over which abundant, violently quick-moving water flowed.
Dry Lake Falls Pothole by Shari Maria Silverman is licensed under a Creative Commons Attribution 3.0 Unported License. The small, circular lake at the top right of the photograph appears to be a pothole (or kettle). The larger lake at left lies at the base of Dry Falls.
Park Lake: Monocline Excavation and Revealed Buried Water Deposit
This stop (at Milepost 92 on State Highway 17), revealed three great views. These views revealed three items: the Grand Coulee floods, the Coulee Monocline, and a possible lake deposit.
Park Lake: The Grand Coulee Floods
Park Lake with Coulee Monocline by Shari Maria Silverman is licensed under a Creative Commons Attribution 3.0 Unported License. The floods through the Grand Coulee tore at the weak points in the basalt, exposing vertical cliffs. Weak points of a monocline are at the bends, so you see the vertical standing cliffs. The islands within Park Lake have tilted tops. These are portions of the monocline’s midsection.
Park Lake: the Unbent Basalt
This view is from across the chasm from the monocline. The view shows two portions of the Wanapum Formation. It serves as scale for the photograph following this one. It also is a nice example of a cataract.
Wanapum Basalts Cataract by Shari Maria Silverman is licensed under a Creative Commons Attribution 3.0 Unported License. This view of the Wanapum Basalts is from the Coulee Monocline across Park Lake. These are cataracts. Cataracts are the vertical walls formed when rapid erosion cuts away at the weak rock below the columns (Bjornstad 2008:19).
Park Lake: The Possible Lake
On the our side of the lake, the cut on the other side of the road is formed by the upper portion of the Coulee Monocline. You can see a slight tilt, indicating that it dipped downwards towards the lake in the canyon, but also to the right (north) a bit. The long white line shows this dip.
The long white line divides the base of one flow from the top of another (one field trip companion suggested that the above flow was the Roza Member of the Wanapum Formation, due to its blocky features).
The white line may be the remnants of a lake that the Roza (?) Member buried. The white color, its texture, and other attributes suggest diatomaceous earth. Diatoms form in shallow water bodies, such as clear, fresh water lakes that were common in the region when the basalt flowed (Bjornstad 2008:183).
Definition of a Diatomaceous Earth and Diatoms
Diatomaceous earth is formed from tiny skeletons formed of silica. Diatoms, which grow in fresh water lakes, are single-celled plants that excrete the tiny skeletons (Bjornstad 2008:183).
Coulee Monocline and Diatomaceous Earth by Shari Maria Silverman is licensed under a Creative Commons Attribution 3.0 Unported License. The white line in this photos tell us two bits of info right away. 1). The monocline is already tilting, not only towards the canyon, but a little to the right (north); and 2). The upper basalt may have covered a fresh water lake, because the white may be composed of diatomaceous earth.
Lake Lenore: Coulee Monocline
We could really see the monocline better from the Lake Lenore Caves park (near Milepost 85 on State Route 17). I understand that the caves themselves are very interesting, but the sun was waning. Below are a couple pictures of the Coulee Monocline.
Lake Lenore: Island with Central Portion of Coulee Monocline’s Tilt
Coulee Monocline Tilted Island by Shari Maria Silverman is licensed under a Creative Commons Attribution 3.0 Unported License. This is one of several islands visible from the Lake Lenore Caves park. The center of the middle portion of the tilt suffers fewer stresses and strains from the bending process. This may be why there are so many surviving islands.
Lake Lenore: Surviving Part of Upper Bend in Coulee Monocline
Coulee Monocline Upper Bend by Shari Maria Silverman is licensed under a Creative Commons Attribution 3.0 Unported License. At the top of the cliff on the right, you can see where the basalt top changes from an almost horizontal level to a slope downwards toward the canyon. This is the upper part of the monocline bend, which is a weak point in the structure.
The last (and most southerly) of the chain of lakes within the Grand Coulee, Soap Lake’s waters feel slippery and watery, hence the name. It lies in a topographic basin, deeply eroded in basalt (Amara and Neff 2008:31).
High concentrations of sodium carbonate salts, some of which line the lakeshore like bubbles, cause the soapy feel. Other minerals, including lithium, copper, tungsten, titanium, vanadium, and iron, fill the lake as well. A thick bed of clay blankets the lake bottom’s cobbles and gravels. The only life in the lake is brine shrimp, and the lake’s pH reading is 9 (7 is neutral [Amara and Neff 2008:31]).
All this attracts tourists as a health destination. Before tourism, Native Americans used it. In fact, they called the lake, “Smokiam.” This means healing waters (Amara and Neff 2008:31).
Soap Lake by Shari Maria Silverman is licensed under a Creative Commons Attribution 3.0 Unported License. Although its deep clay deposits and rich concentration of minerals doesn’t draw huge crowds to Soap Lake, some tourists still visit to “bathe in its foaming waters and use the mud as a poultice (Amara and Neff 2008:31).”
Ephrata Boulder Fields
Ephrata Boulder Fields by Shari Maria Silverman is licensed under a Creative Commons Attribution 3.0 Unported License. At the boulder fields in between Soap Lake and Ephrata, the flood waters through the Grand Coulee slowed down. The decrease caused the boulders to drop from the bedload. As a result, huge boulders, mostly basalt ripped from their foundations upstream, rest on a huge expanse here. Gravel bars are in the distance, showing where the floods slowed down further. However, they are hard to see in a photo, so you are getting the boulder fields.
Link to Part One of Field Trip
First Portion of Basalts and Ice-age Floods Field Trip: Basalts and Ice Age Floods of Grant Co, Washington
Amara, Mark S. and George E. Neff
2008 Banks Lake to Soap Lake. In Geologic Road Trips in Grant County, Washington. Moses Lake Museum and Art Center, Moses Lake, Washington.
2008 On the Trail of the Ice Age Floods: A Geological Field Guide to the Mid-Columbia Basin. Keokee Books, Sandpoint, Idaho.
Bjornstad, Bruce N., R. Scott Babcock, and George V. Last
2007 Flood Basalts and Ice Age Floods: Repeated Late Cenozoic Cataclysms of Southeastern Washington. In Floods, Faults, and Fire: Geological Field trips in Washington State and Southwest British Columbia, Stilling, Pete and David S. Tucker, editors. In The Geological Society of America: Field Guide 9, 2007. The Geological Society of America, Inc., Boulder, Colorado.
Ice Age Floods Institute
2002-2011 Glossary of Technical Terms Related to the Ice Age Floods. Within Ice Age Floods Institute website. Electronic document, http://www.iceagefloodsinstitute.org/glossary.html, accessed April 24, 2013.
Balsalt Monocline and Ice-age Grand Coulee by Shari Maria Silverman is licensed under a Creative Commons Attribution 3.0 Unported License.