Channeled scablands: Spokane to Soap Lake

Steven Dutch, Professor Emeritus, Natural and Applied Sciences, Universityof Wisconsin - Green Bay

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Acknowledgement

I need to make it crystal clear that none of this is my own research. Thepictures on this and associated pages were taken on a GSA field trip in 1994 ledby Richard B. Waitt of the U.S. Geological Survey, and the interpretationspresented here are largely those of Dr. Waitt as presented on the field trip andits guidebook. I thank Dr. Waitt and his colleagues for a stupendous fieldexperience.

1.1 Hangman Creek Rhythmites

	The fact that the Missoula floods discharged first into a glacial lake is profoundly important because quiet arms of the lake received successive deposits of sediment that allow a flood chronology to be developed. Between floods the lake deposited varved sediment that allows flood intervals to be determined. The varves suggest that initial floods were about 80 years apart, becoming smaller and more frequent as time went on, with the last floods only a few years apart.
	Note the large dropstone left of the pine tree. This dropstone is prominent in many other pictures as well. Dropstones were probably lodged in large ice blocks carried by the floods. Each of the "ledges" in the embankment is actually a thin layer of varved clay protecting the sand underneath from erosion.
	Much of the flood sediment is very coarse sand and fine gravel.
	Closeup of the large dropstone, which is about 30 cm in diameter.
	Left and below, a small basalt dropstone resting atop a small pedestal.
	Closeup of the varved clay.
	Enlargement of the photo above. About 22 varves are visible.
	Very large dropstones in the upper part of the sequence.
	View of Hangman Creek from U.S. 195.

Loess Landscape South of Spokane

The thickest loess in the United States, up to 100 meters thick, is in eastern Washington.

1.2 Malden Pit

	A dozen separate flood deposits in this large bar. Since the sediment is largely derived from basalt, it tends to be quite dark.
	1980 Mt. St. Helens ash on top of the embankment.
	The coarse beds are each overlain by a thin layer of finer sand, probably a slack-water deposit deposited as the floods ebbed.

Subaqueous Basalt

	At this locality, lava evidently flowed into a small lake or river, building "foreset beds" of pillow lava enclosed in yellow devitrified glass. These are Columbia River basalts of Miocene age and obviously long predate the Missoula floods.
	Here we see the transition from pillow lava to normal basalt flow.
	The upper part of the flow is crudely columnar with horizontal jointing and some spheroidal weathering.

Channel Forms in Loess

	Across the road from the basalt outcrop above is a nice channel in loess. At left is a streamlined tip of a large bar in mid channel. Below: two views of the channel.

1.3 "Hole in the Ground" 

	This large pit is on top of a ridge, 200 meters long and 30 meters deep. It is excavated in basalt.
	Closed pits in the scablands have been attributed to extremely powerful vortices in the floods. Judging from the unscoured loess uplands nearby, this locality was perhaps 50 meters below maximum flood level.

Rock Coulee

	This 100-meter deep valley has tiered walls, each tier defined by a basalt flow.
	Loess uplands west of Rock Coulee. The loess hill in the distance stood above flood level while the lowland in foreground was mostly scoured free of loess.

1.4 Mima Mounds

	The low humps, a meter or so high and 5-10 metera across, are so-called "mima mounds." They have been ascribed to animal burrowing, cryogenic action and vegetation anchoring coupled with water erosion. They occur in scabland tracts and not nearby thick loess. Just an outlandish thought. Odd circular ground patterns also occur near Mount Shasta, California near the edge of its mega-landslide. Could low-frequency vibrations explain these patterns?
	Left and below: looking across the scoured lowlands toward the loess uplands.

Rock Lake

	Rock lake is an erosional basin whose bottom is over 100 meters below lake level in places. Total relief of Rock Coulee in this area is 250 meters.

Landscapes NW of Rock Lake

1.5 Upper Crab Creek

	Left: view across the valley Below: the loess uplands that were trenched by the floods.
	Left: columnar basalt. Below: The scabland valley floor contrasts with the smooth loess uplands beyond.

Landscapes in Harrington-Odessa Area

	Left and below: loess plains

1.6a Ring Structure in Basalt

 

	Numerous ring-like structures occur in the basalt, looking a bit like old Celtic forts. They have been interpreted as ring-dike like features that form when the solid crust of a flow cracks and allows the still molten interior to extrude.

1.6b Giant Current Dunes

	Wind is slowly reclaiming this abandoned road.

1.7a Bars of Wilson Creek

	Left: View upvalley. Below: the two views below were joined to make the panorama above.
	Looking down the bar. The notched ridge in the distance is a trenched spur, where water simply poured over an obstruction and carved channels through it.
	This road cut shows that the gravel bar is made of grapefruit-sized cobbles.

1.8 Slackwater Facies on Basalt

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Overview
Soap Lake to Chelan
Chelan to Othello
Othello to The Dalles
The Dalles to Seattle

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Created 7 April 2003, Last Update 06 June 2020