Large floods carrying rock debris can carve a path through a broad area and scour parts of it completely clean of loose rock and leave behind many characteristic features, including numerous potholes of various sizes and shapes
Click a link to see original photos
From left to right: Pothole scabland development in eastern Washington; Aerial photo of Dry Falls, Grant County, Washington, WA United States looking downstream - giant plunge pool potholes carved out by flood waters are now dry cataracts. This was once a series of wide waterfalls bigger than Niagara Falls. It formed from an ice dam break at Glacial Lake Missoula about 15,000 years ago. Note the development of flood channels that becomes a butte and basin topography above the dry cataract in right side foreground; Upper Goose Lake (Drumheller Channels) Washington – image credit Bruce Bjornstad - the longitudinal grooves show the direction of flood water flow
From left to right: Scabland Topography - Dry Lakes is near top of image – a developing basin and butte topography that did not develop further after the flood was over (no link – this is not at my pinterest board – my google earth image capture) – I will use this image again in Part VIII; Potholes Coulee dry cataracts that were once large waterfalls during a catastrophic flood over this entire region – located downstream and southwest from Dry Lakes – top of image is east (no link – my Google earth image capture); Potholes Coulee dry cataracts that were once large waterfalls during a catastrophic flood over this entire region - Washington State Scablands – image credit Bruce Bjornstad geocaching page
From left to right: Drumheller Channels – overview of area – big lake at top is Potholes Reservoir (no link – my google earth image capture); Channels just south of Upper Goose Lake – basin and butte topography (seen in image above just to south of the words USA) - (no link – my google earth image capture) – I will use this image again in Part VIII; Drumheller Channels just south of Potholes Reservoir (large lake at upper right) - (no link – my google earth image capture) – I will use this image again in Part VIII
From left to right: Giant Current Ripple - Camas Prairie Montana - one of the many features of flooded scabland terrain; Aerial view of giant current ripples - Kuray Basin, Altai Basin, Russia; Giant current ripples in the Kuray Basin, Altai Republic, Russia - a feature associated with scabland topographies
From left to right: Columbia River Basalt in Drumheller Channels. Columnar basalt forms deep within thick basalt flows - much erosion due to flood waters has taken place to expose these columns. These are seen all over the Pacific Northwest; Drumheller Channels near Othello, Washington. This shows that basin-n-butte topography is associated with other scabland features like potholes and long cliffs along the flood water channels. Thus basin and butte topography can be associated with flooding; Ice age flood channel, eastern Washington. I will use this image again in Part VIII
The “Unique” Case of the Channeled Scablands of Eastern Washington State
A snip from Wikipedia describes what the channeled scablands are – the underlining is mine:
“The Channeled Scablands are a barren, relatively soil-free landscape in eastern Washington, scoured clean by a flood unleashed when a large glacial lake drained. They are a geologically unique erosional feature in the U.S. state of Washington. They were created by the cataclysmic Missoula Floods that swept periodically across eastern Washington and down the Columbia River Plateau during the Pleistocene epoch. Geologist J Harlen Bretz coined the term in a series of papers in the 1920s. Debate over the origin of the Scablands raged for four decades and is one of the great debates in the history of earth science. The Scablands are also important to planetary scientists as perhaps the best terrestrial analog for the Martian outflow channels".
The basic geology of these scablands is described in another snip from Wikipedia:
“The term scabland refers to an area that has experienced fluvial erosion resulting in the loss of loess and other soils, leaving the land barren. River valleys formed by erosional down-cutting of rivers create V-shaped valleys, while glaciers carve out U-shaped valleys. The Channeled Scablands have a rectangular cross section, with flat plateaus and steep canyon sides, and are spread over immense areas of eastern Washington. The morphology of the scablands is butte-and-basin. The area that encompasses the Scablands has been estimated between 1,500 and 2,000 square miles (3,900 and 5,200 km2), though those estimates still may be too conservative.
They exhibit a unique drainage pattern that appears to have an entrance in the northeast and an exit in the southwest. The Cordilleran Ice Sheet dammed up Glacial Lake Missoula at the Purcell Trench Lobe. A series of floods (occurring over the period of 18,000 - 13,000 years ago) swept over the landscape when the ice dam broke. The eroded channels also show an anastomosing, or braided, appearance.
There are also immense potholes and ripple marks, much larger than those found on ordinary rivers. When first studied, no known theories could explain the origin of these features. The giant current ripples are between 3.3 - 49.2 feet (1 - 15 m) high and are regularly spaced, relatively uniform hills. Vast volumes of flowing water would be required to produce ripple marks of this magnitude, as they are larger-scale versions of the ripple marks found on streambeds that are typically only centimeters high. Large potholes were formed by swirling vortexes of water called kolks scouring and plucking out the bedrock.
The Scablands are littered with large boulders called glacial erratics that rafted on glaciers and were deposited by the glacial outburst flooding. The lithology of erratics usually does not match the rock type that surrounds it, as they are often carried very far from their origin.” http://en.wikipedia.org/wiki/Scablands
From left to right: Aerial photograph of Central Washington scablands - basement scouring from a huge flood - the shallow potholes are elongated parallel to the flow of water; Smaller potholes around Deep Lake in the scablands of Washington state; Scabland potholes and flood channels, Washington State
From left to right: Artistic conception of Ice Age floodwaters flowing over the area known today as Dry Falls - depiction of the high flood stage of the Missoula Flood - a flood like this is carrying hundreds of thousands of huge boulders underwater that are violently bouncing all over the place plucking out rock and scouring the land surface; Artistic conception of cataract formation during lesser flood stage of the Missoula flood
From left to right: Aerial photo of Dry Falls, Grant County, Washington, WA State - United States - potholes were formed which then were enlarged as the flooding proceeded - eventually the individual potholes merged into larger potholes and only the upstream sides of the larger potholes now remain - the downstream sides were washed away; Inside the huge basin carved out below Dry Falls cataracts, Washington - person for scale; Eastern half of Dry Falls dry cataracts - aerial shot looking upstream at 400 ft horseshoe shaped escarpments with potholes carved out during the Missoula Flood, ~15000 ya
From left to right: Potholes Coulee plunge-pool view in the Channeled Scablands – looking upstream at the now dry cataract; Looking upstream at Potholes Coulee, Washington State - several giant horseshoe escarpments with potholes remain after flooding; horseshoe potholes form from the impact of vehicle tires combined with moisture in weak areas of roads - this looks just like Pothole Coulee above.
Go here for more excellent pictures: http://hugefloods.com/
© Laura Fitzpatrick 2014