Great Scablands

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Assignments	Peters, Catastrophism Revived: The Example of Pleistocene Outburst Flooding, ch. 6. Chernicoff, Highlight 15-1, Channeled Scablands of Eastern Washington, pp. 310-311.
Reminders	The Planetary Scavenger Hunt has been cancelled. The Mars Show will be presented at 10 a.m. on Friday May 8. Rock collections due Wednesday May 6.

Reading Focus Questions

Read the entire chapter. Focus your attention on methods of reasoning in the earth sciences rather than merely Peters' presentation of information. Be prepared to answer any of these questions should they occur on a quiz or on the final exam.

Unusual Geology in Washington State

1. Plateau basalt
2. ______________ (from French, to flow) are long and deep braided channels which cut down through loess and basalt in Washington state.
3. True or False? The Scablands are barren of soil.
4. Evidence that the coulees were cut by ___________:
   1. U-shaped
   2. Active glaciers in the Washington Cascades
   3. Erratics in the Scablands
5. Evidence that the coulees were of ________ or river origin:
   1. Columbia River would have been larger in the past due to glacial meltwater.

J Harlan Bretz

1. (No period after the J)
2. In the 1920s and 30s, Bretz argued against the glacial theory because:
   1. No glacial till in central Washington
   2. No moraines in the Scablands
3. Bretz argued against the fluvial theory because:
   1. Braided channel pattern incompatible with river flooding; rather suggestive of sudden inundation followed by rapid retreat.
   2. Coulees flat-bottomed, not V-shaped.
   3. Many coulees are dry, with no streams.
4. In 1923 Bretz argued that the Scablands were instead produced by a catastrophic regional flood, composed of a wall of water 1,000 feet high: "Fully 3,000 square miles of the Columbia Plateau were swept by the glacial flood, and the loess and silt cover were removed. More than 2,000 square miles of this area were left as bare, eroded rock-cut channel floors, now the scablands, and nearly 1,000 square miles carry gravel deposits derived from eroded basalt. It was a debacle which swept the Columbia Plateau."
   1. Evidence for his view included:
   2. Braided channel pattern (see above). Normal dendritic drainage is not found in the Scablands. The channels cut across tall divides, forming isolated islands of soft loess surrounded by eroded basalt.
      • Inference: channels were filled to the brim with running water, not formed or overtopped by glaciers.
   3. Deep gravel bars in the coulees.
      1. Unlike glacial moraines, they are well-sorted basalt gravels.
         • Inference: They must have formed from moving water.
      2. One is 100 miles from the Columbia River, 10 miles long.
         • They were not formed by the existing river systems.
      3. Some are 100 feet high.
         • Inference: The water level must have been even higher.
   4. Plunge pools and deeply-gouged, bowl-like hollows in basalt which would not form from gradual erosion.
      1. Some are hundreds of feet in diameter.
         • Inference: They must have formed from rushing water on a huge scale.
      2. Many now completely dry: Dry Falls (book cover) and Palouse River Falls (Fig. 6-6).
         • Inference: present drainage processes inadequate to explain their origin.
   5. Corroborating evidence: Thick strata of fine silts are found upstream in the Columbia and Snake river systems (as far away as Idaho).
      1. Inference: Backflooding; these rivers were made to flow backward by the onrushing wall of water.
   6. Ridges in the gravel bars up to 50 feet high.
      1. Inference: Ripple marks on a gigantic scale require water hundreds of feet high moving at least 50 mph.
   7. Longitudinal bars across hanging valleys.
      1. Inference: Not deposited by glaciers which carved the valleys.
      2. Not deposited by existing rivers because lakes behind them are still undrained.
      3. Resemble natural levees deposited at edges of floodwaters. Inference: gigantic flood.

The Authorities Respond

1. Bretz was attacked in the name of ___________________.
   • For an additional popular interpretation of debates over the Scablands see Stephen Jay Gould, "The Great Scablands Debate," essay published in The Panda's Thumb (Norton, 1980), pp. 194-203.
   • Gould writes that "Instead of testing Bretz's flood on its own merits, they rejected it on general principles" (p. 197).
2. True or False? Most of Bretz's critics had not seen the Scablands.
3. Where did the water come from?
   1. Bretz' theory was a phenomenal theory to establish the past occurrence of an actual event based on descriptive field evidence. Phenomenalists prefer certainty to completeness.
   2. Many critics required a causal theory to explain how any such event could have occurred before they would entertain the field evidence that one such event had in fact happened. Causalists prefer complete theories to immature ones.
      • For example, O. E. Meinzer argued against Bretz's views, insisting that "Before a theory that requires a seemingly impossible quantity of water is fully accepted, every effort should be made to account for the existing features without employing so violent an assumption."
4. In 1942 J. T. Pardee advanced a causal theory for the origin of the water: breaking of an ice dam, draining Lake Missoula.
   1. Ancient shorelines of Lake Missoula (Figure 6-8)
   2. Gravel bars 50 feet high were mega-ripple marks formed during catastrophic draining.
5. In 1965 the International Association for Quarternary Research held its annual meeting in the United States. Participants in a field trip to the Columbia Plateau examined the Grand Coulee and the gravel deposits in the Snake River Canyon. Afterward they telegraphed the aging Bretz, conceding "We are now all catastrophists."
   • What personal and professional qualities made Bretz successful?
6. Bretz' view was not entirely correct: there were multiple episodes instead of a single flood.
   1. 35-40 rhythmites in Lake Missoula sediments (Figure 6-9)
   2. Rhythmites separated by varves for an average of 50 year intervals.
   3. Rhythmites found in Scablands as well.

Human History and the Floods

1. Water wall 50 mph
2. Blast of compressed air

Catastrophic Floods Pop Up Everywhere

1. G.K. Gilbert, Pleistocene Lake Bonneville, Figure 6-2
   1. Pluvial lake
   2. Great Salt Lake, Utah
   3. Drained into Snake River from a direction different than that of the Lake Missoula flooding. "Bretz's large sense of vision is needed to imagine two catastrophic floods coming from different directions, each inundating the Snake River floodplain...." (p. 89)
2. Mars