April 10, 2014 | 7
Sometimes a geological map supports an intriguing idea not by showing the rocks that are there, but by showing the rocks that aren’t there anymore, eroded by a flood of biblical proportions.
“No one with an eye for land forms can cross eastern Washington in daylight without encountering and being impressed by the “scabland.” Like great scars marring the otherwise fair face to the plateau are these elongated tracts of bare, black rock carved into mazes of buttes and canyons. Everybody on the plateau knows scabland……The popular name is a metaphor. The scablands are wounds only partially healed – great wounds in the epidermis of soil with which Nature protects the underlying rock……The region is unique: let the observer take wings of the morning to the uttermost parts of the earth: he will nowhere find its likeness.” (J Harlen Bretz, 1928.)
American geologist J Harlen Bretz (1882-1981), who mapped the scablands, proposed in various papers (here, here and here) published between the years 1919 to 1925 a controversial hypothesis to explain the genesis of this unique landscape: A flood of unprecedented dimensions eroded the scablands into the solid bedrock.
“The volume of the invading waters much exceeds the capacity of the existing streamways. …… Hundreds of cataract ledges, of basins and canyons eroded into bed rock, of isolated buttes of the bed rock, of gravel bars piled high above the valley floors, and of island hills of the weaker overlying formations are left at the cessation of this episode……Everywhere the record is of extraordinary vigorous sub-fluvial action. The physiographic expression of the region is without parallel; it is unique, this channelled scabland of the Columbia Plateau.” (J Harlen Bretz, 1928.)
Already early geologists recognized that the deep incised gorges were formed by the erosive power of running water, however most assumed a slow formation by the Columbia River. Bretz proposed a much faster formation, by a sudden flood. However he couldn’t explain very convincingly the origin of the flood. Bretz speculated that a rise in global temperatures at the end of the last ice age caused the melting of the Laurentian Ice Sheet, or that maybe a volcanic eruption melted the ice age glaciers of the Columbia Plateau. But Bretz’s idea was intriguing enough that in 1977 the Geological Society of Washington organized a meeting to discuss the “Channeled Scabland and the Spokane Flood“. Unfortunately the flood-mechanisms as proposed by Bretz couldn’t provide the needed huge amount of water to explain a sudden, single flood.
However research published some years earlier by geologist Joseph T. Pardee (1871-1960) provided Bretz with a mechanism that maybe could work. Pardee had mapped the shores of a gigantic ice-dammed lake, extending over an area from today’s north-western Washington to Idaho and Montana, which he named Lake Missoula. If this gigantic lake experienced a sudden and catastrophic outburst, the resulting flood would explain the observed deep erosion and the size of the transported erratic boulders.
In 1940, during the meeting of the American Association for the Advancement of Science, in the session dedicated to the “Quaternary Geology of the Pacific Northwest” most contributions were against the flood, but Pardee proposed an interesting paper entitled “Ripple marks (?) in glacial Lake Missoula“, where he described extraordinary large gravel ripples (15 meters high and 150 meters long) discovered in Montana. These ripples, formed by fast running water, were strong evidence supporting a catastrophic flood draining Lake Missoula, providing the necessary huge amount of water end energy to carve the scablands into the bedrock.
In the following decades Bretz continued to collect geologic evidence for his controversial hypothesis and finally between 1960 and 1970 the flood hypothesis was accepted by the geological community.
BAKER, V.R. (2008): The Spokane Flood debates: historical background and philosophical perspective. Geological Society, London, Special Publications Vol. 301: 33-50
BAKER, V.R. (2009): The Channeled Scabland: A Retrospective. Annu. Rev. Earth Planet. Sci. Vol.37(6): 1-19
BRETZ, J. H. (1928): Channeled Scabland of eastern Washington. Geographical Review, 18: 446-477