| What: The accepted model in geology was and is
Uniformitarianism. This idea says that we the
geological features we observe now were formed by processes which occurred in the past, are
occurring now, and will occur in the future. With some obvious exceptions, this model has
served well.
Within this model, there are several processes which form valleys or canyons. They
continue to occur, and they produce typical types of valleys. Stream erosion -- the
wearing away of the bottom of a valley by a stream -- produces V shaped valleys. The
drainage pattern is like a tree, with small streams feeding into a larger ones. The
confluence of such streams doesn't involve much change in level. The stream which made the
valley continues to occupy it and its water flow is in proportion to the valley. Such
valleys are often altered by glaciation -- ice flowing down a stream cut valley. Glacier
carved valleys are usually U shaped. All of these processes are slow.
The Channeled Scablands do not fit either of these processes well. The canyons have
steep, stepped sides and are roughly rectangular in cross-section (when loose rubble is
ignored. The streams which flow in them seem to have little relationship to the canyons
themselves. The canyons are much larger than the streams which occupy them would seem to
be able to erode. The puzzle is "how did they form."
The model we now accept is the work of J. Harlan Bretz. Bretz was born in Michigan in
1882. He was interested in astronomy, but did not pursue that area because he felt weak in
mathematics. He majored in Biology at Albion College (Michigan), and became a high school
biology teacher in Seattle. He became interested in the geology of the Puget Sound area
and studied it extensively as an amateur. He then entered the University of Chicago and
completed his PhD in 1913 with a thesis on the glacial history of the Puget Sound area.
When he returned to the Seattle area to join the University of Washington faculty in
geology and began to study the channeled scablands, he was a recognized expert in the
features of stream and glacial erosion. Even though he soon left the University of
Washington to join the geology faculty at the University of Chicago, he continued to
conduct field work with his students in the channeled scablands each summer. We need to
realize that all of Bretz' work was done from ground level, without aerial or satellite
photos of the sort which we would now have.
It became clear to Bretz that the scablands were not readily explained by either
glacial or ordinary stream erosion. The features of Dry Falls are a case in point. The Dry
Falls are immense. The rim of the falls extends for four miles. There are plunge pools
(now occupied by small lakes) at the base of the falls, as is characteristic of
waterfalls. Had glacial erosion made this feature, the precipice of falls would have been
ground away to form a smooth slope. This leaves water erosion, but there is no obvious
source for enough water to make such an immense waterfall.
Other features which are unusual for water erosion are the drainage patterns. A map of
the Channeled Scablands leaves one with the impression that the water entered from one or
a few points in the northeast and drained to the southwest, but in such volume that it
spread out over the Columbia Basin much like water poured on a dusty driveway spreads into
several channels which merge and split as the water flows downhill. These are called
braided or "anastomosing" channels. Hanging valleys are also features unusual
for stream erosion. These are side canyons which emerge into the main canyon well up its
side, leading to the kind of waterfalls (Multnomah is the classic example) which emerge
high on the side of the main valley. (These are also features of glacial erosion.)
Bretz proposed a flood in which the sudden release of a volume of water much larger
than that which now flows through the area produced the very large scale erosion. This
would require the formation of a dam which would hold back the normal rainfall and
snowmelt for many years, and then suddenly break, releasing this water over a period of
days of weeks. Such a dam could be made by a glacier. If it blocked a stream valley, water
would rise behind it. If it failed, it would do so catastrophically since as it began to
break up, blocks of ice would float away, so the dam would fail from the bottom up. This
would provide the large volumes of water needed to explain erosional features such as Dry
Falls and explain why these features don't conform to the more common characteristics of
stream erosion (V-shaped valleys, streams follow valleys). Since the flood(s) appeared to
originate in the Spokane area, he termed them the Spokane floods.
Bretz essentially proposed a catastrophe -- the direct antithesis of Uniformitarianism.
As we might expect, his proposal was resisted by most geologists. The Channeled Scablands
were attributed to glacial erosion. The issue of the source of so much water was raised.
Bretz kept on with his field work, looking for further evidence which would help settle
the matter.
Two such pieces of evidence were developed. Glaciers are "dirty ice." They
contain rocks plucked from the terrain they erode. Often these rocks are smoothed by the
glacier as it rubs against the ground, but if such a rock is encased ice and
"rafted" somewhere, it may still be rough. In any case, the rock (called a
glacial erratic) will not be like the ground it rests on. Some of these have been found as
far south as Eugene, Oregon.
Another piece of evidence comes from wondering where all that water could go in a short
time. The Columbia Gorge drained off much of this water and was eroded significantly in
the process, but much would have backed up the Yakima and Snake River valleys. There the
flood left sandbars as the water slowed and its load of rock and sand settled out. These
sandbars are unusual in two respects. First they point upstream relative to the current
flow of the river. Second, they are often high on the valley side, suggesting that the
water was very high when they were deposited. In most such bars, there are many layers,
suggesting that the floods recurred at fairly regular intervals up to as many as seventy
times.
This evidence strengthened Bretz' case, but left many unconvinced. What was missing was
evidence for the large body of water and its sudden drainage. This was found in
"beaches" on the mountainsides above Missoula, Montana. These lines can be
followed for miles and allow outlining a lake which extended over much of western Montana.
A glacial dam can be located at the east end of Lake Pend Oreille in the Clark Fork River
valley. Further evidence comes from giant ripple marks. Fast flowing water leaves ripples
in the terrain it flows over. These are larger and farther apart as the more water flows
faster. In fact, they are large enough to be difficult to recognize from ground level, but
become obvious in aerial photographs.
With the source of the water identified and supporting evidence for its volume and
flow, Bretz' idea was accepted. One of his antagonists was heard to say "How could I
have been so wrong" when he visited Palouse Falls. Like many of those who disputed
Bretz, he had never before visited the Channeled Scablands, so the disparity between
the
small stream and the big canyon and plunge pool at Palouse Falls had not hit him.
Bretz was awarded the Penrose Medal (the American Geological Society's highest award)
in 1979 at age 96.
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