Frozen Earth: The Once and Future Story of Ice Ages (18 page)

Figure 12.
The map shows the location of glacial Lake Missoula as the Pleistocene glaciers retreated into Canada.
A lobe of ice blocked the northwestern drainage of the lake; when it gave way a large volume of water flooded westward onto the Columbia Plateau to create the Channeled Scablands.
At the Wallula Gap, the water reached another bottleneck and ponded to great depths.
Lake Bonneville, which also breached a (rocky) dam to flood the Snake and Columbia Rivers, is also shown.

Pardee’s observations removed the major objection to Bretz’s flood hypothesis: the absence of a source that could supply a very large volume of water in a short time.
The giant ripple marks implied just such a source, and the new evidence began gradually to win converts to Bretz’s view.
Still, some of his most vocal opponents took a very long time to come around.
One, Richard Foster Flint, a widely acknowledged expert in glacial geology who had meticulously developed an alternative noncatastrophic theory for the Scablands landscape, found it particularly difficult to discard his own hypothesis.
He had written
the
comprehensive textbook on glacial geology, but it was not until 1971, in his book’s third edition, that he acknowledged the flood origin of the Channeled Scablands.
Even then, he could not bring himself to be effusive about the unique character of the region.
In a book of more than eight hundred pages, he devoted one dry paragraph to a discussion of the Grand Coulee and of Scabland features “widely created east of the Grand Coulee by overflow of an ice-margin lake upstream.”
He makes cursory reference to two of Bretz’s publications.

Bretz lived to a ripe old age (figure 13).
He returned to the Scablands one last time a few months before his seventieth birthday.
By that time, there was abundant new information about the area, gathered through surveys by the U.S.
Bureau of Reclamation, which had initiated a major irrigation project there.
Bretz had access to Bureau of Reclamation excavations,
aerial photographs, and new maps.
One of the features revealed by the excavations was a complex pattern of layering in many of the Scabland sediment deposits, which suggested that there might have been many floods, not just one.
That makes sense.
The end of a glacial period is not abrupt; a few decades of cold weather and the glaciers would have advanced enough to again dam the exit from Lake Missoula, only to crumble as the warming continued, releasing yet another flood.
Exactly how many occurred is still uncertain—some who have looked carefully at the evidence believe there may have been dozens, of varying size, over the several thousand years of the glaciers’ demise.

At one place along the Columbia River’s course, after it has been joined by the Snake River in southern Washington, there is a particularly narrow section called the Wallula Gap.
It is far enough downstream in the Columbia River drainage system for all of the water from each of the Lake Missoula floods to have had to flow through this gorge—it is the only gateway to the west and the Pacific.
Bretz found evidence that water had “ponded” behind this gap, backed up like a traffic jam at a narrow bridge.
Initially, his critics were incredulous, because even at its highest levels, the present-day Columbia flows easily through the narrow valley.
However, Bretz found signs that the floodwaters had risen to heights of at least 300 meters above today’s valley floor and had backed up to flood the valleys of tributary streams such as the Yakima River for tens of kilometers.
Wherever it ponded, the water, no longer traveling swiftly enough to hold its heavy load of clay and sand in suspension, began to deposit sediment.
Each Lake Missoula flood added its contribution; today, in some low-lying regions of southern Washington, there is layer after layer of such sediment.
In an otherwise harsh landscape, fertile, productive soil has developed on these patches of mineral-rich sediment, leaving an unexpected legacy of catastrophic ice age flooding: the burgeoning vineyards of southern Washington.

In 1952, in addition to finding evidence for multiple flooding, Bretz gained some startling new insights into the workings of the floods as he
studied the Bureau of Reclamation aerial photos.
With the bird’s eye perspective they offered, he found clusters of giant ripple marks in many parts of the Scablands, just like the ones Pardee had described near the exit from Lake Missoula.
Although he had crisscrossed most of these areas on foot during his early fieldwork, Bretz had never noticed them.
On the ground, pushing his way through sagebrush and a bit like the proverbial flea on a camel’s back, his close-up view had given
him no clue to the larger picture.
The significance of the regular rise and fall of the land had not sunk in.
The scale of the Scabland ripple marks, like Pardee’s Lake Missoula examples, requires deep and fast-flowing currents.
Bretz added yet another piece of evidence to his list supporting catastrophic flooding.

Figure 13.
J Harlan Bretz at age 95, at home near Chicago.
Two years later, in recognition of his work on the Channeled Scablands, he was awarded a prestigious medal by the Geological Society of America.
“All my enemies are dead,” he said, “I have no one to gloat over.”
Photograph courtesy of the Special Collection Research Center, The University of Chicago Library.

More recent research has shown that most of Bretz’s early conclusions about the processes that formed the Scablands features were essentially correct.
Residual doubts about the ability of water to very rapidly generate erosional features on the scale of those seen in the Scablands have been dispelled by engineering studies.
Turbulent, high-volume flows have tremendous eroding power, especially when they are heavily loaded with rock particles.
Vortices form in the roiling water that can act like a sandblaster, drilling into solid rock, and under some conditions, shock waves produced by breaking bubbles in turbulent flows can generate such large local pressures that they shatter almost any material.
Even features as enormous as the Grand Coulee can be formed rapidly given enough water.
With such knowledge, Bretz’s work was entirely vindicated.
Better late than never, the geological community recognized his contributions by awarding him the Geological Society of America’s highest honor, the Penrose Medal, in 1979.
He was 97.
His one regret, he is reported to have confided to his son, was: “All my enemies are dead, so I have no one to gloat over.”

There was a precedent for Bretz’s flood hypothesis that was rarely mentioned during the debate.
It involved another ice age lake in the western United States, albeit not a glacial lake.
In 1890, G.K.
Gilbert, a geologist with the U.S.
Geological Survey, published a monograph on “Pleistocene Lake Bonneville,” one of many large lakes that formed in low-lying regions of the west during times when the climate in that region was wetter than it is at present.
Lake Bonneville occupied a large tract of land in northwestern Utah; the Great Salt Lake is but a small remnant of it.
Gilbert’s work describes features to the north of the former lake that appeared to him to be the result of flooding caused by a sudden release of water.

Lake Bonneville was too far south to have been dammed by ice, or to be supplied directly by meltwater from the major Pleistocene glaciers.
But it filled and apparently emptied catastrophically toward the end of the most recent glacial episode, in the same time frame as the Lake Missoula floods.
The northern exit from Lake Bonneville was blocked by loosely consolidated rocky rubble that filled in a low-lying area between hills.
Just as the high water stand of Lake Missoula put enormous pressure on the ice dam that blocked its outflow, so a rising Lake Bonneville pushed mightily against its barrier of rubble.
When it broke through, it rapidly cut away the loose material.
To the north lay the Snake River plain.
The waters of Lake Bonneville, reaching depths more than a hundred meters above the present valley floors, rushed northward into the Snake River and eventually, like the Scabland floods, emptied into the Columbia River and the Pacific Ocean.

There appears to have been only one major release of water from Lake Bonneville—once the barrier to its northern exit was eroded away, the lake could no longer fill up to such high levels.
Estimates of the peak water discharge during the flood vary, but most place it at one-tenth of the maximum Lake Missoula rate, or less—still a very large flow.
Potholes, channels, and flood deposits similar to those of the Channeled Scablands can be found for more than a thousand kilometers along the path of the flood.
Gilbert’s investigation focused on Lake Bonneville and its vast extent, rather than on the effects of the flood.
His fieldwork was excellent and his writing clear, and there was little disagreement about his conclusions.
Perhaps surprisingly, his descriptions of the features to the north of the ancient lake, and his interpretation that they were flood-related, did not attract the controversy that Bretz’s work was to generate several decades later.
Gilbert, like Bretz, realized that the landscape he was studying had been formed in a catastrophic process.
That more traditional geologists did not immediately denounce this interpretation was probably because the scale of the erosional features, while significant, was much smaller than that of the Scablands, and because the source of the flooding was so obvious in the
case of Lake Bonneville.
G.K.
Gilbert died in 1918.
Had he been present for the debate about the Channeled Scablands, he would undoubtedly have been on Bretz’s side.

A wonderful thing about science is its interconnectedness, and the fact that one discovery invariably leads to another, often quite unexpectedly.
As the concept of catastrophic glacial flooding gradually gained acceptance, it became reasonable to ask other questions.
Is there evidence for similar glacial floods elsewhere?
Would the rapid addition of all that water to the oceans have had any important consequences?
What about the sediment that was swept along by the floods?
Could it be traced on the ocean floor?
In the past few decades, all of these questions have been answered in the affirmative.

More than a thousand kilometers to the southeast of the point where the Columbia River empties into the Pacific, there is a ridge on the seafloor that has been studied intensively as a possible site of valuable mineral deposits.
The Ocean Drilling Project, a multination project to study the ocean basins, has sent several expeditions to the area, and they have drilled long cores into the sediments near the ridge.
In some of these, there are thick layers of sand that seem to have been laid down almost instantaneously.
This is unusual, because normally, at this distance from land, the dominant type of sediment is a fine-grained mud.
A little geological detective work on the cores showed that the mineral makeup of the sand closely matches that of modern sand from the Columbia River.
It appears that the Scabland floodwaters, heavily laden with sediments, did not immediately mix with the seawater when they debouched into the Pacific, but instead continued to flow along the seafloor as dense “turbidity currents,” only depositing their load of sediment when they eventually slowed down and spread out, very far from their source.
The sandy layer closest to the top of the cores is sixty meters thick, and this is more than a thousand kilometers from the mouth of the Columbia!
How far it extends over the seafloor is not known, but even if it is fairly limited, it represents an enormous amount of sedimentary material.
This uppermost sandy layer is interpreted as
being from the last major Lake Missoula flood, the same flood that gave the Scablands the form they have today.
It seems that as each new aspect of the glacial Lake Missoula floods is uncovered, it underlines the gigantic scale and truly catastrophic nature of the processes involved.

The western part of North America was not the only site of flooding as the glaciers retreated.
Signs of superfloods, as they have come to be known, have been found in northern Sweden, in Siberia, and in central Canada.
Still, on the basis of available evidence, the Lake Missoula flood seems to have been one of the largest, although recent data suggest that it might be edged out of first place by floods that occurred in Siberia.
As in the Channeled Scablands, the evidence for the Siberian floods includes deeply scoured channels, huge sand and gravel bars, and giant ripple marks.
Like the Lake Missoula floods, those in Siberia were caused when an ice dam broke, releasing a huge volume of backed-up melt water from a glacial lake.
Unlike the Scablands, however, the Siberian locality is mountainous, in the Altay Mountains near the northwestern border of Mongolia.
Most of the water coursed along already existing river valleys rather than spilling over a flat plain and forming a new drainage system of its own, as happened on the Columbia Plateau.
In the Siberian floods, the main erosive effect was that the river valleys were deeply scoured.
The best estimates suggest that water levels in the main exit gorge for these floods reached 400 to 500 meters deep.
Those who have studied the field evidence for the Siberian floods bill them as quite possibly “Earth’s greatest floods.”