37.3: Channeled Scablands geomorphology

Figure \(\PageIndex{1}\): Typical Palouse geography, seen near Walla Walla, WA. (CC-by; Photograph by Lynn Suckow, via Wikimedia.)

The area of the Scablands has a three-part geological history. The third part is the one that Bretz elucidated. We will get to that in a moment, but here’s what came first. The area is part of the Columbia Plateau, a vast flood basalt province that mostly formed between 17 Ma and 15 Ma. Mafic lava flows are stacked up like a batch of pancakes. On the very top (like powdered sugar on the pancakes?) is a layer of loess, which is windblown silt. The loess makes for very fertile soils, and it makes for a region of prime agricultural land called the Palouse.

But as we shall soon see, the nurturing soils of the Palouse (and much of the basalt they overlie) were blasted away by the Missoula floods. That’s the third part of the region’s geologic history; the one discovered by J Harlen Bretz. So what did Bretz see instead of the usual basalt and loess? His great contribution to geology was meticulous documentation of a series of superlative landforms. The first of these to consider are the coulees.

Coulees

Figure \(\PageIndex{2}\): A portion of Grand Coulee in Washington holds water impounded by a modern dam (Banks Lake). This highlights the coulee’s shape: long and narrow, with steep cliff-like valley walls. (CC-by license; Photo by Doc Searles, via Flickr.)

Local place names are one of the delights of traveling to different parts of the world. For instance, a small current of running water might be called a stream, a creek, a run, or a kill, depending on what part of the country you find yourself in. An amphitheater-shaped glacial valley head might be called a cirque, a corrie, or a cwm, depending on whether you’re in the Alps, Scotland, or Wales. A valley may be a hollow (or “holler”), a draw, an arroyo, gully, a ravine, a dell or a glen.

In eastern Washington state, some valleys are called coulees. These are not the classic V-shaped valleys with a stream running along the bottom, however: they are rectangular or “boxy” in cross-section, thanks to the architecture of the basalt lava flows they cut through.

As Bretz explored the coulees of eastern Washington, he noticed several things: (1) many of the coulees were bone dry, despite presumably having been formed through erosion by running water, and (2) there was a pronounced common orientation to the coulees, trending from Spokane in the northeast off to the southwest. Bretz and his students documented these landforms across the Scablands, and when presented in a map, the pattern is striking:

Figure \(\PageIndex{3}\): Bretz’s 1923 map of the Channeled Scablands shows the prevailing northeast-to-southwest orientation of dozens of coulees.

The coulees branch and merge, wrapping around isolated mesas of basalt and loess. This is why Bretz dubbed the region the Channeled Scablands. The “Channeled” part of the name refers to these coulees.

Dry waterfalls

Sometimes the coulees featured dramatic downward steps along their downhill course. These “steps” would be dramatic waterfalls if water was flowing over them, but instead they are bone dry, often with dramatic plunge pools below the cliffs. The GigaPan below shows one of these sites; explore it for a sense of the tremendous scale of these features.

Bill Richards GigaPan

The Dry Falls complex separates upper Grand Coulee from lower Grand Coulee. It is 400 feet high and about three miles wide. That is about ten times the size of modern Niagara Falls in New York.

Eroding out these downstream channels and plunge pools would have produced a lot of new clastic sediment, principally basaltic gravel. The size of the clasts of basalt varied from “normal” sizes you could hold in your hand, to truly jawdropping monsters.

Big boulders

The biggest boulders are most worthy of our attention, because they establish an upper bound on how powerful a current of water it must have taken to move them. One giant is found near Soap Lake, Washington, part of a vast deposit of gravel called the Ephrata fan. It is a substantial distance from its source outcrop, but it is Columbia River basalt, so it seems unlikely to have been ice-rafted.

Check it out:

Figure \(\PageIndex{4}\): An exceptionally large boulder of columnar-jointed basalt that was tumbled along in the Missoula outwash floods. It was ultimately deposited near Soap Lake, Washington as part of an underwater fan complex, the Ephrata fan. (Callan Bentley photo.)

That is an enormous boulder, and it must weigh a lot. Though glaciers regularly transport erratics of this size, there is no evidence that glacial ice ever flowed over this part of the country. The nearest glaciers were dozens of miles off to the north... we’ll come back to them in a moment.

Giant ripple trains

Figure \(\PageIndex{5}\): Normal-sized asymmetric (current) ripples in sand. Virginia, USA. (Callan Bentley photo)

As if this weren’t enough, the Channeled Scablands also included vast trains of giant ripples – like the cm-scale ripples that form in sand on a stream bottom, but these were just gargantuan. Each ripple stood 35 feet tall, with a wavelength of 300 feet between ripple crests. They were arranged in large clusters called “trains.” Over one hundred giant ripple trains have been documented in the region.

Figure \(\PageIndex{6}\): Camas Prairie, Montana, has exemplary examples of giant ripples. (Google Earth image.)

Consider this scene from Google Earth, which shows a satellite view of the giant ripples that cover the floor of the Camas Prairie in northwestern Montana. Many places in the Channeled Scablands also feature vast arrays of these ridges. To make ripples this large out of such large sediment would seem to require a lot of very fast moving water. At least, that’s how it seemed to Harlen Bretz.

Instead of being made of sand, like normal ripples, these giants were constructed from boulders and cobbles – sedimentary particles that take vastly more energy to move than sand grains. Here’s a cross-section through one:

Callan Bentley GigaPan

It would be difficult to identify that as a ripple from ground level! This is a case when backing away from the subject yields the critical perspective that gives insight.

Now, let’s zoom out again, and see what these ripples look like from far away. Compare the giant current ripples in the GigaPan below (tan rolling hills above the modern river) to the sheer cliffs of basalt that define the edges of the Columbia River’s gorge:

Callan Bentley GigaPan

These giant ripples could only be deposited by raging currents of immense power and depth.

Potholes

Figure \(\PageIndex{7}\): A typical bedrock pothole in a modern river, with a spiral arrow showing interpreted water flow path (vortex). Field of view is about 1 meter. (Callan Bentley photo.)

When fast-moving water flows over a tough bedrock surface, drag slows the current immediately adjacent to the rock. Meanwhile, less-impeded water flows freely in the main channel, and that sets up a differential rate of flow. Spiral shaped vortices develop, and if these vortices of water have sediment suspended within them (such as sand and silt, under normal circumstances), then they can focus abrasion on the bedrock at the point of the vortex. This creates a cylinder-shaped hole to be bored into the rock: a pothole.

In the Channeled Scablands, there are potholes, too – as you might expect from such violently-rushing waters. The difference between the “normal” situation and that found in eastern Washington is merely size. The potholes in the Channeled Scablands can be a quarter-mile in diameter. Here’s one such location, Potholes Coulee, shown in Google Maps. Zoom in and explore the substantial round holes in the ground. These are potholes – but incredibly enormous potholes!

In summary, Bretz had documented a suite of features that were both very unusual and could all be explained by a common hypothesis. The dry coulees and falls of raw basalt, lacking the mantle of fertile Palouse loess, the giant boulders, potholes, and ripple marks — they all suggested the same thing: the Channeled Scablands had been ravaged by a giant flood beyond anything recorded in human history.

Figure \(\PageIndex{8}\): Icebergs were left perched on dry land after a jökulhlaup drained Iceberg Lake in Wrangell-St. Elias National Park, Alaska. (NPS photo)

Bretz thought the flood might have been similar to big glacial outburst floods documented in Iceland, when volcanic activity melted a glacier from beneath. These jökulhlaups (“yokel-ahps”) were impressive indeed, though not quite on the same scale as what Bretz envisioned for the Channeled Scablands. The jökulhlaups were powerful, but perhaps not quite “catastrophic.” Bretz had cut his teeth working on the glacial geomorphology of Puget Sound, and knew that there was substantial evidence for glaciers not too far away – up by Spokane. Could a jökulhlaup have carved the Channeled Scablands?

Quiz

Exercise \(\PageIndex{1}\)

Which of the following landforms was *not* documented by Bretz in the Channeled Scablands?

a. giant potholes

b. dry waterfalls of vast size

c. giant ripple marks in vast "trains"

d. multiple parallel coulees

e. giant boulders

f. an angular unconformity

Answer

f. an angular unconformity

Exercise \(\PageIndex{2}\)

What event did Bretz infer must have carved the Channeled Scablands?

a. A divinely-induced flood, intended to drown the unworthy sinners of the world

b. A volcanic eruption

c. A tsunami

d. A very high tide

e. A jökulhlaup flood

Answer

e. A jökulhlaup flood