Ancient Flood Beds at White Bluffs


An exotic clast-bearing conglomerate at White Bluffs Overlook was left by an ancient Ice Age flood. The post-Ringold unconformity appears to be a sequence-bounding surface that may correlate with a late-Neogene unconformity in the Rocky Mountains.

Flat-lying Pliocene Ringold lake beds (<5 Ma) are unconformably overlain by a meter-thick conglomerate composed of subangular calcrete rip-ups mixed with exotic clasts (granite, schist, andesite, quartzite). The conglomerate is Pleistocene age, likely a bit younger than 1.5 Ma. Exotic clasts derive from outside the Columbia Basin; they were brought here from the north and east by a combination of ancient rivers (ancestral Columbia, Salmon-Clearwater Rivers) and Ice Age floods. Some of the sediment dropped here by those rivers was later reworked by the cataclysmic floods. Wise not to assume that every exotic clast in Columbia Basin was swept here by floods from western Montana or from beneath ice sheets in British Columbia. Clasts may be reworked from older, local deposits or might be from less-exciting Idaho.

The post-Ringold unconformity is the local expression of a region-scale boundary that separates two of Washington's major packages of rocks, called synthems (unconformity bounded sequences). The erosional surface separates High Cascades Synthem rocks (<3 Ma) above from Walpapi Synthem rocks (13-7 Ma) below (Sloss, 1963; Armentrout, 1987; Cheney, 2016). Four Cenozoic synthems are recognized in the state. The surface has not been precisely dated, but it truncates the youngest Ringold sediments, thus appears to be no older than ~3.2 Ma and no younger than ~1.5 Ma (approximate onset of Ice Age conditions in Washington). If a regional surface, its tied to region-scale tectonism (~7 Ma to ~3 Ma), which is not well understood.

My field notes from a previous visit to a different outcrop in the White Bluffs. If you have ever wondered what Field Geologists do, you can bet sketching and describing layered strata is part of the job. All of the great geologists were pretty good to great sketch artists. A tradition to keep.

Unconformably overlying the conglomerate is a meter-thick, Stage V calcrete paleosol with blocky morphology. Thick calcretes require 10,000s-100,000s years to form, which makes the underlying conglomerate, which I interpret as an "ancient" Ice Age flood deposit (flood gravel), more than 100,000 years old. Likely its much older.

Foreset bedding in the exotic-bearing gravel. Most of the clasts you can see are chunks of calcium carbonate, ripped up from an older calcrete paleosol, but there are plenty of exotics in there, too. South is to the right.

Calcrete rip-ups in the outcrop. Exotic clasts in my hand (and outcrop).

The "ancient" outburst flood record (pre-Missoula floods cycle) in Eastern Washington is not well preserved, but if you develop an eye for it, you'll find it in numerous places. These so-called pre-Late Wisconsin deposits generally include weathered flood-laid sediment, flood-cut surfaces, interfingering loess, and paleosols. The exotic-bearing gravel shown here grades laterally into reworked silty-sandy deposits nearby, also with abundant rip-ups.

A page from my field book. The first draft of any project begins with a sketch.

Uppermost portion of the Ringold Fm and capping Plio-Pleistocene calcretes are well exposed at the White Bluffs Overlook.

Busloads of geologists have visited the White Bluffs Overlook over the decades, most just to enjoy the view. Geologically speaking, there is a lot more going on here than the bland guidebooks and worthless interpretive signs would have you believe. There are beautifully preserved sediments here that record many details about the floods and the older Ringold Formation. The guidebooks provide no detail. But its the details that matter in geology - even guidebook geology.

Detailed stratigraphy in guidebooks on Washington's geology is notably rare, but that isn't the case in every state. The Wyoming Geological Association and Montana's Northwest Geology journal have published original, detailed guidebook content for a 100 years. Neither author nor editor shies away from the details.

But publishing the details means new work must be done by authors and detailed geology can open an author up to professional criticism that he might just as well avoid. Also, some faculty want to keep details about their favorite teaching sites out of the public eye and away from students.

It would be great if the next generation of Columbia Basin guidebook authors (and editors) would see fit to include a few more stratigraphic details in their site descriptions (original, first hand observations). Wouldn't hurt to trim a few pages on the majesty of landscape, too. By now, we're all well aware the Ice Age "thesaurus" floods were Colossal! Stupendous! Cataclysmic! Catastrophic!

Time to move the science and the narrative forward. The familiar old flood story has grown stale because professors have stopped dragging their students out to far-flung valleys to find something new, but there's a lot of new discovery left in this landscape. I believe the reverence for the precious, aging Old Guard of Scabland flood geology is holding people back. Stand on their shoulders, for crying out loud, they're not museum pieces. Tell a new story about your own work, not theirs.

Try this. Next time you visit one of the "famous stops" in the Channeled Scabland, go out and discover something new, something small, some detail that no one wrote about 50 years ago. You almost certainly will. And make a sketch.

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