top of page

Silt-Capped Rhythmites at Old Lady Canyon, WA

The publication of Richard Waitt's paper describing the stratigraphy of Burlingame Canyon in 1980 opened up a can of worms. In particular, a disagreement among geologists over the nature of the thin, silty layers at the tops of Missoula flood rhythmites.

"Are silty tops indicative of deposition by wind or quiet water?"

WIND OPTION -- If the silt was blown in by wind, then surfaces between floods were dry and exposed. A significant period of time must have passed between floods with windblown tops since the floodwaters had time to drain away. Thus, each rhythmite represents a single flood event.

WATER OPTION -- If the silt was deposited by settling through a column of standing water, then the landscape remained wet between flood events. If the silt is water-laid, then the argument could be made that little time passed between flood events and that some rhythmites might record surges within a single flood.

A conspicuous light-colored silt layer with distinctive vertical fractures caps a Touchet Bed rhythmite at Old Lady Canyon in the Columbia Gorge near Roosevelt, WA.

From the beginning, the problem has been that people find it difficult to a.) determine at the outcrop if silt caps are present in a given set of rhythmites, and b.) resolve the fine-scale features within a silt cap with the naked eye - especially when the tools available are a shovel and (God forbid!) a rock hammer. Seeing the fine features within a silt cap is crucial to determining whether deposition was by wind or water (subaerial vs. slackwater).

Nine silt layers cap a stack of fine-grained rhythmites at Old Lady Canyon. The repetition and similar thickness suggests that all are water-lain. There's an oxidation and mottling overprint throughout. This drainage was recently swept by some huge floods (1964?). I suspect the mottling is a recent modification, based on the height of the pre-flood alluvial fill surface.

In the years following Waitt's publication, numerous workers confirmed through several lines of evidence (varves, burrows, mottling, roots, charcoal, etc.) that scabland floods were separated by significant periods of time, often several decades in length. Therefore, the larger question is now settled: windblown silts are possible (if difficult to see) and water-laid silts are common. Waitt (1985) finds loess and ash caps on rhythmites at Mabton, an outcrop which is now covered.

However, consider the plight of the junior geologist put on the spot by his superiors. If asked to show evidence of wind vs. water at a given outcrop, he may find the challenge remains as thorny as ever. How to proceed?

Get one of these brushes and see what you can uncover.

After doing field work all over the Columbia Basin for the past 20 years, I am convinced very, very few geologists a.) leave the roadside, and b.) visit out-of-the-way places like this. Admittedly, fear of being shot by the landowner or being arrested in front of students is inconvenient, but these are stochastic events and, therefore, cannot be planned for. Outcrops must be visited and documented. Be bold. You will not likely return to this spot anytime soon. Field Geologists were put here to visit outcrops. I prefer to make discoveries the old fashioned way: on foot. Fencelines be damned. The key is going early in the day.

Last 50 Posts
All Posts by Month
    bottom of page