'Stratigraphy' is essentially the study of rock and sediment layering. 'Classic' sediment deposits form in perfectly planar, parallel layers. It is when these layers get compressed, scorched, warped, punctured, folded over, and uplifted that geologists really start to get interested. And when sedimentary layers become folded-over to the point where they form rolling dynamic aggregating nodules, its time to call the mechanical engineers!
Spent most of the morning of October 20th, 2010 at Goods Quarry #2 near Napavine, WA. Also briefly visited Goods Quarry #3, and documented the day's findings in this geotagged photo set. After ruminating on these first-hand observations, in my mind the debate about whether or not Missoula Floods had any influence here is over but for the details in the report writing.
Study of fluid mechanics teaches that aggregating nodules such as these must be formed dynamically, in a viscous fluid flow. Essentially the process can be described as a 'snowball effect', where a 'kernel' becomes ever-larger as it rolled by a viscous fluid through a layer of sticky material.
In the case of Missoula Flood flows applied to this location, both of these conditions are predictably expected as floodwaters receded. After observing the strata of undisturbed in-situ weathered basalt at Goods Quarry #3, my hypothesis has now come to expect a layer of native clay, predictable after the roughly 16 million years that this likely Grande Ronde Basalt has been sitting here at the foot of Mt. Rainier saturated with moisture. But depending upon location within the spillway and elevation with respect to the saddle, there is evidence to suggest that some floodwaters were powerful enough to strip the in-situ clay down to bedrock.
Strong evidence was provided by a multitude of spheroid and ovoid clay nodules with an evident 'dynamic stratigraphy' suggesting a rolling motion as they were deposited. Nodules were observed most commonly in the lowest clay strata, varying in size from a few centimeters to the more than half-meter specimen shown in this photo:
Geologists have cautioned that what I am describing as 'dynamic stratigraphy' may in fact be 'spheroidal weathering' of the underlying basalt bedrock. At first, I was certain that what I was seeing could not be in-situ weathered clay due to the fact that erratics may be observed both within the strata containing the large nodules, and within the large nodules themselves.
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| Half-meter long clay nodule, More Photos |
But after witnessing actual in-situ basalt weathering at Goods Quarry #3, in a strata which appeared unaffected and thus containing no erratics, I've devoted both careful thought and some initial investigation into the subject of sedimentary structures. And I can now envision a scenario where we may both be right, which is the focus of the next sub-section.
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| Extraction site of GQ2001, an example of gray sandstone, More Photos |
Formative Engineering Conjecture
Two conditions help the nodule maintain its integrity as it becomes larger:
- Rolling it at an ever-slowing rate, and
- Gradually increasing the viscosity of the surrounding fluid
As floodwaters rose quickly and reached maximum elevation, the topsoil was scoured from the spillways down to the in-situ weathered basalt, and thus the in-situ clay may have provided a significant portion of the material observed in the photos above, as it was peeled-off and rolled by the current. In the process of formation, erratics would fall from ice-rafts and become trapped within structures comprised significantly of the in-situ clay.
This elevation with respect to Lake Allison was at the very maximum achieved by a Missoula Floods event, and thus the only material expected from floodwaters here are the suspended ultra-pulverized clay and organic material, and ice-rafted erratics ranging two orders of magnitude in size -- from grains of sand to bowling ball cobbles. Larger erratics are not excluded, but they have not yet been found at this location.
Then almost as quickly as it had risen, Lake Allison's altitude began falling, trapping the suspended clay material in a decelerating flow along with innumerable ice rafts. As the flow decelerated, the viscosity increased as the suspended material became ever-more concentrated.
Conclusion
Here and at the upper inundation elevations of the Willamette Valley are the only areas to date where Missoula Floods clay has been identified. But above 122m in the Willamette Valley, geologists seem to favor the explanation that clay deposits were formed from aeolian (wind-blown) clay. This explanation is difficult to rebut with a Missoula Floods conjecture, since around the ancient shoreline at the high-stand of Lake Allison, any Missoula Floods clay deposit will likely not contain erratics due to ice raft scattering.
However, now that it has been demonstrated that Missoula Floods clay was in fact deposited above 122m elevation and thus representative sampling can be done with confidence, geologists may perform laboratory analysis to identify signature constituents. With this new information, there may be more Missoula Floods clay yet positively identified at some surprising locations and elevations in both Washington and Oregon.
And while the final chapter of how they were created needs more careful study and input from sedimentology experts, it is predictable that here and only here with respect to Missoula Floods will be found the highly-unique 'dynamic stratigraphy' observed in these enormous clay nodules.
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