Dry Hollow Site Geology near Clyde, WA
Outcrop sketch: For large outcrops, I will take a panoramic photo, draw a quick map, and make a sketch in my field book. If I have the time, I will print the color pano photo at full-page size, return to the outcrop, and transfer my field notes to the photograph using a pen. This step is where the rubber meets the road. I usually double the amount of information obtained up to that point. New questions will arise. The final interpretation is created in Adobe Illustrator (like this one). At each step, I'm learning something new as I wrestle with the stratigraphic details, crosscutting relationships, etc.. The most important thing is to patiently do as much of the sketching and labeling work at the outcrop as possible.
Loess in North America - In areas of the world where loess is thick, a downwind transition from dunes to sheet sands to loess is commonly observed. Loess is essentially the farther-traveled, finer-grained facies of eolian sand; both sand and silt are derived from the same source. The Palouse of WA-ID-OR is one of North America's five loess regions, which include 1.) Alaska-Yukon, 2.) Palouse Hills, 3.) Snake River Plain, 4.) Great Plains, and 5.) Central Lowlands (Muhs and Bettis, 2003). Loess is sparse in Canada due to ice cover during the Pleistocene, when considerable fine grained sediment was generated. Thick loess is not reported in Mexico.
Palouse loess - The Palouse loess was deposited in tandem with Pleistocene scabland flooding beginning sometime after 2 million years ago. Loess derives from flood-laid sediments that was remobilized and transported to the north and east by wind (Busacca, 1989; Sweeney et al., 2017). Tephras from Cascade volcanoes, fossils, paleosols, flood-cut unconformities, and other crosscutting relationships have helped establish a chronostratigraphic framework for the Palouse. The loess is divided into three units (L1, L2, L3), where L1 is the youngest unit and L3 is the oldest. L2 and L3 contain conspicuous and widespread paleosols, a few of which are named: Washtucna Soil, Old Maid Coulee Soil, Devils Canyon Soil.
Palouse region geologists - Early ideas about Palouse soils and geology came from field investigations by Roald Fryxell, Donald Meinig, Kirk Bryan, Henry Smith, Joseph Pardee, Pierce Lewis, Ray Treasher, Israel Russell, Frank Calkins, Harold Culver, Rollin Salisbury, Robert Newcomb, Bruce Foxworthy, and others. Recent publications on the geology of the Palouse (1988-present) are by Alan Busacca (WSU, retired), Eric McDonald (DRI), Mark Sweeney (USD), David Gaylord (WSU), Paul McDaniel (U of I), their graduate students, and collaborators. Their work has focused mainly on the thickest loess deposits, paleosols within them, and sand dunes of the central Columbia Basin.
Map: Dry Hollow Site is located in a flood-swept coulee in the southern Palouse along the margin of Eureka Flat.
The Dry Hollow Site - The Dry Hollow Site is located in the southern Palouse Hills south of Clyde, WA. The outcrop contains an interesting contact between loess and flood rhythmites. L2 and L3 paleosols are truncated by Missoula flood deposits. The contact is abrupt and steeply-inclined.
Clastic dikes - At least two generations of sheeted, uncemented clastic dikes are present, too. The dikes don't go all the way to the top; they are truncated by the erosional surface, by certain bedding contacts in the rhythmites above, and by L1 loess. One dike, 4th from the right, pinches upward and dies out near the base of the L1. All others pinch downward.
Fuzzy geography - Some confusion exists concerning the location of the "CLY Site", a large double-sided roadcut at a pass on Lyons Ferry Road. The roadcut at the pass is either the CLY-1 or the CLY-2 named in various papers by the Busacca-McDonald-Sweeney team. Professor Nick Bader of Whitman College tells me its CLY-2.
Sketch map: Field notes from a recent visit.
Nodular loess - The two nodular loess units appear to be the Washtucna Soil above and an older paleosol below, mostly because of the vertical distance that separates them. Visually darker horizons, each ~1m thick, parallel and overlie both nodular zones. Nodules are cemented fossil feeding burrows of cicada nymphs. High sand content of the L2 loess (Sandy Loess between the two nodular units) is due to the site's proximity to Eureka Flat, a major source of dust and fine sand.
Modern soil - The youngest loess (L1 loess) that caps the outcrop is unusually thick - about twice as thick as seen in the Walla Walla Valley - but maintains a familiar profile and prismatic parting. Its thickness, still increasing, reflects the influence of nearby Eureka Flat.
Scabland influence - The valley of Dry Hollow, which today contains an underfit stream, clearly carried the water of a few late Wisconsin floods. Benches composed of flood sediment line the valley and a trimline is visible at ~320m elevation (1050'). The source of the water was probably a spillover out of the Snake River canyon (Cheney-Palouse scabland tract) somewhere between Lyons Ferry and Starbuck. Other flat-bottomed valleys in the vicinity contain similar geomorphic evidence of Pleistocene flooding.
Map: Dry Hollow merges with Winnett Canyon. I found a few large mammal bones in silty rhythmites in Winnett Canyon in 2007.
Deformation - Sandy-silty Touchet Bed material in the exposure is deformed and, in places, appears swirled where thin layers of coarse black sand grains provide contrast. Bedding contacts between the rhythmites are obscure, which is unusual. Bedding in Touchet Beds is typically clear. The dikes appear folded near the loess-Touchet Bed contact. The nodular loess that follows that contact is also a bit weird. There's >5m of relief on the contact. Is there slip on that surface in addition to erosion? Apparently so. A freshly bladed roadcut at the nearby intersection of Lyons Ferry Rd-Wiseman Rd (310m, 1017') contains at least 6 rhythmites, all quite thin. Several dikes with truncated tops cut that section, too.
Similar to Touchet Valley outcrops - Relationships at Dry Hollow closely resemble those found at Plucker Historical Marker Site and other locations in the Touchet River Valley. I believe the prominent erosional-deformational contact observed at Dry Hollow and Plucker is a correlative surface. See my sketch and notes on the Plucker outcrop in another post, "Two Generations of Clastic Dikes in Ice Flood Deposits" available at this link:
Local loess stratigraphy - This stratigraphic column for the CLY Sites (Sweeney et al., 2004, Fig. 4; Sweeney et al., 2017, Fig. 3) depicts the Washtucna Soil (L2) as welded. At Dry Hollow, located some 60m lower, more than a meter of sandy loess separates the two nodular soils. Is the sandy loess (L2) an eolian sand sheet (from Eureka Flat) or flood-laid sand (from a spillover flood moving down DH)? Is the nodular paleosol exposed at the base of the outcrop Washtucna, Old Maid Coulee, or Devils Canyon?
Soil terminology falls short - Palouse research is inherently interdisciplinary, falling somewhere between Geology, Soil Science, Paleoclimatology, and Botany. However, most articles are published in Geology journals. Where most of us will readily learn and adopt new terms introduced by other disciplines (i.e., phytolith, Artemisia, MIS 4, etc.), many geologists draw the line at Soil Taxonomy. Soil Taxonomy, with its byzantine terminology and horizon notation codes, remains a source of frustration for many readers with Geology backgrounds (Vitrandic Fragixeralf? 2Bkqm? Paralithic contact?). While an enormous amount of information is contained in soil profile descriptions, many routinely skip past it because the language of soils is a foreign language. Frankly, Soil Taxonomy has not proven relevant across the Geosciences despite decades of development by NRCS Soil Survey Staff. It is full of needless jargon - an intellectual stiff-arm to non-soil folks. Journal editors could solve the problem by simply requiring the use of both soils-specific and standard geological terminology in figures they publish.
Clastic dikes and Ice Age floodways - The CLY strat column (the outcrop at the pass), shown above, is entirely eolian; it contains no flood deposits and no clastic dikes. I've documented thousands of clastic dikes at hundreds of locations in Eastern Washington, but not one in purely eolian deposits. The dikes are restricted to Ice Age floodways, whether carved by pre-late Wisconsin floods or those of the Missoula cycle. The elevation of the pass is 378m (1240'), nearly that of Lake Lewis at its full-pool stage (380m, 1247'). We're about 45km NE of Wallula Gap. What should we expect to find way out here, at the shoreline of a highstand Lake Lewis, where waters were shallow and in competition with upland hillslope processes?
Paleosol interpretation - I've come up with three ways to interpret the nodular paleosols exposed at Dry Hollow. The upper nodular unit is almost certainly the Washtucna Soil (L2). The lower nodular unit may be a second Washtucna Soil (L2), the Old Maid Coulee Soil (L3), or the Devils Canyon Soil (L3). The last case implies the OMC Soil is present, but hidden (from my eyes anyway).
Flood trimlines in Dry Hollow - Oblique aerial photo-map of the Dry Hollow Site. The roadcut lies below the flood-scoured trimline (~320m elevation). Missoula floods moved west down the coulee toward Eureka Flat (left to right in photo). The CLY Site is located at the pass to the south. Stratigraphy there consists of loess-soil sequences typically of this area. At the Dry Hollow Site, both eolian hillslope sediments (Palouse loess) and flood-laid valley deposits (Touchet Beds) occur. I found no clastic dikes above the 320m trimline in roadcuts between Dry Hollow and CLY.
Ancient scabland floods - If ancient south-flowing floods ever did overtop the tidy passes of nearby Skyrocket Hills canyons (Piper, McFeely, Donnelly, Skyrocket) before emptying to Eureka Flat or the Touchet Valley, then an awful lot of loess has been deposited in the area since then. Evidence of floods at pass levels in the Skyrocket Hills (north of Prescott) might be buried.
Go outside - Dry Hollow is located in the southern Palouse on Lyons Ferry Road near the eastern margin of Eureka Flat (just W of Touchet Valley). The site is a 22 mile drive from downtown Walla Walla (30 minutes). Not a lot of great parking right at the roadcut, so look around. Traffic isn't heavy here, but does move fast. Don't block the driveway of the shooting preserve.
I was a college sophomore taking my first real Geology course when Professor Spencer first hauled us out here. We were to describe the CLY roadcut in detail despite the miserable cold and stiff winds of an early Spring day. It was my first encounter with the Palouse loess and its wild surfaces and paleosols. I recall thinking, "Loess ain't easy". Snot (and possibly tears) obscured my field notes.
Let me know what you find at Dry Hollow: skyecooley@gmail.com
