Rulo Site - Walla Walla Valley, WA

Rulo is a classic Palouse outcrop. The Rulo site is a large, double-sided roadcut located a few miles northwest of Walla Walla along Sudbury Road. Roadcuts and nearby outcrops expose what at first appears to be the same-old, same-old innards of a Palouse loess hill, but upon closer inspection proves to be much more interesting. Faulted Miocene basalt bedrock is overlain by locally-derived gravel and an unusual micaceous, fossiliferous fluvial deposit of Pliocene age. Above, lie a number of cross-lapping beds of windblown silt - Pleistocene Palouse loess in its classic form, overprinted by calcic paleosols and interfingered with sandy flood rhythmites deposited by both Missoula floods and floods far older. Elevation of the site is ~285m, well below the maximum level of Lake Lewis (366m). Distinct horizons were chewed through by innumerable cicada during the Pleistocene. Waves of cicada thrived beneath a long-lived sagebrush plant community and left behind a clear record of their presence in thousands of backfilled burrows. Several whispy tephras can be seen in the stack, few of which have their age well constrained. Four generations of sheeted clastic dikes, each truncated by local erosional surfaces, cut conspicuous paths through the high exposure. A tumbleweed-choked gully just west of the road reveals a slug of reworked and over-thickened ash from the Holocene Mount Mazama eruption. Perhaps most remarkable thing about Rulo is that the entire Miocene to Holocene vertical succession occupies just a few 10s of meters and lacks a major unconformity. The landscape has not changed all that much in the past few million years here along the northern margin of Walla Walla Valley, where the Palouse Hills meet the Channeled Scabland.

West side of roadcut.

The Rulo five. Five unconformity-bounded units recognized at Rulo by Bader et al. (2016, Fig. 3). Note how clastic dikes truncate at erosional surfaces - a phenomenon I've documented at numerous other outcrops in the Channeled Scablands.

Easy access. The roadside exposure accommodates both standers and climbers. Good views are had from the road shoulder to those of the former group, while clearer details are had by more intrepid geologists willing to kick a few steps and eventually slide back down (with varying amounts of grace).

Just grade it. Seasonal collapses and the County's team of enthusiastic grader operators regularly refresh the cut faces at Rulo.

Local unconformities separate units. Four generations of sheeted Touchet-type clastic dikes are associated with packages of sediment at Rulo. The dike sets are truncated by erosional surfaces (a,b,c,d), highlighting the fact diking was repetitive. Note the dikes never crosscut younger horizons, only older ones; they are not liquefaction features. Sheeted clastic dikes in Eastern Washington are found exclusively within the margins of scabland floodways. My sketch condenses the stratigraphy described by Bader et al. (2016) into a fieldbook-friendly column.

Water-worked. Channel fills containing small cobbles hint at alluviation in wet swales, common in the Palouse both then and now. Weakly-developed to non-existant bedforms are par for the course when only silt is the only sediment transported by flowing water.

Thrust fault. A small thrust displaces Miocene basalt and an overlying gravel containing locally-derived basaltic clasts. A thin, mica-bearing sandstone unit lies just above and is also involved in faulting. The reddened, silty material above remains undeformed. Timing of the rupture predates deposition of at least a portion of the Pleistocene section, indicating the fault formed at least a few thousand years ago and likely more than 10 ka.

Basaltic gravel. Angular to subrounded basalt clasts comprise a gravel (stratified stream deposit) laid down atop the weathered surface of a Miocene basalt flow. The clasts were certainly derived from one or more basalt flows nearby and likely from this flow - once loose detritus and colluvium picked up and transported a ways by flowing water. Micaceous, crossbedded fluvial deposits are found deep in some Palouse hills; they form fairways across the Palouse and follow an east-to-west, pre-Pleistocene drainage pattern. The mica is from Idaho.

Whitman College contributions. We understand the geology at Rulo in large part due to the work of Dr. Patrick Spencer and several of his students from Whitman College. His most important article is Bader et al. (2016). I hope its not the last.

Rip-ups not nodules. Rip-ups of calcrete in a weakly-stratified, silt-sand diamict attest to reworking of loess hills (hillslope deposits) by flowing water (channels). It can be difficult to pick these units out at first, but once you see diamict, you cannot unsee it.

Calcrete thoroughly cements (overprints) densely cicada-burrowed loess.

Delicate details in many cicada feeding burrows (trace fossils) remain.

Feeding burrows sometimes weather out as clasts.

An unconsolidated, sheeted clastic dike cuts weakly-cemented, cicada-burrowed silt. East side cut.

A cemented, sheeted dike cuts cemented cicada-burrowed silt. West side cut.

Clastic dike cuts calcrete.

Truncated top. A sheeted dike with two branches extend downward through silty, oxidized, slightly-cemented sediment, its top truncated by an erosional surface mantled by white calcrete. Oxidation overprints both dike and host sediment, indicating they aged together at least for some period of time. Sediments below the erosion surface are middle to early Pleistocene age (many tens to a few hundred thousand years old). Sediments above the surface are mostly late Pleistocene age. lies a stack of calcic paleosols developed in loess (mostly). Pencil for scale.