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Reese Coulee - Walla Walla Valley, WA

The Reese Coulee site is a two-sided roadcut along Hwy 12 between Wallula Junction and Touchet, WA. The cut is important because Missoula flood deposits (Touchet Beds, <20 ka) and older deposits (cemented, reddened diamicts of probable flood origin) are exposed together. Flood-cut coulees and an isolated, conical hill are found nearby.

South-facing roadcut at Reese Coulee (Ninemile Hill) exposes rubbly, cemented diamicts filling one of several channels cut into the basalt. The diamicts - here labeled 'alluvial fan-loess-calcrete complex' - are unconformably overlain by flat-lying Touchet Beds and some Holocene loess. The way the diamicts drape the bedrock gives the impression flow was to the west, but the configuration of local coulees suggests deposition was oblique to the cut face.

Angular and rounded clasts interbedded with loess with caliche horizons (calcic paleosols) are interpreted as either ancient overland flood deposits (process = glacial outburst flooding, wind, soil development) or an alluvial fan-loess-calcrete complex (hillslope and eolian processes with calcic soil development). Ancient flood deposits and hillslope complexes exhibiting identical characteristics are found in numerous places elsewhere in the floodscape.

Reddish diamict beds are a mix of broken, angular basalt, a few non-basaltic clasts, and reworked silt. Ancient overland floods swept rocky colluvium and old Palouse loess from local hillslopes, depositing it here. Calcium carbonate (calcic horizons) cement several of the beds. Sheeted clastic dikes, sourced in the overlying Touchet Beds, descend through the pre-late Wisconsin diamicts. Older clastic dikes within the diamicts and truncated by the Touchet Beds are present along with opal-filled fractures. I interpret all of the dikes as syn-depositional structures triggered by forces generated during deposition of the sediment, not seismites.

Reworked silt comprises a significant portion of some beds in the diamict unit. Also, note how angular many of the clasts are. Good evidence that bedrock - and more importantly its broken, looser, colluvial cover - is nearby. Angular clasts were picked up and transported from nearby hillslopes. Rounded clasts attest to longer distance transport or a two-stage depositional history. Some rounded clasts might be remnants of an older landscape - fluvial gravels originally deposited by Tertiary streams and later reworked by Pleistocene currents. Both angular and rounded clasts occur at Reese Coulee.

North-facing roadcut exposes a truncated basalt surface draped by diamict beds, but fewer Touchet Beds. The truncation surface on the basalt is fairly low angle here in contrast to the steeper matching surface in the south-facing cut.

Flood-cut troughs truncate Miocene basalt. The orientation of 3 trough axes indicates overland floods flowed NW-SE and roughly parallel to the Walla Walla River, but in the upvalley direction. Do crosscutting relationships show us how old these troughs are? The Walla Walla River flows west, joining the Columbia at Wallula Junction. An isolated hill of basalt, similar to that noted by J Harlan Bretz at Hooper (Washtucna Coulee), is found uphill and NE of the roadcut (figure below). Profiles made in Google Earth.

Yellow arrows are upvalley flow features (backfloods) at Reese Coulee according to me. Arrows point downhill or across slopes. The Walla Walla River valley existed prior to Ice Age flooding, a pre-glacial drainage with a similar amount of incision/relief as it has today. Reversing the direction of the yellow arrows (i.e., reinterpret the gullies as features formed by westward/down-valley flow) makes less sense. The surface of a gravel bar along the Walla Walla River appears ornamented with semi-giant current ripples. Also, a strong (NE-SW) fracture set in the basalt to the north (off the image) controls a set of gullies oriented differently than the yellow arrows at Reese Coulee. Did gullies to the north carry spillover floods from the Snake River into Walla Walla Valley prior to the late Wisconsin Missoula floods? Base image from Washington Lidar Portal.

J Harlan Bretz noted this isolated, cone-shaped hill of loess that resembles the isolated, cone-shaped hill of basalt north of the roadcut at Reese Coulee (see map above). Lots of similar landforms elsewhere in the Scablands.

Richard Waitt and I visited Reese Coulee on a rainy morning in Spring 2021. I first came here on college field trips (early 1990s), but didn't learn much, so I've returned a few times over the years - mostly when in-route to a margherita slice at Sweet Basil Pizzeria.


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