Harder Road - Calcrete Over pre-Wisconsin Flood Gravel

Harder Road Site. Harder Rd crosses the Cheney-Palouse scabland tract. This roadcut is mentioned in Baker (1973) and other articles on scabland flooding, but nowhere well photographed. Its not a big exposure, but contains an advanced-stage calcrete (>Stage III) developed in an upward-fining deposit of sandy loess over >4m of pre-Missoula flood gravel. What we see at Harder is a remnant of a former landscape, one where this blanket-like calcrete crust (a dryland paleosol) was present across a broad, lowland swath of Eastern Washington. Younger outburst floods have removed much of it.

Loess islands. Dark areas of windblown silt (Palouse loess), which have strong NE-SW topographic grain to them, are coursed by mostly south-trending scabland channels. The modern channel of Rock Creek appears in white at right. Cow Creek is on the left. The Harder Rd site lies at the edge of a loess island. Basemap is an excerpted lidar relative elevation image by Daniel Coe of the Washington Geological Survey.

A diminutive exposure. The 4m of loose scree covers a thick flood gravel deposit. Just uphill, to the east, gullies expose loess and caliche paleosols. Several other roadcuts in the area reveal similar strata, sometimes with gravel below (Calloway, Marengo, Revere, Beckley, Benge, etc.).

The gravel. The flood gravel is cross-stratified with imbrication consistent with southward transport seen in the lidar image. Mostly basaltic clasts, pebbles to boulders, sandy matrix.

Old gravel, old floods. Calcretes take tens to hundreds of thousands of years to form, which means the underlying gravel is even older. That's why this site is important. It shows scabland flooding began well before the Missoula cycle (18-14 ka).

Overprinted gravel. Pedogenic carbonate engulfs the top of the gravel. Clasts are entirely surrounded by secondary CaCO3. Platy calcrete, probably in fine grained parent material (loess) is seen above in sunlight. Morphology of the calcrete changes with grainsize of the parent material and position in the soil profile. It is common to find the uppermost 20cm or so is fully plugged and very hard in thick calcretes of the region.