Stoss slope trenches at Coral Pink Sand Dunes State Park, USA

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Locale: Coral Pink State Park, Utah, USA
GPS coordinates: 37.052627, -112.710652 (WGS84)

Fig 1. Three trenches on the stoss slope of a transverse dune.

The finer grains in pinstripe laminae preferentially conduct water, which can be exploited to make them more visible (see Methods: Making pinstripe laminae visible). But how to apply an even layer of water over a large area? Rainfall during a visit to Coral Pink Sand Dunes State Park did the job, evenly moistening the top few centimeters of sand over a large area, creating an opportunity to dig trenches to reveal the internal structure of the dunes.

Three trenches were dug on the stoss surface of a transverse dune, perpendicular to its travel (as indicated by erosion-exposed lineations on the stoss surface of former slipfaces), as shown in figures 1 and 2. Figure 1 was photographed from the brink of the next upwind transverse dune (estimated 189m brink-to-brink). The heights of the trenches and dune were estimated using photogrammetry.

Based on historical Google Earth imagery, the dune travelled an average of 2.9 meters per year from 1997 to 2015 (see Dune migration at Coral Pink Sand Dunes). That permits rough estimates of when the slipface was at the location of each of the trenches, as indicated by the year labels in figure 2.

Each trench exposes sand that was deposited at that elevation on the slipface of the dune at the time of deposition. The heights labelled in figure 2 assume a near-horizontal interdune surface upon which the dune migrated.

Fig 2. Trench locations. The dune migrated right-ward. Estimated year of deposit, at each trench location, is indicated, based on an average migration rate of 2.9 m/yr (from historical imagery).

The dune was likely disturbed by park visitors (although this section of the park is less visited).

Below, figure 3, are photos of segments of the vertical sections exposed by the three trenches. The three are approximately the same scale; there is a red rope running along the edge of each trench, marked with white paint at one meter intervals.

The middle and lowest trench sections have abundant grainflow/pinstripe laminae, but the highest trench section appears to have none. Likely the highest trench was in sand that was deposited as wind ripple upon the crest region.

All the trenches are viewed from the side on the right in figure 1; in each trench photo, the brink is in the direction to the right.

Fig 3a. Highest trench, at 7.3 meters. No grainflow or pinstripes are visible. Presumed to be wind ripple deposits.

Fig 3b. Mid-level trench, at 4.2 meters. Abundant grainflow/pinstripes present.

Fig 3c. Lowest trench, at 0.6 meters. Abundant grainflow/pinstripes present.

The moist region around pinstripe layers diminished in thickness as the sand dried after being exposed by excavation, becoming thinner and eventually disappearing. The thicker pinstripes in figure 3c, compared with figure 3b, is mostly due to a difference how long they were exposed; the exposure in figure 3b had slightly more time to dry.

The extent of the moistened sand is probably proportional to the thickness of a pinstripe. Two pinstripes close together probably enhance conduction into the gap between them.

Below, figure 4, is a photo of a trench cutting through the dune brink. The lineation about a meter back from the brink is probably a reactivation bounding surface caused by reverse-direction wind which eroded the brink. Subsequent dominant-direction wind rebuilt the brink, with little grainflow until the brink reached its former position.

Fig 4. A trench through the brink of a transverse dune. The low-angle lineation about a meter back from the brink is probably a reactivation surface caused by reverse-direction wind.

Fig 5. Rain-moistened sand on a slipface, broken by human footsteps. Dry sand underlays the approximately 2 cm of moist sand.

Figure 5 shows foot tracks on a slipface, breaking the cohesive rain-moistened sand overlaying dry sand. The cohesion probably arises from surface tension in water at grain contact points. The grains may also have a clayey coating, as residual cohesion has been observed even after the sand dries.

Full-size versions of these photos are available.