South Slough Reserve, Oregon

Geology

The South Slough lies along a geologic fold, or syncline, which bears its name. Due to this downwarp, the watershed's eastern and western sides are of distinct geologic types, with different elevations and gradients. The eastern shore formation, which rarely exceeds 250 feet, is typical of the larger Coos estuary watershed. Its highly erodible Quaternary marine terraces of semi-consolidated sand, silt and clay are gently sloping and worn down along creek beds to sandstone and siltstone overlain by loamy sand and sandy and silty loam.

The western side's Empire formation, with scattered Quaternary terraces, is unique to the South Slough. Its hard, impermeable marine sandstone rises 370 feet above sea level, in a long, steeply sloping north-south ridge. The western slopes are mantled with sandy and silty loam and loamy sand.

Soil Types

The predominant soil type throughout the slough is Templeton silt loam with medium to high runoff and erodibility. Bullards sandy loam is present on the northeastern highlands. Like much of the Coos estuary, the tidal and intertidal portions of the slough are primarily soft sediments, with concentrations of coarse marine sand and silts in more exposed regions.

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