Abstract

Surficial folds have formed by rock creep on dip slopes in siltstone and diatomaceous shale. These folds occur on slopes ranging from 20 to 50 degrees (2.75:1–0.8:1). Their axes are commonly parallel to each other and to the strike of the slope. The down-slope limbs usually dip more steeply and are frequently faulted. The amplitude is as great as 18 feet, although the average is about 6 feet. Many of the folds are separated from the underlying and uniformly dipping shale by a décollement. Although the folding mechanism is now essentially inactive, owing to increased aridity, this creep phenomenon should not be ignored in slope-stability analyses.

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