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Pacific Coastal & Marine Science Center

Bedform Sedimentology Site: “Bedforms and Cross-Bedding in Animation”

Cross-Bedding, Bedforms, and Paleocurrents

Photographs

Photo of rock or sand showing pertinent structure or structures; see caption below.

FIG. 20.  Structure formed by reversing ripples; modern fluvial deposits, Colorado River, Grand Canyon National Park, Arizona. The computer-generated image of this depositional situation is shown in Figure 19.

RECOGNITION: Most of this example was deposited by symmetrical ripples that reversed migration direction.  Symmetry of the ripples is demonstrated by the shape of the rippleform laminae, and reversals in migration direction are demonstrated by the millimeter-thick ripple-crest zig-zags produced by deposition on alternating flanks of the ripples.  At least one flow reversal occurred on a longer time scale and caused some ripples to become asymmetric and migrate several centimeters to the left (indicated by arrow). Horizontal sections cut into this bed showed the crestline to have a slight three- dimensionality that is not detectable in this vertical section. Flow reversals in the Colorado River result from waves generated by rapids, from migrating eddies and boils, from fluctuations in the point of eddy reattachment along the river banks, and from the creation and destruction of eddies as river stage changes (Schmidt, 1986).

 

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