Pacific Coastal & Marine Science Center
Bedform Sedimentology Site: “Bedforms and Cross-Bedding in Animation”
FIG. 54. Horizontal section through structures deposited by bedforms with migrating lee-side spurs or superimposed bedforms; fluvial deposits, Colorado River, Grand Canyon National Park, Arizona. The area shown is approximately 30 cm from left to right.
RECOGNITION: The crestlines of the main bedforms (fluvial bars or dunes) that deposited these beds trended from the upper right of the photograph to the bottom center, as indicated by imaginary lines connecting the fingertips of adjacent scour-pit paths. Scour pits are inferred to have been bounded by the main bedforms and by the crests of a set of superimposed bedforms, such as ripples, or bounded by lee-side spurs. Scour-pit paths are controlled by three vectors: migration of the main bedforms (normal to their crestlines in a left-to-right direction), migration of the ripples or spurs along the trough of the main bedforms (bottom to top in the photograph), and deposition (upward through the horizontal section). The scour-pit paths (indicated by axes of the trough- shaped sets, which are seen in horizontal section) are oriented at approximately 45 degrees to the main crestlines, which indicates that the rate of along-crest migration of the scour pits was approximately equal to the migration speed of the main bedforms.