Ripple marks and cross-bedding have been studied at more than 160 localities in the lower part of the Morrison Formation (Jurassic), Rio Arriba County, New Mexico. The cross-bedding represents running water, and hence ground slope, during the time of Morrison deposition. As during the times of Chinle (Triassic) and Entrada (Jurassic) deposition, the ground slope was down toward the west-northwest. (A check of cross-bedding higher in the Morrison Formation, and in overlying Dakota sandstones, showed a slope toward the northeast.)

Average ripple-mark spacing in the lower Morrison sandstones was 3.43 cm (maximum, 7.0 cm; minimum, 0.7 cm). Approximately 60 percent of the examples exhibited perfect symmetry. With four exceptions, ripple indices were in the range, RI = 5.0 to RI = 15.0. A study of ripple-mark straightness and parallelism also was made. All of the computed parameters indicate a wave origin, except for the 40 percent of the examples having slight-to-moderate asymmetry. This latter observation cannot be used, despite other evidence, to deny a wave origin. Modern ripple marks of this type commonly appear in water less than 2 or 3 m deep.

Wave-tank observations indicate that similar ripple marks are formed by waves having heights of less than 5 cm and lengths (between crests) of 2 or 3 m or less. The fetch necessary to generate such waves can be measured in hundreds of meters. The lower Morrison ripple mark fields are, therefore, assigned to a shallow lake or pond origin.

Ripple-mark crest orientations are approximately north-south, and the direction of transport, deduced from asymmetry, was toward the west-northwest; hence a current bias (down the regional slope) may have been important in providing asymmetry to these wave-formed features. The orientation requires, however, that the wind blew from either east or west. This is not in agreement with wind direction deduced from eolian cross-bedding and eolian ripple marks in the Entrada Formation in the same area.

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