Each of the images created for this publication is the result of a geometry experiment conducted by computer. For each experiment, the code uses a list of parameters that define the morphology and behavior of a particular bedform or assemblage of bedforms. The parameters specified for each experiment include the spacing, steepness, asymmetry, migration direction, migration speed, plan-form shape, and along-crest migration speed of plan-form sinuosities of each set of bedforms (Appendix A). Because bedform morphology and behavior can vary through time, the code is capable of varying most of these parameters cyclically through time; the magnitude, period, and phase of variations in any changing parameter must be specified. Specifying these parameters for as many as three sets of simultaneously or alternately active bedforms, specifying how the various bedforms are to be superimposed, specifying the rate of deposition, and specifying changes through time in the rate of deposition requires 75 geometric parameters.
With these input specifications, the code uses sine curves to create surfaces that approximate the shape of bedforms. Di splacement of the sine curves simulates bedform migration, changing amplitude simulates changing bedform height, and combining separate sets of sine curves simulates superpositioning of bedforms. The code is capable of combining the curves according to several different rules: simple addition, adding a percentage of the superimposed curve that is either proportional or inversely proportional to the local elevation of the main curve, or selecting the curve that locally has the greatest elevation (Appendix A). All rules except the latter produce bedforms that generate foresets with tangential basal contacts; the latter rule produces foresets with angular basal contacts.
Figures & Tables
The computer modeling that forms the basis for this publication was undertaken to relate the geometry of cross-bedding to the morphology and behavior of bedforms. Using computers for this purpose is necessary only because sedimentologists cannot adequately visualize the geometry arising when complex, changing surfaces move through space. Images range from the complicated, which will be of interest to sedimentologists who have had experience interpreting cross-bedding or studying the behavior of bedforms, to simpler illustrations the can convey an understanding of the origin of cross-bedding geometry even to those not experienced in the field.