BROADENING OUR CONSIDERATION OF BED FORMS

Two basic aspects of bed forms need consideration in the future. (1) combined flows: In nature, many flows that generate bed forms are multidirectional rather than unidirectional. Most experiments and observations of bed forms have been in unidirectional flow, for reasons having to do with practical difficulties. Combined-flow bed forms must be common in nature, but it is difficult to recognize such bed forms in the sedimentary record. (2) Density ratio: we have dealt almost entirely with only two points in the spectrum of ratios of sediment density to fluid density: quartz-density sand in water, and quartz-density sand in air. Can we hope to gain a full understanding of the dynamics of bed forms with little knowledge of the extremely wide range of density ratios? We earthlings are limited in our ability to set up flows with a wide range of density ratios, but there must be zillions of places out there in the universe where bed forms are being produced in flows with greatly different density ratios.