Abstract

The development of a new method to measure a behavioral shape property of sand grains is presented. This property "rollability" is related to the angle of slope on which a grain will roll down in air. It is measured on the inside of a slightly inclined revolving cylinder mantle. The grains emerge from the lower end of the cylinder and the time it takes them to reach this lower end is a measure for their rollability. The property "rollability" correlates well with the influence of grain shape in other processes such as settling in water. The rollability is measured on 1/4 phi sieve fractions. The measurements are transferred to relative rollability values. For a certain area, the mean of all the rollability values is calculated per sieve fraction and the individual measurements are expressed as percentile deviations of this mean value. In this way, the strong influence of the source of the material on the shape characteristics is largely excluded and the observed shape differences between samples can be attributed exclusively to shape sorting phenomena. This facilitates the comparison between sediments of same depositional circumstances but different source. Per sample, graphs are plotted which show the distribution of the relative rollability values over the size fractions. This property is called the Shape Distribution Character (S.D.C.). It appears to be characteristic for the mode of origin. Shape selection phenomena are best understood in connection with Kuenen's deposit-repository concept. When a population is split by a transport mechanism into two (or more) different populations, these populations have complementary characteristics as regards size, shape and density. Such a twin relationship comes out very clearly in the S.D.C.'s of the deposit and its repository. Examples are shown of such genetic relationships in the complementary character of the shallow off-shore zone with the beach, the beach and adjoining dunes, and the eolian cover sands with the areas from which the sands have been blown out.

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