Introduction

The size of a sedimentary particle is commonly described in terms of a characteristic length. If sedimentary particles were all the same regular shape and composed of the same material, the choice of an operational definition of size (length, area, volume, sieve, settling velocity) would not be a problem because the various definitions could be mathematically related. Sediments, however, are variable in shape and composition, causing the choice of a definition of size to be more critical.

A complex three-dimensional description of populations of irregular particles is currently impractical on a routine basis. For simplicity and ease of analysis, particle size is currently related to an “equivalent” particle such as a circle or a sphere. This permits the use of a single length to be a complete but idealized descriptor. Unfortunately, because there are multiple techniques to arrive at a characteristic length, the reported size of the particle becomes partially dependent on the technique employed.

Commonly a complete size description of many sedimentary units cannot be achieved by one measurement technique because of the great range of sizes in the sample. This necessitates the physical separation of the sample into two or more fractions and often involves the mathematical combination of two or more measurement techniques. Standard analytical procedure for the determination of grain size distributions (Krumbein & Pettijohn, 1938; Folk, 1974) usually consists of separating the population into a coarse and a fine fraction by wet sieving at 62 µm.