6.13: Unique Characteristics of Lithogenous Deposits and Rounding of Sediment Grains
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Unique characteristics of lithogenous deposits
Sediments preserve other characteristics that may tell information about the environment where they occur. Sediment particle shapes (rounding), degree of sorting, and bedding characteristics are typically unique to different geologic settings.
Rounding of sediment grains
When particles are moved by running water they become rounded ("roundness" is illustrated in Figure 6.51). The corners hit first and are worn down. The sharp edges are also pounded. The particles may become round boulders or pebbles. Bits of sand move with them. As the water slows the largest particles drop out first, making deposits of round boulders and pebbles called conglomerate. The smaller particles are swept away downstream (unless they are trapped between or beneath the large particles).
Figure 6.51. "Roundness" of sediment grains: The farther a particle is moved, the more rounded and spherical it should become. Angular particles tend to be deposited close to their source, they become more rounded the farther they travel downstream. Grains of beach sand are typically well rounded. Dune sand is typically even more rounded and better sorted (Image from Powers, 1959).