Particle Roundness

Particle roundness is the degree of sharpness of the corners and edges of a sediment grain. ^[1] Grains with smooth corners are described as well rounded; grains with sharp, angular corners are poorly rounded. Roundness is the second-order aspect of particle shape - it describes the corners of the grain outline rather than the overall gross form. A grain can be highly spherical in overall form while still being poorly rounded at its corners, or vice versa; roundness and sphericity are independent properties that must be assessed separately.

Measurement

Wadell (1932) developed a mathematical formula that yields a value of 1 for perfectly rounded particles and smaller, fractional values for less rounded ones, analogous to the Krumbein sphericity formula. ^[1] Because measuring roundness mathematically requires tracing and analyzing every corner of a grain - a laborious process - most workers instead compare grains visually against a standard grain roundness scale such as the Powers scale, which displays reference images for each roundness class from very angular through rounded.

Controls on Roundness

Roundness is controlled by four main factors: grain composition, grain size, type of transport process, and distance of transport. ^[1]

• Composition: Hard, resistant minerals such as quartz and zircon resist rounding far more stubbornly than weaker minerals such as feldspars and pyroxenes.
• Grain size: Pebble- to cobble-sized grains are more easily rounded by abrasion than sand-sized grains. Resistant grains smaller than 0.05-0.1 mm do not appear to become rounded by any transport process.
• Transport type: The contrast between wind and water is dramatic (see below).
• Distance: Most rounding occurs early in transport; rounding does not increase proportionally with distance traveled.

Because composition and size both strongly influence how quickly a grain rounds, roundness comparisons between samples are only meaningful when made on particles of the same size and composition. Mixing grain sizes or mineralogies in a roundness study will produce misleading results.

Wind Versus Water Transport

Experimental studies in flumes and wind tunnels show that wind transport is 100 to 1,000 times more effective at rounding sand-sized quartz grains than water transport. ^[1] In fact, almost no rounding occurs during as much as 100 km of water transport. Studies of quartz grains in natural rivers confirm this: no measurable increase in rounding was detected across a transport distance of 1,100 mi (1,775 km) along the Mississippi River between Cairo, Illinois, and the Gulf of Mexico.

The difference arises because wind transports grains by saltation in air, where grain-to-grain collisions are far more energetic than collisions between grains in water. Surf action on beaches falls between the two extremes - more effective at rounding grains than river transport but less effective than wind transport - though its effectiveness is not well understood.

Inherited Roundness and Multi-Cycle Sediments

Once quartz grains acquire roundness, they do not easily lose it. ^[1] Rounded quartz grains can persist through multiple cycles of erosion, transport, and redeposition. This is a crucial interpretive complication: well-rounded quartz grains in an ancient sandstone may record an eolian episode somewhere in the grain’s history, but it may be impossible to determine whether rounding occurred during the last transport event or in a much earlier cycle.

Roundness of Pebbles

Pebble roundness is strongly tied to pebble composition and size. ^[1]

Soft lithologies such as shale and limestone round far more rapidly than hard, resistant types like quartzite or chert. Stream transport - which does relatively little to round sand-sized grains - is effective for pebbles: the distance required to achieve well-rounded pebbles ranges from as little as 11 km (7 mi) for limestone to 300 km (186 mi) for quartz. Most rounding occurs early in the transport history, within the first few kilometers; incremental rounding with continued distance is limited. ^[1]

Several important interpretive caveats apply: ^[1]

• Not a reliable distance indicator: The greatest rounding happens early; later transport adds little.
• Not exclusive to rivers: Pebbles can become rounded on beaches and possibly on lake shores too.
• Post-fluvial reworking: Well-rounded fluvial pebbles may later be carried into nearshore marine settings, entrained by turbidity currents, and redeposited in deep-ocean environments - carrying their rounded form into an entirely different setting.

Well-rounded pebbles in ancient sedimentary rocks are a general indicator of fluvial transport, but they cannot be treated as unequivocal environmental proof without supporting sedimentary structures and stratigraphic context.

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