Pick up a handful of sand from a beach and one from a landslide, and you are holding two different histories. Moving water and weathering change sediment the longer they work on it, and three features record how long: sorting, rounding and composition.
- Sorting: how alike the grain sizes are in one sample. Waves and wind carry away grains that are too fine and leave those too coarse, so beach and dune sand is well sorted. Glaciers and landslides dump everything together: poorly sorted.
- Rounding: how worn the corners are. Grains knocking together lose their sharp edges, slowly, over long distances or many trips.
- Composition: weathering destroys feldspar and dark minerals over time, while quartz survives. A sand that is over 90% quartz is mature.
Why it works
Each feature measures time and energy. Sorting needs a process that picks grains by size again and again, like waves sloshing back and forth for years. Rounding needs countless collisions. Mineral maturity needs time for slow chemical reactions. So a well-sorted, well-rounded, quartz-rich sand has had a long, busy history, while an angular, poorly sorted, feldspar-rich one came quickly from a nearby source.
Key idea
The farther and longer sediment travels, the better sorted and rounder it gets and the richer in quartz. Well sorted means the grains in one sample are about the same size.
Worked examples
Worked example: Which traveled farther?
Sample X: angular grains of many sizes, about half feldspar. Sample Y: rounded, well-sorted grains, nearly all quartz. Which traveled farther?
- Rounding, sorting and quartz-richness all grow with transport.
- Y shows all three; X shows none. Y traveled farther; X probably came from granite close by.
Answer: Sample Y.
Worked example: Counting quartz
Of 400 grains counted under a microscope, 372 are quartz. Mature or immature?
- $372 \div 400 = 0.93 = 93\%$ quartz.
- Over 90%: a mature quartz sand.
Answer: $93\%$ quartz: mature.
Common mistake
"Well sorted" sounds as if the sediment were sorted into separate layers. It means something else: within one sample, the grains are nearly all the same size. A single thin layer of dune sand is well sorted; a thick, layered pile of glacial till is still poorly sorted. Check by looking at one handful, not at the layers.