In contrast, clean, bright looking shades of gray, olive green, brown, red, yellow, and white are associated with inorganic soils. . Red, yellow, and yellowish brown result from the presence of iron oxides. . White to pink may indicate considerable silica, calcium carbonate, or aluminum compounds. The maximum particle size of each sample considered should always be estimated if not measured. This establishes the upper limit of the gradation curve.
Gravels range down to the size of peas.
Sands start just below this size and decrease until the individual grains can barely be seen by the naked eye. The eye can normally see individual grains about 0.05mm in size or about the size of the No. 200 screen. While the sample for grain sizes is being examined, the shapes of the visible particles can be determined. Particles may not be completely angular or completely rounded. These particles are called subangular or subaounded, depending on which shape predominates.