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Soil Survey Manual - Table 3-11

Terms for rock fragments

Shape and size1  Noun  Adjective
Spherical, cubelike, or equiaxial:    
2-75 mm diameter  Pebbles  Gravelly
     2-5 mm diameter  Fine  Fine gravelly
     5-20 mm diameter  Medium  Medium gravelly
     20-75 mm diameter  Coarse  Coarse gravelly
75-250 mm diameter  Cobbles  Cobbly.
250-600 mm diameter  Stones  Stony.
>600 mm diameter  Boulders  Bouldery.
Flat:    
2-150 mm long  Channers  Channery.
150-380 mm long  Flagstones  Flaggy.
380-600 mm long  Stones  Stony.
>600 mm long  Boulders  Bouldery.
1. The roundness of the fragments may be indicated as angular (strongly developed faces with sharp edges), irregular (prominent flat faces with incipient rounding or corners), subrounded (detectable flat faces with well-rounded corners), and rounded (flat faces absent or nearly absent with all corners.