The Wentworth Scale Phi (φ) = -log 2 (Diameter in mm) Best way to remember: – 1mm: φ = 0 – φ increases as diameter decreases – Every factor of 2 change in diameter = one step in φ
Particle Descriptions NamePhiDiameter mm Boulder< -8256 + Cobble-6 to -864 - 256 Pebble-2 to -64 - 64 Granule-1 to -22 - 4 Sand-2 to 41/16 - 2 Silt4 - 81/256 - 1/16 Clay> 8< 1/256
Sediment Sizes and Clastic Rock Types Sedimentary rocks made of silt- and clay-sized particles are collectively called mudrocks, and are the most abundant sedimentary rocks. Rock TypeSedimentGrain Size ShaleClayless than 0.001 mm SiltstoneSilt.001-0.1 mm SandstoneSand.01-1 mm ConglomerateGravel1mm +
Super-Size Me Blair and McPherson, J. Sed. Res., v. 69, no. 1, Jan. 1999, p. 6-19. Block: 4-65 m (φ = -12 to -16) Slab: 65-1000 m (1 km) (φ = -16 to -20) Monolith: 1-33 km (φ = -20 to -25) Megalith: 33-1000 km (φ = -25 to -30)
Some Special Clastic Rock Types Arkose Feldspar-Rich Breccia Angular Fragments Graywacke Angular, Immature Sandstone
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