Sediment Particle Size Classifier (Wentworth Scale)
Classify sediment by the Wentworth scale from diameter in mm or micrometers.
Returns phi (φ) value, class from clay to boulders, and Stokes settling velocity.
The Wentworth scale — a century-old standard
Chester K. Wentworth published his sediment size classification in 1922, building on earlier work by Johan Udden (1898). The Udden-Wentworth scale remains the global standard in sedimentology, geology, and engineering for describing particle size from microscopic clay to room-sized boulders.
The scale is logarithmic — each major class boundary represents a doubling of grain size:
| Class | Size range (mm) | Notes |
|---|---|---|
| Boulder | > 256 | Largest unconsolidated sediment; can require machinery to move |
| Cobble | 64 - 256 | Fist to head sized; common in glacial deposits, riverbeds |
| Pebble | 4 - 64 | Pea to fist sized; “gravel” in everyday usage |
| Granule | 2 - 4 | Smallest gravel class |
| Very coarse sand | 1 - 2 | Very gritty; high-energy environments |
| Coarse sand | 0.5 - 1 | Clearly gritty; river bars and high beaches |
| Medium sand | 0.25 - 0.5 | Clearly visible grains; classic beach sand |
| Fine sand | 0.125 - 0.25 | Individual grains barely visible without lens |
| Very fine sand | 0.0625 - 0.125 | Boundary between sand and silt |
| Silt | 0.004 - 0.0625 | Gritty between fingers but no visible grains |
| Clay | < 0.004 | Smooth, plastic when wet; sticks to fingers |
The phi (φ) scale
For statistical analysis, sedimentologists use the phi scale:
φ = −log₂(d in mm)
Each whole phi unit equals one doubling. The reason: grain size data is logarithmically distributed (small grains are far more numerous than large), so taking logs makes statistics tractable.
Key reference values:
| Size (mm) | Phi (φ) | Class |
|---|---|---|
| 256 | -8 | Cobble/boulder boundary |
| 64 | -6 | Pebble/cobble boundary |
| 4 | -2 | Granule/pebble boundary |
| 2 | -1 | Sand/granule boundary |
| 1 | 0 | Coarse sand boundary |
| 0.5 | 1 | Medium sand boundary |
| 0.25 | 2 | Fine sand boundary |
| 0.125 | 3 | Very fine sand boundary |
| 0.0625 | 4 | Sand/silt boundary |
| 0.004 | 8 | Silt/clay boundary |
A grain at phi 3 (very fine sand, 0.125 mm) is 8 times larger than a grain at phi 6 (silt, ~0.016 mm). Working in phi lets sedimentologists treat sediment as a normal distribution and use standard statistics (mean, sorting/standard deviation, skewness).
Sorting — how uniform is the grain size
Beyond mean size, sedimentology cares about sorting (how spread out the grain sizes are):
| Sorting | Standard deviation (φ) | Examples |
|---|---|---|
| Very well sorted | < 0.35 | Beach sand, eolian dune sand |
| Well sorted | 0.35 - 0.50 | Most beach sand |
| Moderately well sorted | 0.50 - 0.71 | River sand |
| Moderately sorted | 0.71 - 1.00 | Glacial outwash, some river deposits |
| Poorly sorted | 1.00 - 2.00 | Most glacial till, some debris flows |
| Very poorly sorted | > 2.00 | Glacial till, mass-flow deposits |
Sorting tells you about the depositional environment:
- Well sorted = stable single-energy environment (constant wind, calm beach)
- Poorly sorted = chaotic transport (glaciers carry everything, regardless of size)
- Bimodal = two distinct sources (e.g., flood deposit with sand + cobbles)
A well-sorted, fine-grained sandstone in the rock record almost certainly indicates beach or dune origin. A poorly-sorted, angular, mixed deposit suggests glacial or landslide origin.
Stokes’ law and settling velocity
For fine particles in still water, settling velocity follows Stokes’ law (George Stokes, 1851):
vs = (ρs − ρf) × g × d² ÷ (18 × μ)
Where:
- vs = settling velocity (m/s)
- ρs = particle density (~2,650 kg/m³ for quartz)
- ρf = fluid density (~1,000 kg/m³ for water)
- g = 9.81 m/s²
- d = particle diameter (m)
- μ = fluid viscosity (~0.001 Pa·s for water at 20°C)
Some typical settling rates for quartz in water at 20°C:
| Particle | Size (mm) | Settling rate | Time to settle 1 m |
|---|---|---|---|
| Boulder | 256+ | turbulent | seconds |
| Cobble | 64-256 | turbulent | seconds |
| Pebble | 4-64 | turbulent | seconds |
| Coarse sand | 0.5 | 6 cm/s | 17 seconds |
| Medium sand | 0.25 | 1.6 cm/s | 1 minute |
| Fine sand | 0.125 | 0.4 cm/s | 4 minutes |
| Very fine sand | 0.0625 | 0.1 cm/s | 16 minutes |
| Silt (coarse) | 0.03 | 0.07 cm/s | 24 minutes |
| Silt (fine) | 0.01 | 0.007 cm/s | 4 hours |
| Clay | 0.001 | 0.0007 cm/s | 16 days |
| Very fine clay | 0.0001 | turbidity stays suspended | months to years |
The dramatic range — boulders settle instantly, while clay stays suspended for weeks — is why lakes and oceans accumulate sediment in layered patterns. The biggest, densest grains drop first; the finest particles settle last (or never, until aggregating into floccules via salt water).
Stokes’ law breaks down at larger grain sizes (above ~0.1 mm), where flow around the particle becomes turbulent. Larger particles settle by more complex relationships involving Reynolds number and shape factor.
Why grain size matters in geology
Particle size affects almost every geologic process:
- Erosion potential: silt and fine sand erode at lower wind/water speeds than clay (counterintuitively — clay particles cohere). The Hjulström curve maps this.
- Permeability: grain size correlates with how easily fluids flow through. Pebbles = very permeable; clay = nearly impermeable
- Porosity: well-sorted spheres have ~36% porosity regardless of size; poorly sorted sediment has much less
- Sediment color / mineralogy: smaller particles disproportionately concentrate certain minerals (clay minerals, iron oxides)
- Fossil preservation: smaller grain sediments (silt, clay) preserve finer fossils
- Soil agriculture: clay-rich soils hold water and nutrients but drain poorly; sandy soils drain fast but lose nutrients
- Construction: foundation soil properties depend heavily on grain size distribution
The 12-bin engineering classification
For engineering applications (geotechnical, civil), the AASHTO and USCS (Unified Soil Classification System) systems use slightly different bins:
| USCS class | Boundary (mm) | Notes |
|---|---|---|
| Gravel (coarse) | > 19 | Above sieve #3/4 |
| Gravel (fine) | 4.75 - 19 | Above sieve #4 |
| Sand (coarse) | 2.0 - 4.75 | Above sieve #10 |
| Sand (medium) | 0.425 - 2.0 | Above sieve #40 |
| Sand (fine) | 0.075 - 0.425 | Above sieve #200 |
| Silt | 0.005 - 0.075 | Inert silt and rock flour |
| Clay | < 0.005 | Cohesive, plastic |
The sieve numbers refer to the number of openings per inch of mesh. Sieve #200 (0.075 mm) is the boundary between “fines” (silt + clay) and “sand.”
Practical field grain size estimation
You don’t always have a sieve set. Field shortcuts:
| Test | Result |
|---|---|
| Particles visible to the naked eye | At least sand size |
| Pass through a screen window mesh (~1-2 mm) | Sand or finer |
| Visible only with 10x hand lens | Probably very fine sand or silt |
| Smooth between fingers, no grit at all | Clay |
| Slightly gritty between fingers | Silt |
| Crunchy/gritty between fingers | Sand |
| Shows individual grains in palm | Coarse sand or larger |
The “rub the sediment between your fingers” test is surprisingly accurate after a little practice. Combined with appearance, you can field-classify any sediment within 15 seconds.
Sediment in the rock record
Lithified versions of sediments retain their original size classification:
| Sediment | Rock |
|---|---|
| Clay | Mudstone, shale, claystone |
| Silt | Siltstone |
| Sand | Sandstone |
| Granule + small pebble | Conglomerate (rounded clasts) or Breccia (angular clasts) |
| Mixed | Conglomerate (if rounded) or Diamictite (if poorly sorted) |
Bottom line
The Wentworth scale, 100+ years old, remains the standard for sediment grain size. The phi (φ) scale allows statistical work. Sorting tells you about the depositional environment as much as the grain size does. Stokes’ law predicts settling velocity for fine particles. Grain size determines everything downstream: erodibility, permeability, fossil preservation, agriculture potential, and engineering behavior.