Wentworth-Udden Particle Scale
A common system of grading the size of grains in a rock or sediment. If the grains can be seen the grade is at least silt, if it is not gritty between the teeth it is clay.
| >256mm | boulder |
| 64 to 256mm | cobble |
| 4 to 64mm | pebble |
| 2 to 4mm | gravel |
| 1/16 to 2mm | sand |
| 1/256 to 1/16mm | silt |
| <1/256mm | clay |
