Definitions

arkose - A quartzose sandstone with greater than 25% feldspar grains. Usually interpreted to indicate a provenance with a granitic composition (granite, gneiss, others), and commonly interpreted as being deposited fairly close to the provenance region (ie., an immature sandstone).














boulder - A largish rock, generally greater than 256 mm along its long axis. The 256 mm value comes from the Wentworth particle size classification, originating in the 1920's. According to the Howell scale of grain sizes, a boulder is a largish rock most comfortably picked up with two hands (or larger).













conglomerate - A coarse-grained sedimentary rock, dominated by grains larger than 2 mm in diameter (pebbles or larger - see grain size).












detrital - referring to loose grains of minerals or rocks that have been eroded from other rocks or earth materials. A detrital sedimentary rock is one that is composed of grains that derive from preexisting rocks. A quartz sandstone, for example, is typically composed of quartz grains that were eroded away from an extant body of rock (igneous, metamorphic or sedimentary).














grain size - A reference to the nominal diameter of the particles that comprise a rock or other earth material. Particles may be interlocking crystals (as in igneous or chemical sedimentary rocks such as halite) or discrete detrital grains, as sand grains in a sandstone.

Fine vs Coarse grain size:  As a general rule, "fine-grained" refers to rocks with grains that are small or invisible to the naked eye, "coarse-grained" refers to grains easily visible (greater than 2 mm in diameter), and "medium-grained" refers to grain sizes between those two. This is strictly a general rule -- in an area dominated by very fine-grained shales, for example, a worker may refer to a locally silty layer as "coarse-grained", an informal, relative term that would bring tears of laughter to the eyes of a geologist more used to working in boulder conglomerates.

Wentworth grain size scale:  Charles Wentworth in 1925 published a classification scheme for detrital sedimentary grain sizes as follows.

Wentworth Grain Size Chart

Howell grain size scale:  An unpublished, widely disputed attempt to make intuitive the major subdivisions of the Wentworth scale.  Boulder is a largish rock most comfortably picked up with two hands (or larger).

Howell Grain Size Chart











sandstone - A sedimentary rock composed primarily of sand-sized grains. The sand grains may be of any composition -- specific types of sandstones are generally named according to the composition of the grains. Some examples:

  • Pebbly sandstone (contains a few pebbles, but not enough to make it a conglomerate)
  • Fossiliferous sandstone (contains fossils)
  • Arkose (a special name given to sandstones rich in feldspar grains)
  • Auriferous sandstone (a gold-bearing sandstone, such as the XXX placer deposit of S. Africa)














    • Last updated August 14, 1996.