Particles larger than sand size are easily identified with the naked eye. The third digit in the Ferm code stands for the composition of the pebbles in pebbly sandstones and conglomerates. Seven pebble types are commonly found:

  • 7X2 Shale
  • 7X3 Ironstone
  • 7X1 Shale and Ironstone
  • 7X4 Quartz
  • 7X5 Rock
  • 7X8 Coal Bands
  • 7X9 Coal Spars

Pebbles and other large particles in sandstones and conglomerates are common near the base of thick, crossbedded, and massive sandstones with sharp, scoured bases. Shale pebbles are gray to black, with irregular frayed or worn edges. Ironstone pebbles are orange to brown, and have rounded or oval shapes. Quartz pebbles are milky white or gray and have smooth, rounded edges. Rock pebbles may be igneous or volcanic, metamorphic, or sedimentary, and typically have a variety of different colors and textures. Coal bands are thin layers of fragmented coaly material comprising a significant percentage of a sandstone lithology. The individual particles are visible on the butt end of the core and resemble charcoal. Coal spars are irregular, frayed fragments of shiny coal. They commonly occur at angles to the horizontal plane. Coal bands and spars may occur in conjunction with other pebble types and the other types take precedent for classifying the rock.

Shale pebbles in a conglomerate (742).
Ironstone pebbles in a conglomerate (743).
Quartz pebbles in a conglomerate (754).
Assorted rock pebbles in a conglomerate (745).
Thin coal bands or laminae in core (748). These are not rooted in place, so they must be transported coal particles.
Coal spars in sandstone core (749).

 

 

Last Modified on 2017-06-30
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