At what size does rock become gravel?
-280 mesh (280 openings to the inch is clay sized particles
+280 mesh to -200 mesh is silt size particles
+200 mesh to -40 mesh is fine sand
+40 mesh to -#8 is med sand
+8 screen to -#4 screen is coarse sand
+#4 screen to 3" is gravel
+3" to -12" are cobbles
+ 12" are boulders
This is by the Unified Soils Classification System the professional Engineering firms use.
Natural gravel comes from colluvial deposits where gravity has carried rock that is decomposing on a mountain side, gravity and runoff transport it and it loses it's angularity from abrasion as it travels to where it stops in what is referred to as colluvial fan. Most of this rock is still angular to sub angular in shape. Colluvial rock that makes it to the creek or river becomes alluvial in nature, it is transported by moving water and depending on the energy of the water movement it can really be reduced in size and shape. The more movement the more abraision reducing it's angularity the farther it travels the more rounded it gets as the water movement slows,! the energy to carry heavier rock disappears and the heavier rock settles in place. By the time you get to fairly flat terrain the water velocity only has enough energy to carry silt to clay sized particles. Not all rock is crushed ,much is just sorted by size by screening and / or is recombined to give optimum structural performance. 36 years of working in the earth materials field, private and public sector, BS degree in Earth Science / Geology, and 25 years as a registered Professional Geologist with the WPGA. Any more questions on gravel ?.Mb
