The difference between sand and gravel is simply the size of the material in question. In this article we will expand on this size classification, as well as reasons behind variations in size. Sand is a granular material derived from the erosion of rocks, ranging in size from 0.075 mm to 4.75 mm. Sand particles are larger than silt but smaller than gravel. Gravel is a granular material derived from the erosion of rocks, ranging in size from 4.75 mm to 75 mm. Gravel particles are larger than sand but smaller than boulders.
Erosion is the gradual destruction of a landform through natural physical or chemical processes. Cyclical freezing and thawing during the winter can heave and crack bedrock, transported downslope by ice and flowing water in the spring. Eventually, water and pounding wave action can abrade even the most resistant rock to smooth round shapes. This is an example of physical erosion. Chemical erosion includes rock being dissolved by naturally-occurring sulphuric and organic acids, especially carbonate-rich rock such as limestone.
Deposition of transported, eroded material occurs when material enters a lower-energy depositional environment. For example, when transported material reaches the mouth of a river or stream (at its entrance to an ocean/lake) a river delta or outwash plain can form. As water speeds decrease, the force necessary to transport material is lost. The largest and densest material such as boulders and cobbles will be deposited first, then smaller material such as stones and sand, finally ending in silt and clay. This type of water-lain action will effectively sort material by size and density forming sand and gravel deposits that may be useful in building and road construction.
As briefly defined in the table above, sand and gravel are both loose aggregations of rounded products of erosion that differ only by grain size. Size classifications vary, but generally well-sorted sand has individual grain sizes of between 0.0625 mm and 4 mm, whereas gravel has individual grain sizes of between 4 mm and 75mm. The qualifiers ‘coarse’, ‘medium’, and ‘fine’ often precede ‘sand’ and ‘gravel’ to subdivide these size ranges further.
Shaw Resources derives the following two aggregate products from the same source (rocks eroded and deposited by glaciation into esker landforms, but screens the material in order to produce various gradations of products. The first product, Brown Play Sand, is classified as a sand whereas the second product, Natural Pea Stone is classified as a gravel.
Depending on the application, the degree of sphericity (roundedness) of sand and gravel is also important. When increased angularity is needed and cannot be sourced from naturally-occurring gravel or sand deposits, the material can be made through industrial crushers and screeners which source material from a rock quarry. Road and railway track ballast often requires angular gravel due to its resistance to compaction and movement.
If you have an upcoming project that requires either sand or gravel, you can request a quote from Shaw Resources online here.