Gravel refers to rocks or mineral debris with sharp edges and corners with an average particle size of more than 2 mm and less than 64 mm. Geologically, natural rocks with smooth surfaces and no sharp edges and corners are defined as pebbles, and those with a particle size of less than 2 mm are defined as sand.
Sand refers to a loose mixture of sand. Geologically, minerals or rock particles with a particle size of 0.074~2mm are called sand, and those with a particle size larger than 2mm are called gravel or breccia (the difference between them lies in their smoothness, see the entry conglomerate and sandstone for details).
Gravel can be subdivided into fine gravel (particle size is 2- 10 mm), coarse gravel (10- 100 mm) and giant gravel (greater than 100 mm). Typical gravel specific gravity is 1800kg/m3.
Gravel is formed by weathering of rocks exposed to the surface; Usually deposited in foothills and foothills; Or because rocks are eroded and broken by water and washed and deposited by rivers; Gravel cementation forms conglomerate or breccia.
Extended data:
Conglomerate is a kind of sedimentary rock, which is formed by carrying, depositing, compacting and cementing the debris with particle size larger than 2 mm after the parent rock is broken.
If the average diameter of gravel is 2-4 mm, it is fine gravel, 4-50 mm is called medium gravel, 50-200 mm is coarse gravel, and more than 200 mm is huge gravel. Conglomerate is a kind of clastic rock. Its cement often contains minerals, and conglomerate can be used as building materials.
The conglomerate is rough, and obvious gravel can be seen. If the gravel cemented in the rock is angular, it is called breccia.
Breccia is located at the bottom of the transgression sequence, and it is in pseudo-conformity contact with the underlying strata, representing the sedimentary discontinuity in a certain geological period. The bottom conglomerate is usually formed by quartzite conglomerate.
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