"The pit sand does not contain any marine impurities like seashells, chemical salts, silt deposits, which are more common in the river or sea sand. Sometimes you may find them associated with the clay materials, or some organic impurities which can be removed effectively by sieving and washing them as said earlier.
The pit sand is usually found in the regions having more quartz and sandstones, which gets eroded slowly forming the deposits of the sand layer."