A hard mass of mineral, usually rounded, found in various forms in soil created by the deposition of minerals from solution. The way nodules are formed can assist in palaeoenvironmental reconstruction and the age of the conditions under which they formed. Nodules are often elongate with a knobby irregular surface; they usually are oriented parallel to the bedding. Chert and flint often occur as dense and structureless nodules of nearly pure silica in limestone or chalk, where they seem to be replacements of the carbonate rock by silica.