Rock flour, orglacial flour, is a kind offlour which consists ofclay-sized particles of rock, generated byglacial erosion[broken anchor] or by artificial grinding to a similar size. Because the material is very small, it issuspended in river water making the water appear cloudy. If the river flows into aglacial lake, the lake may appear turquoise in color as a result. Examples of this areLake Louise, Canada andGjende lake in Norway.
Someagronomists believe that rock flour has a powerful effect in restoring trace minerals to soil. An early experimeter was theGerman millerJulius Hensel, author ofBread from Stones, who reported successful results withsteinmehl (stonemeal) in the1890s. His ideas were not taken up through technical limitations and, according to proponents of his method, opposition from the champions of conventionalfertilisers.