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Rock flour flowing into Peyto Lake

The river Muru flowing into Gjende lake in Norway

Rock flour, or glacial flour, consists of clay-sized particles of rock, generated by glacial erosion or by artificial grinding to a similar size. Because the material is very small, it is suspended in river water making the water appear cloudy. If the river flows into a glacial lake, the lake may appear turquoise in color as a result. Examples of this are Lake Louise, Canada and Gjende lake in Norway.

Natural rock flour is typically formed during glacial migration, where the glacier grinds against rock beneath it, but is also produced by freeze thaw, where the act of water freezing and expanding in cracks helps break up rock formations.

Although clay-sized, its particles are not clay minerals but typically ground up quartz and feldspar. Rock flour is carried out from the system via meltwater streams, where the particles travel in suspension. Rock flour particles can travel great distances either suspended in water or by the wind, in the latter case forming deposits called loess.

Some agronomists believe that rock flour has a powerful effect in restoring trace minerals to soil. An early experimeter was the German miller Julius Hensel, author of Bread from Stones, who reported successful results with steinmehl (stonemeal) in the 1890s. His ideas were not taken up through technical limitations and, according to proponents of his method, opposition from the champions of conventional fertilisers.

John D. Hamaker argues that widespread remineralization of soils with rock dust is required to reverse soil depletion by current agriculture and forestry practice.

While this was originally an alternative concept, increasing mainstream research has been devoted to soil amendment and other benefits of rock flour application: for instance, a pilot project on the use of glacial rock, granite and basaltic fines by the U.S. Department of Agriculture at the Henry A. Wallace Beltsville Agricultural Research Center. The SEER Centre in Scotland is a leading source of information on the use of rock dusts and mineral fines. The Soil Remineralization Forum was established with sponsorship from the Scottish Environment Protection Agency and has commissioned a portfolio of research into the benefits of using mineral fines. The Forum provides an interface between research, environmentalists and industry.

• On a Fad Diet of Rock Dust, How the Garden Does Grow Henry Homeyer, June 24, 2004, New York Times.
• Remineralize the Earth - "non-profit organization incorporated to disseminate ideas and practice about soil remineralization".
• Soil remineralization: a future market for aggregate industry fines, Robert J. Able, June 1995, Pit & Quarry.
• Julius Hensel
• [1]Downloadable document on co-utilisation of mineral fines and composts