rock flour

noun

Origin:
1880–85

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World English Dictionary
rock flour
 
n
very finely powdered rock, produced when rocks are ground together (as along the faces of a moving fault or during the motion of glaciers) and are thus chemically unweathered

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Example sentences
The glaciers in the background grind the rocks underneath as they melt in the
  summer and produce rock flour.
We also learned the secret of the mountain lakes' brilliant turquoise color: a
  substance called rock flour.
It is found beneath old swamps, as mud on the bottom of the sea, and as the
  ground-up rock flour left by melting glaciers.
Rock debris is crushed and ground into fine grains, called rock flour.
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