Glacial flour, also known as rock flour, is finely ground rock and mineral sediment created by the mechanical grinding of bedrock during glacial erosion.
Because the particles are extremely fine - similar in consistency to flour - they remain suspended in meltwater, making the water appear cloudy. This fine sediment scatters sunlight, creating the milky, cloudy, or vibrant turquoise colours often seen in glacial lakes and rivers. Examples include Lake Tekapo and Lake Pukaki in the South Island of New Zealand.