If you walk up on a mountain stream, crystal clear but for a mysterious streak of milky blue threaded through its ﬂow, you’ve likely found the telltale sign of a rock ﬂour deposit, a collection of silt-sized particles generated by glacial erosion and swept along by glacial meltwater, sometimes over very great distances. When the rock-studded base of a glacier scrapes its way across a valley ﬂoor, pieces of the ice-embedded rock are ground to a ﬁne powder. These tiny particles are mostly quartz, a mineral that stays suspended in water long after other, heavier material has dropped out, and it’s this suspended quartz powder that creates the milky blue tint and gives rise to the term glacial milk. A simple test to determine whether what you’re seeing is rock ﬂour is to dip a container in the ﬂow, shake it up, and look for light sparkling in the swirling dust. Dried rock ﬂour is sometimes carried great distances by the wind, and then it may be deposited as one of the components of a layer of ﬁne, rich soil called loess. The parts of western Idaho and eastern Washington called the Palouse, as well as parts of Iowa, are known for their fertile, gently undulating loess hills.