Collision of cosmic rays with the atmosphere produces rare atoms, including beryllium-10, which falls out in precipitation and binds to soils. Measurements on glacial deposits from the last ice age in Colorado, USA show that millions of beryllium-10 atoms fall on each square centimeter of land each year. The rate of delivery increases with elevation and precipitation, which allowed us to make maps of the beryllium-10 flux for an entire watershed that will be used to measure erosion rates.