IT has been proposed by McCrea1, Shapley2 and Hoyle and Lyttleton3 that passages of the Solar System through interstellar clouds have appreciable effects on the Earth. McCrea argues that the recurrence of ice epochs4 every ∼ 250 Myr coincides with the passage of the Solar System through a galactic spiral arm approximately every 108 yr. We report here studies on the character and grain-size distribution of texturally-mature lunar soils which support these views. The evidence shows the flux of micrometeoroids (≲ 10−6 g) at the lunar surface has remained in quasi-equilibrium near the present-day value over the past 2×109 yr, but that significant increases have occurred. Three near-cyclical enhancements are superimposed on the drill core stratigraphy, with separations of ∼ 108 yr. The magnitudes, durations, and periodicity of the flux increases suggest their origin may be the passage of the Solar System through dust lanes in the galactic spiral arms.