McCREA has revived the idea that the Earth's ice ages may have been caused by the interaction of the Sun with dense interstellar clouds1. In this theory, originally formulated by Hoyle and Lyttleton2, the solar luminosity is temporarily increased by the accretion of gas on to the Sun when the Solar System passes through a dense interstellar cloud. The increased insolation is assumed to cause an increase in precipitation and ice accumulation. McCrea suggests that the recurrence and duration of ice epochs can be attributed to the passage of the Solar System through compression lanes in the spiral arms. Relative velocities between the Solar System and the clouds range from 5–20 km s−1, but cloud densities of 105–107 hydrogen molecules per cm3 are called for by the model. A study of the character and grain size distribution of texturally mature lunar soils supports this model3. Here we examine several consequences of the passage of a dense cloud through the Solar System and find severe problems for this glaciation mechanism.