Universidade Estadual Vale do Acaraú (UVA) 2014 - Inglês

TEXTO

Ice ages, those periods when ice covered extensive areas of the Earth, are known to have occurred at least six times. Past ice ages can be recognized from rock strata that show evidence of foreign materials deposited by moving walls of ice or melting glaciers. Ice ages can also be recognized from land formations that have been produced from moving walls of ice, such as U-shaped valleys, sculptured landscapes, and polished rock faces. 

According to the passage, what in the rock strata is a clue to geologists of a past ice age?