An understanding of geologic time is comprised of 2 facets. Events in Earth's history can be placed in relative and absolute temporal succession on a vast timescale. Rates of geologic processes vary widely, and some occur over time periods well outside human experience. Several factors likely contribute to an understanding of geologic time, one of which is an individual's ability to perceive the relative size of large time periods and to move multiplicatively through quantities that differ by many orders of magnitude. Thirty-five US students aged 13-24 years participated in task-based interviews to assess their understanding of large temporal periods. Fewer than half of the students performed well enough to indicate that their knowledge of large numbers was robust enough to enable them to understand processes in geologic time. Some students were confused about relationships between quantities in the thousands and millions, while others had difficulty showing proportional relationships among relatively small temporal units (up to 100 years). Students differed in their ability to perceive the entire scale upon which numbers were to be placed as well as broader problem-solving strategies. Spatial mapping of numbers was evident. Implications for future research are discussed.