Past research has evaluated how preservice teachers, high school students, and introductory college students conceptualize geologic or deep time. The ability to think on geologic timescales is regarded as an important skill in the study of geology, yet little work has specifically addressed how student understanding of this concept evolves during the undergraduate experience and beyond. We describe an exploratory, preexperimental study conducted at a large research university investigating student knowledge of various Earth history and geologic time concepts among geology majors. A 21-question pre/posttest was constructed to assess concepts related to landscape identification, landscape formation rates, Earth history, the geologic timescale, and relative and absolute dating. Pre/posttesting was conducted across four semesters within a variety of courses including historical geology, structural geology, geomorphology, and a geology field camp course. Data were also collected from a group of nonmajors in an introductory physical geology course to obtain a novice score and from a group of faculty expert geoscientists at the Geological Society of America annual meeting to obtain an expert score on the instrument. Additionally, interviews were conducted with 11 senior geology majors to gain a deeper understanding of their conceptions related to a sample of the concepts assessed. Results show that students make the largest gains after taking physical and historical geology courses, but no significant changes were seen in upper-level geology courses. Many geology students lacked familiarity with the geologic timescale and had difficulty estimating formation rates of landscapes formed on intermediate timespans.