Deep Thinking Over Geologic Time is a seven-part unit consisting of hands on activities and graphical analysis designed to investigate the role of fossils and rock layers in determining the Earth’s age and geologic history. Individual lessons begin with an investigation of relative dating (Days 1 and 2) in which students apply the Laws of Superposition and Original Horizontality. On Day 2, students analyze a Stratigraphic Succession Map of Michigan. To make this more relevant to their students, teachers may want to substitute a map of their own state. Next, students participate in a hands on activity using information from “Earth cups” to develop a relative biological timeline of Earth’s history. The unit then shifts into scientifically-based activities, tasking students with creating a 4.6 billion year biological timeline and investigating connections among the stratigraphic succession maps of several US national parks. Finally, students utilize the concept of index fossils in their analysis of a simple geologic timeline provided in their student guide. The authors estimate that the unit can be completed in seven class periods. A handout of fossils to complete the Day 4 activity, a student guide and teacher guide are included and can be found at www.nsta.org/scope1217 . The authors include a lengthy list of supplies necessary to implement the unit; teachers should allow themselves adequate time to gather all materials.