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It was during the time of the Greek philosopher Aristotle (384–322 BC) that someone may have first argued that rocks can be viewed as recorders of ancient environments, although just how ancient was still not clearly understood in his time. By walking along the seashore and observing the types of sediments present and the various organisms living there, philosophers as early as Aristotle were able to deduce that the rocks which made up the landscape shared many characteristics of the modern ocean. From this, Aristotle would argue that “where there is sea, there is at another time land.” This was perhaps the first application of the now-foundational dictum of geology: “the present is the key to the past” – the idea that we can apply our understanding of the natural world today to gain an understanding of the past. This chapter reviews some of the historical milestones and key people in the development of geological reasoning and concludes with a discussion of how modern geological science works as an academic and scientific discipline.
This chapter is specifically focused on the events that occurred at the boundary between the Mesozoic and Cenozoic eras, commonly referred to as the K–Pg extinction because it marks the boundary between the Cretaceous (K) and Paleogene (Pg) geologic periods. It is the most recent of the five mass extinctions introduced in Chapter 9 and has been a topic of considerable public interest because it also marks the end of the “Age of Dinosaurs” (see Chapter 12). The two main hypotheses for the cause of the extinction center on a giant asteroid impact and massive volcanic eruptions in India.