MAPS, CROSS-SECTIONS AND THE GEOLOGY OF CHARLES DARWIN

Charles Darwin’s geological publications focus on his descriptions of geologic units and structure and his theorizing about geologic processes, perhaps most notably about aspects of what we now call tectonics. His thinking was influenced by, and his publications illustrated by, a variety of maps and geologic cross-sections. As a self-assigned exercise before he received the invitation to join the HMS Beagle, he prepared an unpublished geologic map of a region near his family home in Shropshire, UK. One of the principal missions of the Beagle expedition and its captain, Robert FitzRoy, was the preparation of nautical charts. These charts also influenced Darwin’s thinking and remain a valuable resource for historical, geomorphic, and tectonic studies. Perhaps most well known of Darwin’s graphic depictions of geologic structure are three beautifully colored geologic sections that he made along his traverses into and across the Andes and that he included in his book on the geology of South America. Less well known is his unpublished bedrock map of southern South America, which he may have decided not to publish because of the prior publication of a map of much the same region by his French contemporary (and perceived rival), Alcide d’Orbigny, notwithstanding the greater detail on Darwin’s map. His book on coral reefs, describing his theory for the formation of coral islands by the subsidence of the ocean basins, contains a map showing the distribution of coral islands by type as well as a map and section of the Keeling (now Cocos) Islands in the Indian Ocean. Apparently at the urging of a reviewer, Adam Sedgwick, Darwin’s publication on the origin of the Parallel Roads of Glen Roy in Scotland includes a map of these ancient shorelines. Thus while Darwin’s geologic legacy is not centered around geologic mapping per se, maps and cross-sections were important parts of his geologic thinking, and he was comfortable communicating in this geologic idiom.