Humankind has entered another lithic epoch. Concrete is the modern stone. Since taking its contemporary form nearly two centuries ago, over five hundred billion tons of the gray matter have been deposited on the earth’s crust. If this amount of concrete was used to build a sidewalk that was six feet wide and three inches thick, it could wrap around the equator over thirty-eight thousand times. The scale of production is tremendous, but only part of the story. Due to being fire-resistant, waterproof, plentiful, durable, malleable and relatively cheap, concrete has become the primary material usedto transform the possibilities of human geography. Such megalithic environmental manipulations would be impossible without the sustained mass production of cement, concrete’s essential ingredient. This dissertation explores the origins of the contemporary concrete cornucopia through an environmental history of the cement transitions that manifested it. Abundant fuel and raw materials as well as robust building regimes and demand for large-scale building on land and under water are necessary conditions for such cement transitions—defined as occurring whenever the production process and properties of cement are altered in a way that significantly changes construction possibilities. A central claim of this dissertation is that these requirements were met in southeastern Great Britain at the turn of the nineteenth century with the discovery of the cementitious properties of the natural cement stones in the London Clay at the moment of British imperial consolidation and industrial take-off. Ironically named “Roman cement,” this natural cement substitute differed from its ancient namesake that had determined the building possibilities of western Europe for roughly two millennia. The British cement production system soon spread to other industrial regions with similar raw material deposits, notably the northern United States, in a process of technology transfer that has since transformed the world. It is argued that this method of mass producing durable, quick-setting and waterproof cement with fossil fuels and its worldwide diffusion was foundational to the built environment’s divergence from the organic economy. Thus began the Second Stone Age.