One of the underlying assumptions of the authors is that the environment, including physical structures such as mountains and plains, and climatic conditions, provide the context for civilisational developments. They document the evidence for physiographic and climatic changes. These are then connected with the decline of the Harappan civilisation which is partially explained in terms of changes in river courses and decreasing rainfall. The authors suggest that this triggered off population movements into first, the upper Ganga-Yamuna doab, and subsequently to the south and east. This in turn provided the context for what is known as the second urbanisation, that of the Ganga Valley.