Hilltops surrounded by farmland in southern Wisconsin turn out to be the eroded remnants of an ancient archipelago. An island in the Yellow Sea where Korean tourists flock is the peak of a flooded mountain rising from a drowned continental shelf. From a mountaintop shrine to Genghis Khan in Inner Mongolia, the silhouette of a Silurian seascape can be spotted. On the shores of Hudson Bay, where polar bears patrol the Arctic tundra, a close look unveils what was a tropical coastline encrusted with corals nearly 450 million years ago.

The geologist Markes E. Johnson invites readers on a journey through deep time to find the traces of ancient islands. He visits a dozen sites around the globe, looking above and below today’s waterlines to uncover how landscapes of the past are preserved in the present. Going back 500 million years to the Cambrian through the Pleistocene 125,000 years ago, this book reconstructs how “paleoislands” appeared under different climatic conditions and environmental constraints. Finding vestiges of prehistoric ecologies, Johnson emphasizes the complexity of island ecosystems and the importance of preserving these significant sites.

Inviting and accessible, this book is a travelogue that takes readers through time as well as space. Islands in Deep Time shares the adventure of exploring striking locations across geologic eras and issues a passionate call for their conservation.

Markes E. Johnson is the Charles L. MacMillan Professor of Natural Science Emeritus at Williams College. His most recent book is Baja California’s Coastal Landscapes Revealed: Excursions in Geologic Time and Climate Change (2021).