Book excerpt: Acclaimed geologist Robert Thorson has been fascinated by kettle lakes ever since his youth in the upper Midwest. As with historic stone walls, each kettle lake has a story to tell, and each is emblematic of the interplay between geology and history. Beyond Walden covers the natural history of kettle lakes, a band of small lakes that extends from the prairie potholes of Montana to the cranberry bogs of Cape Cod. Kettle lakes were formed by glaciers and are recognizable by their round shape and deep waters. Kettles are the most common and widely distributed "species" of natural lake in the United States. They have no inlet or outlet streams so they are essentially natural wells tapping the groundwater. Isolated from one another, each lake has its own personality, and is vulnerable to pollution and climate warming. The most famous kettle lake is Walden Pond in Concord, Massachusetts; but northern Wisconsin, Michigan, and Minnesota are most closely associated with them. These lakes have had a tremendous impact on the livelihood and lifestyles of peoples of the area--Native Americans, early explorers and settlers, and the locals and tourists who now use the lakes for recreation. Thorson explores lake science: how kettle lakes are different from other lakes, what it takes to keep all lakes healthy, how global warming and other factors affect lakes. Beyond Walden has a strong environmental message, and will do for the kettle lakes of America's Heartland--and beyond--what Stone by Stone did for the historic stone walls of New England.