"Let us settle ourselves, and work and wedge our feet downward," Thoreau invites his readers in Walden, "till we come to a hard bottom and rocks in place, which we can call reality." Walden's Shore explores Thoreau's understanding of that hard reality, not as metaphor but as physical science. Robert M. Thorson is interested in Thoreau the rock and mineral collector, interpreter of landscapes, and field scientist whose compass...