As glaciers melt and retreat, they leave behind rocks and boulders, some of which have been carried great distances. These erratics, aliens in their new geologic communities, stumped scientists until the mid–nineteenth century, when Louis Agassiz realized they provided strong evidence for his new theory of glaciation: they were blazes marking the former trail of the ice. Those boulders most easily linked to the geology of some more distant region are called “indicator blocks,” and have been found 750 miles or more from their source. According to one history of New England, patches of great erratic boulders are called bear dens. Erratics often give further testimony to their past in the crisscrossing striations that mark their surface, a result of their tumbling, churning tenure in the ice.

Bill McKibben