sheepback rock

Gray humps of rock poking up through the soil are a common sight in northern New England, where the soil covering is typically less than thirty feet deep. The formation is known as a sheep rock, sheepback rock, whaleback, boulderback, or roche moutonnée. The latter French term means “fleecy rock,” a rock that looks like a sheep lying prone. Properly defined, these knobs are erosion-resistant outcroppings of bedrock, all of them shaped by glacial movement in a characteristic way—a smooth, gentle slope on the upstream side, and “plucked” or left ragged on the steeper, downstream side. (Sheepback rocks are always found with their long axes parallel to the direction of the glacier’s movement.) Sheepbacks are not random rocks lying loose atop the soil but exposures of the bedrock below; that is, not rocks dropped by retreating glaciers but features cut in place by the grind and push of glacial ice, moving over the uneven surface of a rock that stood fast before such forces. In the many places where these rock forms appear across New England, you may also find obvious scratch marks and gouges in the parent bedrock. Called striations, these lines were etched by bits of harder rock, embedded like styluses on the underside of a passing glacier.

Gretchen Legler