Why Geologists Think Glacial Mountains Look Like Sheep

Thank the French. An Object Lesson.

A short mountain stands beyond a lake, surrounded by trees, in Maine's Acadia National Park
Beehive Mountain in Acadia National Park, Maine (Mircea C / Shutterstock)

Two dozen mountains drape themselves diagonally across the middle of Acadia National Park on Mount Desert Island in Maine. The peaks start near Bar Harbor in the northeast, tumble down across Jordan Pond, Somes Sound, and Echo Lake, and end with Mansell and Bernard Mountains in the southwest. Every one of these mountains, like the state of Maine, all of New England, and much of the continent of North America, has been scoured by glaciers during multiple ice ages of the past. The most recent glacier began its retreat about 17,000 years ago.

In the mid-19th century, the Swiss-born geologist Louis Agassiz visited Maine on a vacation from Harvard University. The glacier-scoured bedrock formations on Mount Desert Island gave him plenty of evidence to support his theory of the Ice Age: that enormous glaciers, rather than a flood, had once covered much of Europe and North America. A few years after his visit, in 1867, Agassiz published two essays on “Glacial Phenomena in Maine” in The Atlantic (part I, part II). He used a French term to name what he saw, calling the glacially shaped formations roches moutonnées. The meaning of this term has caused confusion in geology ever since.

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To this day, many geologists follow Agassiz in considering all the rounded peaks and outcrops on Mount Desert Island to be roches moutonnées. Other geologists, including Duane and Ruth Braun, the co-authors of Guide to the Geology of Mount Desert Island and Acadia National Park, reserve this curious French term for the most distinctly asymmetrical of the mountains, ones with a long gradual slope along the northern “up-ice” side (the direction from which the glacier advanced) and cliffs on the lee or “down-ice” side. A mountain known as the Beehive, whose steep, plucked, south-facing cliffs terrify hikers with vertigo, illustrates the accepted American definition of roche moutonnée.

Indeed, the “Glossary of Glacial Terms” published in 2013 by the U.S. Geological Survey (USGS), defines roche moutonnée in just this way, as “an elongated, rounded, asymmetrical, bedrock knob produced by glacier erosion. It has a gentle slope on its up-glacier side and a steep to vertical face on the down-glacier side.” Interestingly, the USGS definition of the landform makes no attempt to translate the French roche moutonnée—and perhaps for good reason. It’s tricky, it turns out, to turn the expression into sensible English.

Roche means “rock.” Moutonnée is an antiquated word. Derived from the French word for sheep, mouton, it has no exact English equivalent. Some dictionaries have no listing for it; others say that the adjective means “sheep-like” or “fluffy.” In the 17th and 18th centuries, the word was used to describe hair that was curled and styled to resemble lamb’s wool.

As a geological term, roche moutonnée has been used for more than two centuries in a widespread, if not consistent way. The translation often given for the expression on websites and in textbooks is “sheepback,” a reasonable compromise. In a beautiful mid-19th century painting by Rosa Bonheur, for example, it’s possible to see asymmetrical “mountains” in the contours of her sheep’s hindquarters.

Stephen Marshak, an author of popular college geology textbooks, notes the resemblance to a sheep’s hindquarters in his definition of roche moutonnée. When one looks at the “granite and metamorphic rock hills of Acadia National Park,” he explains in Earth: Portrait of a Planet, “the hill’s profile resembles that of a sheep lying in a meadow.” Similarly, Julia A. Jackson’s comprehensive Glossary of Geology gives as synonyms for roche moutonnée both “sheepbackrock” and “sheeprock,” suggesting that if in doubt about a glacial landform, one should simply imagine a sheep’s contour.

The similarity between glacially scoured rocks and sheep is even more intriguing if one interprets the expression roches moutonnées more broadly, as did the 19th-century French geologist Albert de Lapparent. In his book Les Anciens Glaciers (“Ancient Glaciers”), Lapparent explained the origin of the term this way:

All the unevenness of the rock appears rounded on the upstream side, and [. . .] when one turns one’s eyes downstream, one has the impression of a cluster of sheep backs, whence comes the name “roches moutonnées,” used to designate these landforms so very characteristic of glacial action.

For Lapparent in 1893, the expression makes sense in the plural: A cluster of glacially sculpted, asymmetrical rocks look like a “suite” of sheep, whose backs are similarly curved and plunging. Lapparent assumes that the term came into being because of this resemblance, and his authority on the matter influenced scientists throughout the following century.

Some geology websites, however, understand a different sort of similarity between sheep and rocks. They translate the French expression not as “sheepback” but as “fleecy rock,” shifting attention from contour to texture. The rounded “backs” of glacially polished rock hardly look fleecy or “frizzy” (another translation). “Moutonnée” does mean something closer to “fleecy” than it does to “sheepback”—but one has to strain to see these sculpted landforms as woolly. They look smooth and slick instead.

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It takes a bit more detective work to figure out what is going on. Many geology textbooks and glossaries, including the current entry in Wikipedia, note that the term roche moutonnée originated with Horace-Bénédict de Saussure, an 18th-century Swiss geologist who traveled extensively through the Alps. He made his observations about 50 years before the advent of modern glacial theory, but he noticed a pattern that would become important for future glaciologists. According to Saussure, in some spots in the Swiss Alps, one could find smoothed rocks with a peculiar appearance: They looked to him like wigs. The “rounded, contiguous, repeated rocks (rochers) resemble,” wrote Saussure in 1786, “a thick mane of hair, or the wigs we also call moutonnées.”

It’s not entirely clear which wigs Saussure has in mind, as it’s hard to find a wig actually called a “moutonnée.” One mid-18th century hairstyle worn by French women was called the tête de mouton; it involved a smooth back and rows of curls in the front, in what one might imagine to be the same sort of asymmetrical arrangement as the roche moutonnée.

The tête de mouton hair style in “Mlle. Huquier with a Cat,” a 1745 painting by Jean-Baptiste Perronneau. (Artist / Musée du Louvre)

Most geologists, though, have assumed that Saussure had in mind a more dramatic mane of hair. They assert that he was referring to wigs shaped and attached with mutton grease pomade. In that case, the slick, hairy waviness explains the name’s origin—not a resemblance to the contour of a sheep’s back. This second theory is the one that the British geologists Douglas Benn and David Evans promote in their 1998 study Glaciers and Glaciations. Saussure named these landforms, the authors explain, “based on a fancied resemblance to the wavy wigs of [the late-18th century], which were called moutonnées after the mutton fat used to hold them in place.” Authoritative indeed, though Benn and Evans give no source.

Since its first coinage in 1786, Saussure’s name for glacially sculpted rocks and mountains has taken on a life of its own. In the 19th century, when the modern theory of glaciers was introduced, glaciologists plucked two terms—rochers and moutonnées—from Saussure’s famous studies of the Alps and brought them together in the expression roches moutonnées, losing sight of the essence (and essential wit) of the analogy he had used. Saussure’s happy observation that certain landforms looked like big hair or mutton wigs was put aside; wigs had gone out of fashion by the 19th century, and it must have seemed obvious to geologists that Saussure meant for us to see the contour of ovine bodies in roches moutonnées, not their fleece. At one point in the late 19th century, a scientist named Grenville Cole reread the original passage in Saussure and, alarmed, publicized the mistaken translation. But his attempt to reinstate the wig analogy was only partly successful, and through the decades the two not quite compatible explanations have coexisted in strange and wonderful ways.

Even now, geology websites make heroic attempts to reconcile the wig and the sheep-hindquarters theories. A Candian earth sciences website gives my favorite example. Describing Old Fort Point hill in Alberta, the website tells us that Saussure, who was “the first to clearly define the term geology,” also coined roches moutonnées to name landforms that “reminded him of the mutton wigs worn by the French aristocracy.” But then it continues:

Smooth and rounded on the back, combed upward in the front, such a wig looked rather like a sheep lying down. Which is pretty much the shape of any bedrock hill carved by moving glacial ice. Old Fort Point is a classic roche moutonnée.

As a crowning touch, the website displays an image of a fluffy lamb’s wool wig, whose resemblance to a resting sheep and to the asymmetrical Old Fort Point hill pictured next to it can be seen—with some effort, anyway. Recognizing the humor in the fanciful, multi-layered analogy, the site goes on to note that “a sheep-back hill” is also the home of bighorn sheep. Thus, the two readings of Saussure are wedded together, making a reader unlikely to forget this piece of geological lore.

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The Old Fort Point website has it mostly right: The glacially sculpted boulders and hills known today as roches moutonnées look like 18th-century wigs, which apparently got their name from mutton fat, and which also happen to look like the backs of sheep whose fat was used to slick them down. But there’s one last twist. The rounded up-ice surface of a roche moutonnée has on its back parallel scratched lines running roughly north-south, in the direction of the advancing ice sheet. Rocks lodged under the heavy ice scored the bedrock underneath, and from a distance, these parallel scratches (called striations) could be said to resemble combed hair.

The explanation of the roche moutonnée is surely more complicated than Saussure meant it to be. It has adapted itself nicely, though, to the needs of geologists who try to make sense of the formations they study. Saussure himself, I like to think, would have admired the bald peaks of Mount Desert Island, whose name, too, comes from the French. In 1607 the explorer Samuel de Champlain noted the place on his map of the Maine coast, calling it l’île des monts déserts, the island of bare mountains. Had Saussure been with him on that voyage, the mountains that looked so stark might have sprung to life—as a suite of grazing sheep or a well-oiled wig.